ALBERT

Description: Nature Geoscience 6, 828 (2013). doi:10.1038/ngeo1931 Authors: Shang-Ping Xie, Bo Lu & Baoqiang Xiang Spatial variations in ocean warming have been linked to regional changes in tropical cyclones, precipitation and monsoons. But development of reliable regional climate projections for climate change mitigation and adaptation remains challenging. The presence of anthropogenic aerosols, which are highly variable in space and time, is thought to induce spatial patterns of climate response that are distinct from those of well-mixed greenhouse gases. Using CMIP5 climate simulations that consider aerosols and greenhouse gases separately, we show that regional responses to changes in greenhouse gases and aerosols are similar over the ocean, as reflected in similar spatial patterns of ocean temperature and precipitation. This similarity suggests that the climate response to radiative changes is relatively insensitive to the spatial distribution of these changes. Although anthropogenic aerosols are largely confined to the Northern Hemisphere, simulations that include aerosol forcing predict decreases in temperature and westerly wind speed that reach the pristine Southern Hemisphere oceans. Over land, the climate response to aerosol forcing is more localized, but larger scale spatial patterns are also evident. We suggest that the climate responses induced by greenhouse gases and aerosols share key ocean–atmosphere feedbacks, leading to a qualitative resemblance in spatial distribution.

Description: Nature Geoscience 6, 856 (2013). doi:10.1038/ngeo1928 Authors: Chloé Michaut, Yanick Ricard, David Bercovici & R. Steve J. Sparks Eruptions at active silicic volcanoes are often cyclical. For example, at the Soufrière Hills volcano in Montserrat, Mount Pinatubo in the Philippines, and Sakurajima in Japan, episodes of intense activity alternate with repose intervals over periods between several hours and a day. Abrupt changes in eruption rates have been explained with the motion of a plug of magma that alternatively sticks or slides along the wall of the volcanic conduit. However, it is unclear how the static friction that prevents the plug from sliding is periodically overcome. Here we use two-phase flow equations to model a gas-rich, viscous magma ascending through a volcanic conduit. Our analyses indicate that magma compaction yields ascending waves comprised of low- and high-porosity bands. However, magma ascent to lower pressures also causes gas expansion. We find that the competition between magma compaction and gas expansion naturally selects pressurized gas waves with specific periods. At the surface, these waves can induce cyclical eruptive behaviour with periods between 1 and 100 hours, which compares well to the observations from Soufrière Hills, Mount Pinatubo and Sakurajima. We find that the period is insensitive to volcano structure, but increases weakly with magma viscosity. We conclude that observations of a shift to a longer eruption cycle imply an increase in magma viscosity and thereby enhanced volcanic hazard.

Description: Nature Geoscience 6, 866 (2013). doi:10.1038/ngeo1933 Authors: Suzanne M. Carbotte, Milena Marjanović, Helene Carton, John C. Mutter, Juan Pablo Canales, Mladen R. Nedimović, Shuoshuo Han & Michael R. Perfit The global mid-ocean ridge is segmented in its seafloor morphology and magmatic systems, but the origin of and relationships between this tectonic and magmatic segmentation are poorly understood. At fast-spreading ridges, tectonic segmentation is observed on a fine scale, but it is unclear whether this partitioning also occurs in the magmatic system. Fine-scale tectonic segmentation could have a deep origin, arising from the distribution of upwelling mantle melt, or a shallow origin, linked to offset intruding dikes from long, more continuous crustal reservoirs. Here we use seismic reflection data from the fast-spreading East Pacific Rise, between 8° 20′ N and 10° 10′ N, which includes a unique area where two documented volcanic eruptions have occurred, to image the crustal magma bodies in high resolution. We find that the magma reservoirs form 5- to 15-km-long segments that coincide with the fine-scale tectonic segmentation at the seafloor and that three lens segments fed the recent eruptions. Transitions in composition, volume and morphology of erupted lavas coincide with disruptions in the lens that define magmatic segments. We conclude that eruptions at the East Pacific Rise are associated with the vertical ascent of magma from lenses that are mostly physically isolated, leading to the eruption of distinct lavas at the surface that coincide with fine-scale tectonic segmentation.

Description: Nature Geoscience 6, 871 (2013). doi:10.1038/ngeo1911 Authors: Judith A. Coggon, Ambre Luguet, Geoffrey M. Nowell & Peter W. U. Appel Partial melting of the Earth’s mantle is a key process in the generation of crustal material and the formation of continents. Crustal samples record the generation of crust up to 4.4 billion years (Gyr) ago, yet the complementary record in the mantle extends to only 3.3 Gyr ago, with sparse evidence for differentiation occurring 3.9–4.1 Gyr ago. Here we use the Pt–Os isotope chronometer to show that a Hadean record of mantle depletion is preserved in Earth’s oldest known ultramafic rocks, the Ujaragssuit Nunât intrusion of southwest Greenland. We identify two distinct age populations at approximately 4.1 and 2.9 Gyr. We suggest that the younger age population records a regional metamorphic event and the older one records mantle depletion. We also identify individual sample ages of up to 4.36 Gyr old, thus extending the record of large mantle-melting events into the Hadean. Furthermore, the preservation of Hadean model ages in Os-rich mantle-derived rocks supports the theory that re-enrichment of Os in the mantle during the Late Heavy Bombardment—after expected partitioning into the Earth’s core—occurred at least 0.2 Gyr earlier than previously thought. This also implies that the Earth could have been habitable by 4.1 Gyr ago.

Description: Nature Geoscience 6, 875 (2013). doi:10.1038/ngeo1903 Authors: Jun Wu & Peter R. Buseck A significant fraction of Earth’s carbon resides in the mantle, but the mode of carbon storage presents a long-standing problem. The mantle contains fluids rich in carbon dioxide and methane, carbonate-bearing melts, carbonate minerals, graphite, diamond and carbides, as well as dissolved carbon atoms in metals. However, it is uncertain whether these can sufficiently account for the total amount of carbon thought to be stored in the mantle and the volume of carbon degassed from the mantle at volcanoes. Moreover, such carbon hosts should significantly affect the physical and chemical behaviour of the mantle, including its melting temperature, electrical conductivity and oxidation state. Here we use in situ transmission electron microscopy to measure the storage of carbon within common mantle mineral analogues—nickel-doped lanthanum chromate perovskite and titanium dioxide—in laboratory experiments at high pressure and temperature. We detect elevated carbon concentrations at defect sites in the nanocrystals, maintained at high pressures within annealed carbon nanocages. Specifically, our experiments show that small stacking faults within the mantle analogue materials are effective carbon sinks at mantle conditions, potentially providing an efficient mechanism for carbon storage in the mantle. Furthermore, this carbon can be readily released under lower pressure conditions, and may therefore help to explain carbon release in volcanic eruptions.

Description: Nature Geoscience 6, 885 (2013). doi:10.1038/ngeo1922 Authors: Lars Möller, Todd Sowers, Michael Bock, Renato Spahni, Melanie Behrens, Jochen Schmitt, Heinrich Miller & Hubertus Fischer

Description: Nature Geoscience 6, 847 (2013). doi:10.1038/ngeo1905 Authors: Marcello Campione & Gian Carlo Capitani Earthquakes generated in subduction zones are caused by unstable movements along faults. This fault-slip instability is determined by frictional forces that depend on the temperature, pressure, morphology and deformation state of the fault rocks. Fault friction may also be influenced by preferred mineral orientations. Over-thrusting of rocks at the interface between a subducting slab and the overlying mantle wedge generates shear deformation that causes minerals to align, and this preferred mineral orientation affects the propagation of shear seismic waves. Here we use laboratory experiments to simulate fault slip in antigorite, the most abundant hydrous mineral phase within Earth’s upper mantle. Using atomic force microscopy, we show that antigorite single crystals possess strong frictional anisotropy on their basal slip surface and that preferred mineral alignment extends this property to a regional scale. Depending on the alignment, fault movements can occur along a high-friction direction, creating stick-slip behaviour that generates earthquakes. In contrast, if movements occur along a low-friction direction, the mantle wedge will deform aseismically. Our results imply that mantle rocks in subduction-zone thrust faults can exhibit two opposite frictional behaviours, seismic and aseismic.

Description: Magnetic field and dynamic pressure ULF fluctuations in coronal-mass-ejection-driven sheath regions Annales Geophysicae, 31, 1559-1567, 2013 Author(s): E. K. J. Kilpua, H. Hietala, H. E. J. Koskinen, D. Fontaine, and L. Turc Compressed sheath regions form ahead of interplanetary coronal mass ejections (ICMEs) that are sufficiently faster than the preceding solar wind. The turbulent sheath regions are important drivers of magnetospheric activity, but due to their complex internal structure, relatively little is known on the distribution of the magnetic field and plasma variations in them. In this paper we investigate ultra low frequency (ULF) fluctuations in the interplanetary magnetic field (IMF) and in dynamic pressure ( P dyn ) using a superposed epoch analysis of 41 sheath regions observed during solar cycle 23. We find strongest fluctuation power near the shock and in the vicinity of the ICME leading edge. The IMF and P dyn ULF power have different profiles within the sheath; the former is enhanced in the leading part of the sheath, while the latter is increased in the trailing part of the sheath. We also find that the ICME properties affect the level and distribution of the ULF power in sheath regions. For example, sheath regions associated with strong or fast ICMEs, or those that are crossed at intermediate distances from the center, have strongest ULF power and large variation in the power throughout the sheath region. The weaker or slower ICMEs, or those that are crossed centrally, have in general considerably weaker ULF power with relatively smooth profiles. The strong and abrupt decrease of the IMF ULF power at the ICME leading edge could be used to distinguish the ICME from the preceding sheath plasma.

Description: Cluster observation of few-hour-scale evolution of structured plasma in the inner magnetosphere Annales Geophysicae, 31, 1569-1578, 2013 Author(s): M. Yamauchi, I. Dandouras, H. Rème, R. Lundin, and L. M. Kistler Using Cluster Ion Spectrometry (CIS) data from the spacecraft-4 perigee traversals during the 2001–2006 period (nearly 500 traversals after removing those that are highly contaminated by radiation belt particles), we statistically examined the local time distribution of structured trapped ions at sub- to few-keV range as well as inbound–outbound differences of these ion signatures in intensities and energy–latitude dispersion directions. Since the Cluster orbit during this period was almost constant and approximately north–south symmetric at nearly constant local time near the perigee, inbound–outbound differences are attributed to temporal developments in a 1–2 h timescale. Three types of structured ions at sub- to few keV range that are commonly found in the inner magnetosphere are examined: – Energy–latitude dispersed structured ions at less than a few keV, – Short-lived dispersionless ion stripes at wide energy range extending 0.1–10 keV, – Short-lived low-energy ion bursts at less than a few hundred eV. The statistics revealed that the wedge-like dispersed ions are most often observed in the dawn sector (60% of traversals), and a large portion of them show significant enhancement during the traversals at all local times. The short-lived ion stripes are predominantly found near midnight, where most stripes are significantly enhanced during the traversals and are associated with substorm activities with geomagnetic AL 〈 −300 nT. The low-energy bursts are observed at all local times and under all geomagnetic conditions, with moderate peak of the occurrence rate in the afternoon sector. A large portion of them again show significant enhancement or decay during the traversals.

Description: Nature Geoscience 6, 801 (2013). doi:10.1038/ngeo1983 The latest report on global warming brings yet another rise in confidence that human actions are altering the Earth's climate. But in contrast to its 2007 predecessor, it is unlikely to cause a stir.

Description: Nature Geoscience 6, 802 (2013). doi:10.1038/ngeo1971 Authors: Thomas Shea, Julia Hammer & Emily First

Description: Nature Geoscience 6, 803 (2013). doi:10.1038/ngeo1970 Authors: Melissa D. Rotella, Colin J. N. Wilson, Simon J. Barker & Ian C. Wright

Description: Nature Geoscience 6, 805 (2013). doi:10.1038/ngeo1981 Author: Amy Whitchurch

Description: Nature Geoscience 6, 807 (2013). doi:10.1038/ngeo1960 Author: Julie Brigham-Grette During the Last Glacial Maximum, ice sheets in Eurasia terminated at the edge of the Laptev Sea. Seismic data now suggest that a separate ice sheet was repeatedly centred further east, in the East Siberian Sea, during previous glacial periods.

Description: Nature Geoscience 6, 805 (2013). doi:10.1038/ngeo1979 Author: Tamara Goldin

Description: Nature Geoscience 6, 811 (2013). doi:10.1038/ngeo1951 Author: W. Roger Buck Intrusions of magma into the crust help accommodate the divergence between tectonic plates. A magnetotelluric survey of the crust and mantle beneath Afar, Ethiopia, has identified enough magma to accommodate plate separation there for about 50,000 years.

Description: Nature Geoscience 6, 805 (2013). doi:10.1038/ngeo1978 Author: Anna Armstrong

Description: Nature Geoscience 6, 813 (2013). doi:10.1038/ngeo1955 Authors: Stefanie Kirschke, Philippe Bousquet, Philippe Ciais, Marielle Saunois, Josep G. Canadell, Edward J. Dlugokencky, Peter Bergamaschi, Daniel Bergmann, Donald R. Blake, Lori Bruhwiler, Philip Cameron-Smith, Simona Castaldi, Frédéric Chevallier, Liang Feng, Annemarie Fraser, Martin Heimann, Elke L. Hodson, Sander Houweling, Béatrice Josse, Paul J. Fraser, Paul B. Krummel, Jean-François Lamarque, Ray L. Langenfelds, Corinne Le Quéré, Vaishali Naik, Simon O'Doherty, Paul I. Palmer, Isabelle Pison, David Plummer, Benjamin Poulter, Ronald G. Prinn, Matt Rigby, Bruno Ringeval, Monia Santini, Martina Schmidt, Drew T. Shindell, Isobel J. Simpson, Renato Spahni, L. Paul Steele, Sarah A. Strode, Kengo Sudo, Sophie Szopa, Guido R. van der Werf, Apostolos Voulgarakis, Michiel van Weele, Ray F. Weiss, Jason E. Williams & Guang Zeng

Description: Nature Geoscience 6, 805 (2013). doi:10.1038/ngeo1980 Author: Alicia Newton

Description: Nature Geoscience 6, 809 (2013). doi:10.1038/ngeo1953 Author: Klaus Hasselmann

Description: Nature Geoscience 6, 804 (2013). doi:10.1038/ngeo1965 Author: Emily Lakdawalla

Description: Nature Geoscience 6, 833 (2013). doi:10.1038/ngeo1887 Authors: J. N. Bassis & S. Jacobs Iceberg calving has been implicated in the retreat and acceleration of glaciers and ice shelves along the margins of the Greenland and Antarctic ice sheets. Accurate projections of sea-level rise therefore require an understanding of how and why calving occurs. Unfortunately, calving is a complex process and previous models of the phenomenon have not reproduced the diverse patterns of iceberg calving observed in nature. Here we present a numerical model that simulates the disparate calving regimes observed, including the detachment of large tabular bergs from floating ice tongues, the disintegration of ice shelves and the capsizing of smaller bergs from grounded glaciers that terminate in deep water. Our model treats glacier ice as a granular material made of interacting boulders of ice that are bonded together. Simulations suggest that different calving regimes are controlled by glacier geometry, which controls the stress state within the glacier. We also find that calving is a two-stage process that requires both ice fracture and transport of detached icebergs away from the calving front. We suggest that, as a result, rapid iceberg discharge is possible in regions where highly crevassed glaciers are grounded deep beneath sea level, indicating portions of Greenland and Antarctica that may be vulnerable to rapid ice loss through catastrophic disintegration.

Description: Nature Geoscience 6, 824 (2013). doi:10.1038/ngeo1923 Authors: C. T. Adcock, E. M. Hausrath & P. M. Forster If the chemistry essential to life was present in water-containing environments on Mars, the processes that led to life on Earth may have also occurred on the red planet. Phosphate is one of the chemical nutrients thought to be essential for life and is also considered critical to reactions that may have led to life on Earth. However, low prebiotic availability of phosphate may have been a complicating factor in terrestrial abiogenesis, suggesting that a similar hurdle may have confronted the development of life on Mars. Phosphate available for biological reactions can be introduced into aqueous environments through dissolution of primary phosphate minerals during water–rock interactions, but little is known about the dissolution of the dominant phosphate minerals found in martian meteorites and presumably on Mars. Here we present dissolution rates, phosphate release rates and solubilities of phosphate minerals found in martian rocks as determined from laboratory measurements. Our experimental findings predict phosphate release rates during water–rock interactions on Mars that are as much as 45 times higher than on Earth and phosphate concentrations of early wet martian environments more than twice those of Earth. We suggest that available phosphate may have mitigated one of the hurdles to abiogenesis on Mars.

Description: Nature Geoscience 6, 837 (2013). doi:10.1038/ngeo1924 Authors: María Fernanda Sánchez Goñi, Edouard Bard, Amaelle Landais, Linda Rossignol & Francesco d’Errico A period of continental ice growth between about 80,000 and 70,000 years ago was controlled by a decrease in summer insolation, and was among the four largest ice expansions of the past 250,000 years. The moisture source for this ice sheet expansion, known as the Marine Isotope Stage (MIS) 5a/4 transition, has been proposed to be the warm subpolar and northern subtropical Atlantic Ocean. However, the mechanism by which glaciers kept growing through three suborbital cooling events within this period, which were associated with iceberg discharge in the North Atlantic and cooling over Greenland, is unclear. Here we reconstruct parallel records of sea surface and air temperatures from marine microfossil and pollen data, respectively, from two sediment cores collected within the northern subtropical gyre. The thermal gradient between the cold air and warmer sea increased throughout the MIS5a/4 transition, and was marked by three intervals of even more pronounced thermal gradients associated with the C20, C19 and C18’ cold events. We argue that the warm ocean surface along the western European margin provided a source of moisture that was transported, through northward-tracking storms, to feed ice sheets in colder Greenland, northern Europe and the Arctic.

Description: Nature Geoscience 6, 842 (2013). doi:10.1038/ngeo1904 Authors: Frank Niessen, Jong Kuk Hong, Anne Hegewald, Jens Matthiessen, Rüdiger Stein, Hyoungjun Kim, Sookwan Kim, Laura Jensen, Wilfried Jokat, Seung-Il Nam & Sung-Ho Kang During the Pleistocene glaciations, Arctic ice sheets on western Eurasia, Greenland and North America terminated at their continental margins. In contrast, the exposed continental shelves in the Beringian region of Siberia are thought to have been covered by a tundra landscape. Evidence of grounded ice on seafloor ridges and plateaux off the coast of the Beringian margin, at depths of up to 1,000 m, have generally been attributed to ice shelves or giant icebergs that spread oceanwards during glacial maxima. Here we identify marine glaciogenic landforms visible in seismic profiles and detailed bathymetric maps along the East Siberian continental margin. We interpret these features, which occur in present water depths of up to 1,200 m, as traces from grounding events of ice sheets and ice shelves. We conclude that the Siberian Shelf edge and parts of the Arctic Ocean were covered by ice sheets of about 1 km in thickness during several Pleistocene glaciations before the most recent glacial period, which must have had a significant influence on albedo and oceanic and atmospheric circulation.

Description: Nature Geoscience 6, 861 (2013). doi:10.1038/ngeo1925 Authors: M. Desissa, N. E. Johnson, K. A. Whaler, S. Hautot, S. Fisseha & G. J. K. Dawes Shallow magma reservoirs exist in the crust beneath volcanoes and mid-ocean ridges, yet there are no reports of extensive magma bodies within the uppermost mantle. Indeed the buoyancy of magma should cause it to intrude into the crust, preventing it from ponding in the mantle below. The Dabbahu magmatic segment in Afar, Ethiopia, marks the late stages of continental rifting. This segment has been active since 2005 and has experienced repeated magma intrusions. Here we use magnetotelluric data to image magma bodies beneath it. We identify a 30-km-wide region of very high electrical conductivity that reaches down to about 35 km depth. We interpret this region as a large volume of magma of at least 500 km3 that extends well into the mantle and contains about 13% melt fraction. The magma volume is orders of magnitude larger than that intruded during a typical rifting episode, implying that the magma reservoir persists for several tens of thousands of years. This is in marked contrast to the situation beneath mid-ocean ridges, where melt supply is thought to be episodic. Large magma reservoirs within the mantle may therefore be responsible for the localization of strain that accompanies the final stages of continental break-up.

Description: Nature Geoscience 6, 852 (2013). doi:10.1038/ngeo1927 Authors: Pascal Audet & Susan Y. Schwartz Subduction zones can exhibit variable seismic behaviour, ranging from great earthquakes to slow slip. This variability may be linked to fault frictional properties, and the rheology and structure of the upper plate. The subduction zone beneath the Nicoya Peninsula, Costa Rica, is characterized by strong variations in fault-slip behaviour and a lateral change in the origin of the subducting plate. In the northwest, the plate interface is locked, and experiences large, infrequent earthquakes, and the subducting plate is formed at the East Pacific Rise. In contrast, in the southeast, slow-slip events occur frequently and the subducting plate is formed at the Cocos–Nazca spreading centre. Here we use seismic receiver-function data to analyse the structure of the subduction zone beneath the Nicoya Peninsula. We find extremely high P–S seismic-velocity ratios within the entire subducting oceanic crust that we interpret as high pore-fluid pressure. Velocity ratios in the overriding continental crust, however, change from lower values in the northwest to higher ones in the southeast, indicating a disparity in fluid accumulation. We infer that this disparity is caused by a higher supply of fluid from the subducting slab in the southeast, owing to the permeability structure of oceanic crust formed at the Cocos–Nazca spreading centre. We suggest that the spatial gradient in fluid content influences upper-plate strength and controls the segmentation of seismogenic behaviour in this subduction zone.

Description: Nature Geoscience 6, 806 (2013). doi:10.1038/ngeo1929 Author: Matthew Pasek Phosphorus is an important element for biogeochemical development. According to a set of experiments, martian phosphate minerals dissolve more quickly than terrestrial ones, possibly providing nutrients in aqueous environments for early martian life.

Description: Nature Geoscience 6, 810 (2013). doi:10.1038/ngeo1967 Author: Daniel Cossa The neurotoxin methylmercury can accumulate in marine food webs, contaminating seafood. An analysis of the isotopic composition of fish in the North Pacific suggests that much of the mercury that enters the marine food web originates from low-oxygen subsurface waters.

Description: Nature Geoscience 6, 879 (2013). doi:10.1038/ngeo1918 Authors: Joel D. Blum, Brian N. Popp, Jeffrey C. Drazen, C. Anela Choy & Marcus W. Johnson

Description: Influence of the terrestrial magnetic field geometry on the cutoff rigidity of cosmic ray particles Annales Geophysicae, 31, 1637-1643, 2013 Author(s): K. Herbst, A. Kopp, and B. Heber Studies of the propagation of charged energetic particles in the Earth's magnetic field go back to Carl Størmer. In the end, his investigations finally lead to the definition of the so-called cutoff rigidity R C ; that is, the minimum momentum per charge a particle must have in order to reach a certain geographical location. Employing Monte Carlo simulations with the PLANETOCOSMICS code we investigate the correlation between the geomagnetic field structure and the cutoff rigidity. We show that the geometry of the magnetic field has a considerable influence on the resulting cutoff rigidity distribution. Furthermore, we will present a simple geometry-based parameter, δ B , which is able to reflect the location-dependent cutoff rigidity. We show that this correlation is also visible in the temporal evolution of the Earth's magnetic field, at least over the last 100 yr. Using latitude scans with neutron monitors, changes of the relative counting rates at different positions are calculated, showing small variations for, e.g., Kiel and Moscow, while large ones occur at Mexico City as well as on the British Virgin Islands.

Description: Electron acceleration at Jupiter: input from cyclotron-resonant interaction with whistler-mode chorus waves Annales Geophysicae, 31, 1619-1630, 2013 Author(s): E. E. Woodfield, R. B. Horne, S. A. Glauert, J. D. Menietti, and Y. Y. Shprits Jupiter has the most intense radiation belts of all the outer planets. It is not yet known how electrons can be accelerated to energies of 10 MeV or more. It has been suggested that cyclotron-resonant wave-particle interactions by chorus waves could accelerate electrons to a few MeV near the orbit of Io. Here we use the chorus wave intensities observed by the Galileo spacecraft to calculate the changes in electron flux as a result of pitch angle and energy diffusion. We show that, when the bandwidth of the waves and its variation with L are taken into account, pitch angle and energy diffusion due to chorus waves is a factor of 8 larger at L -shells greater than 10 than previously shown. We have used the latitudinal wave intensity profile from Galileo data to model the time evolution of the electron flux using the British Antarctic Survey Radiation Belt (BAS) model. This profile confines intense chorus waves near the magnetic equator with a peak intensity at ∼5° latitude. Electron fluxes in the BAS model increase by an order of magnitude for energies around 3 MeV. Extending our results to L = 14 shows that cyclotron-resonant interactions with chorus waves are equally important for electron acceleration beyond L = 10. These results suggest that there is significant electron acceleration by cyclotron-resonant interactions at Jupiter contributing to the creation of Jupiter's radiation belts and also increasing the range of L -shells over which this mechanism should be considered.

Description: The coupling between the solar wind and proton fluxes at GEO Annales Geophysicae, 31, 1631-1636, 2013 Author(s): R. J. Boynton, S. A. Billings, O. A. Amariutei, and I. Moiseenko The relationship between the solar wind and the proton flux at geosynchronous Earth orbit (GEO) is investigated using the error reduction ratio (ERR) analysis. The ERR analysis is able to search for the most appropriate inputs that control the evolution of the system. This approach is a black box method and is able to derive a mathematical model of a system from input-output data. This method is used to analyse eight energy ranges of the proton flux at GEO from 80 keV to 14.5 MeV. The inputs to the algorithm were solar wind velocity, density and pressure; the Dst index; the solar energetic proton (SEP) flux; and a function of the interplanetary magnetic field (IMF) tangential magnitude and clock angle. The results show that for lowest five energy channels (80 to 800 keV) the GEO proton fluxes are controlled by the solar wind velocity with a lag of two to three days. However, above 350 keV, the SEP fluxes, accounts for a significant portion of the GEO proton flux variance. For the highest three energy channels (0.74 to 14.5 MeV), the SEPs account for the majority of the ERR. The results also show an anisotropy of protons with gyrocenters inside GEO and outside GEO, where the protons inside GEO are controlled partly by the Dst index and also an IMF-clock angle function.

Description: A statistical study on O + flux in the dayside magnetosheath Annales Geophysicae, 31, 1005-1010, 2013 Author(s): R. Slapak, H. Nilsson, and L. G. Westerberg Studies on terrestrial oxygen ion (O + ) escape into the interplanetary space have considered a number of different escape paths. Recent observations however suggest a yet insufficiently investigated additional escape route for hot O + : along open magnetic field lines in the high altitude cusp and mantle. Here we present a statistical study on O + flux in the high-latitude dayside magnetosheath. The O + is generally seen relatively close to the magnetopause, consistent with observations of O + flowing primarily tangentially to the magnetopause. We estimate the total escape flux in this region to be ~ 7 × 10 24 s −1 , implying this escape route to significantly contribute to the overall total O + loss into interplanetary space.

Description: Simulation of the influence of historical land cover changes on the global climate Annales Geophysicae, 31, 995-1004, 2013 Author(s): Y. Wang, X. Yan, and Z. Wang In order to estimate biogeophysical effects of historical land cover change on climate during last three centuries, a set of experiments with a climate system model of intermediate complexity (MPM-2) is performed. In response to historical deforestation, the model simulates a decrease in annual mean global temperature in the range of 0.07–0.14 °C based on different grassland albedos. The effect of land cover changes is most pronounced in the middle northern latitudes with maximum cooling reaching approximately 0.6 °C during northern summer. The cooling reaches 0.57 °C during northern spring owing to the large effects of land surface albedo. These results suggest that land cover forcing is important for study on historical climate change and that more research is necessary in the assessment of land management options for climate change mitigation.

Description: IMF effect on the polar cap contraction and expansion during a period of substorms Annales Geophysicae, 31, 1021-1034, 2013 Author(s): A. T. Aikio, T. Pitkänen, I. Honkonen, M. Palmroth, and O. Amm The polar cap boundary (PCB) location and motion in the nightside ionosphere has been studied by using measurements from the EISCAT radars and the MIRACLE magnetometers during a period of four substorms on 18 February 2004. The OMNI database has been used for observations of the solar wind and the Geotail satellite for magnetospheric measurements. In addition, the event was modelled by the GUMICS-4 MHD simulation. The simulation of the PCB location was in a rather good agreement with the experimental estimates at the EISCAT longitude. During the first three substorm expansion phases, neither the local observations nor the global simulation showed any poleward motions of the PCB, even though the electrojets intensified. Rapid poleward motions of the PCB took place only in the early recovery phases of the substorms. Hence, in these cases the nightside reconnection rate was locally higher in the recovery phase than in the expansion phase. In addition, we suggest that the IMF B z component correlated with the nightside tail inclination angle and the PCB location with about a 17-min delay from the bow shock. By taking the delay into account, the IMF northward turnings were associated with dipolarizations of the magnetotail and poleward motions of the PCB in the recovery phase. The mechanism behind this effect should be studied further.

Description: A comparison of bow shock models with Cluster observations during low Alfvén Mach number magnetic clouds Annales Geophysicae, 31, 1011-1019, 2013 Author(s): L. Turc, D. Fontaine, P. Savoini, H. Hietala, and E. K. J. Kilpua Magnetic clouds (MCs) are very geoeffective solar wind structures. Their properties in the interplanetary medium have been extensively studied, yet little is known about their characteristics in the Earth's magnetosheath. The Cluster spacecraft offer the opportunity to observe MCs in the magnetosheath, but before MCs reach the magnetosphere, their structure is altered when they interact with the terrestrial bow shock (BS). The physics taking place at the BS strongly depends on Θ Bn , the angle between the shock normal and the interplanetary magnetic field. However, in situ observations of the BS during an MC's crossing are seldom available. In order to relate magnetosheath observations to solar wind conditions, we need to rely on a model to determine the shock's position and normal direction. Yet during MCs, the models tend to be less accurate, because the Alfvén Mach number ( M A ) is often significantly lower than in regular solar wind. On the contrary, the models are generally optimised for high M A conditions. In this study, we compare the predictions of four widely used models available in the literature (Wu et al., 2000; Chapman and Cairns, 2003; Jeřáb et al., 2005; Měrka et al., 2005b) to Cluster's dayside BS crossings observed during five MC events. Our analysis shows that the Θ Bn angle is well predicted by all four models. On the other hand, the Jeřáb et al. (2005) model yields the best estimates of the BS position during low M A MCs. The other models locate the BS either too far from or too close to Earth. The results of this paper can be directly used to estimate the BS parameters in all studies of MC interaction with Earth's magnetosphere.

Description: Investigation of radiative effects of the optically thick dust layer over the Indian tropical region Annales Geophysicae, 31, 647-663, 2013 Author(s): S. K. Das, J.-P. Chen, M. Venkat Ratnam, and A. Jayaraman Optical and physical properties of aerosols derived from multi-satellite observations (MODIS-Aqua, OMI-Aura, MISR-Terra, CALIOP-CALIPSO) have been used to estimate radiative effects of the dust layer over southern India. The vertical distribution of aerosol radiative forcing and heating rates are calculated with 100 m resolution in the lower atmosphere, using temperature and relative humidity data from balloon-borne radiosonde observations. The present study investigates the optically thick dust layer of optical thickness 0.18 ± 0.06 at an altitude of 2.5 ± 0.7 km over Gadanki, transported from the Thar Desert, producing radiative forcing and heating rate of 11.5 ± 3.3 W m −2 and 0.6 ± 0.26 K day −1 , respectively, with a forcing efficiency of 43 W m −2 and an effective heating rate of 4 K day −1 per unit dust optical depth. Presence of the dust layer increases radiative forcing by 60% and heating rate by 60 times at that altitude compared to non-dusty cloud-free days. Calculation shows that the radiative effects of the dust layer strongly depend on the boundary layer aerosol type and mass loading. An increase of 25% of heating by the dust layer is found over relatively cleaner regions than urban regions in southern India and further 15% of heating increases over the marine region. Such heating differences in free troposphere may have significant consequences in the atmospheric circulation and hydrological cycle over the tropical Indian region.

Description: Low level jet intensification by mineral dust aerosols Annales Geophysicae, 31, 625-632, 2013 Author(s): O. Alizadeh Choobari, P. Zawar-Reza, and A. Sturman Modification of the intensity of a low level jet (LLJ) and near-surface wind speed by mineral dust is important as it has implications for dust emission and its long-range transport. Using the Weather Research and Forecasting with Chemistry (WRF/Chem) regional model, it is shown that direct radiative forcing by mineral dust reduces temperature in the lower atmosphere, but increases it in the layers aloft. The surface cooling is shown to be associated with a reduction of turbulent kinetic energy (TKE) and hence vertical mixing of horizontal momentum. Changes in the vertical profile of temperature over the regions that are under the influence of a LLJ are shown to result in an intensification of the LLJ and near-surface wind speed, but a decrease of winds aloft. These changes in the wind speed profile differ from results of previous research which suggested a decrease of wind speed in the lower atmosphere and its increase in the upper boundary layer.

Description: Parametric validations of analytical lifetime estimates for radiation belt electron diffusion by whistler waves Annales Geophysicae, 31, 599-624, 2013 Author(s): A. V. Artemyev, D. Mourenas, O. V. Agapitov, and V. V. Krasnoselskikh The lifetimes of electrons trapped in Earth's radiation belts can be calculated from quasi-linear pitch-angle diffusion by whistler-mode waves, provided that their frequency spectrum is broad enough and/or their average amplitude is not too large. Extensive comparisons between improved analytical lifetime estimates and full numerical calculations have been performed in a broad parameter range representative of a large part of the magnetosphere from L ~ 2 to 6. The effects of observed very oblique whistler waves are taken into account in both numerical and analytical calculations. Analytical lifetimes (and pitch-angle diffusion coefficients) are found to be in good agreement with full numerical calculations based on CRRES and Cluster hiss and lightning-generated wave measurements inside the plasmasphere and Cluster lower-band chorus waves measurements in the outer belt for electron energies ranging from 100 keV to 5 MeV. Comparisons with lifetimes recently obtained from electron flux measurements on SAMPEX, SCATHA, SAC-C and DEMETER also show reasonable agreement.

Description: Coherent amplitude modulation of electron-beam-driven Langmuir waves Annales Geophysicae, 31, 633-638, 2013 Author(s): K. Baumgärtel A linear approach to the phenomenon of irregular amplitude modulation of beam-driven Langmuir waves, developed in a previous paper, is extended to explain periodic modulation as well. It comes about by beating of the fastest growing mode of the instability with beam-aligned plasma oscillations. They are naturally generated in a uniform domain of beam–plasma interaction prior to the onset of the instability. Particle-in-cell (PIC) simulations support the results of the linear analysis.

Description: Seasonal variation of the ion upflow in the topside ionosphere during SAPS (subauroral polarization stream) periods Annales Geophysicae, 31, 1521-1534, 2013 Author(s): H. Wang and H. Lühr A statistical study has been performed by using two years of DMSP (Defense Meteorological Satellite Program) plasma observations to investigate the seasonal effect of SAPS (subauroral polarization stream) on the ion upflow in the duskside ionosphere of the Northern Hemisphere. There are obvious upflows occurring in the topside ionosphere around the SAPS region, exceeding 200 m s −1 at winter solstice, indicating an important relationship between SAPS and the local plasma upward motion. Both SAPS and ion upward velocities show similar seasonal variations, largest in winter and smallest in summer, irrespective of geomagnetic activity. A good correlation is found and a linear relationship is derived between SAPS and the ion upflow velocities. During December solstice the average upflow flux can reach about 2 × 10 8 cm −2 s −1 for more disturbed periods, which is comparable to the typical upflow flux in the dayside cusp region. The depression of the ion temperatures around the peak SAPS region can be understood in terms of the adiabatic cooling. The hot ion cools down when expanding into the low ion concentration region. The electron temperature elevates around the SAPS region because of the reduced Coulomb cooling in the low ion density region. Both the changes of ion and electron temperatures are larger in winter than in summer, however, for Kp 〈 4 the electron temperatures are almost seasonably independent. The present work highlights the important role of the SAPS-related frictional heating at mid-latitudes on the local formation of the strong upward flow, which might provide a direct ionospheric ion source for the ring current and plasmasphere in the duskside sector.

Description: Magnetic field generation in a jet-sheath plasma via the kinetic Kelvin-Helmholtz instability Annales Geophysicae, 31, 1535-1541, 2013 Author(s): K.-I. Nishikawa, P. Hardee, B. Zhang, I. Duţan, M. Medvedev, E. J. Choi, K. W. Min, J. Niemiec, Y. Mizuno, A. Nordlund, J. T. Frederiksen, H. Sol, M. Pohl, and D. H. Hartmann We have investigated the generation of magnetic fields associated with velocity shear between an unmagnetized relativistic jet and an unmagnetized sheath plasma. We have examined the strong magnetic fields generated by kinetic shear (Kelvin–Helmholtz) instabilities. Compared to the previous studies using counter-streaming performed by Alves et al. (2012), the structure of the kinetic Kelvin–Helmholtz instability (KKHI) of our jet-sheath configuration is slightly different, even for the global evolution of the strong transverse magnetic field. In our simulations the major components of growing modes are the electric field E z , perpendicular to the flow boundary, and the magnetic field B y , transverse to the flow direction. After the B y component is excited, an induced electric field E x , parallel to the flow direction, becomes significant. However, other field components remain small. We find that the structure and growth rate of KKHI with mass ratios m i / m e = 1836 and m i / m e = 20 are similar. In our simulations saturation in the nonlinear stage is not as clear as in counter-streaming cases. The growth rate for a mildly-relativistic jet case (γ j = 1.5) is larger than for a relativistic jet case (γ j = 15).

Description: Ionospheric response to total solar eclipse of 22 July 2009 in different Indian regions Annales Geophysicae, 31, 1549-1558, 2013 Author(s): S. Kumar, A. K. Singh, and R. P. Singh The variability of ionospheric response to the total solar eclipse of 22 July 2009 has been studied analyzing the GPS data recorded at the four Indian low-latitude stations Varanasi (100% obscuration), Kanpur (95% obscuration), Hyderabad (84% obscuration) and Bangalore (72% obscuration). The retrieved ionospheric vertical total electron content (VTEC) shows a significant reduction (reflected by all PRNs (satellites) at all stations) with a maximum of 48% at Varanasi (PRN 14), which decreases to 30% at Bangalore (PRN 14). Data from PRN 31 show a maximum of 54% at Kanpur and 26% at Hyderabad. The maximum decrement in VTEC occurs some time (2–15 min) after the maximum obscuration. The reduction in VTEC compared to the quiet mean VTEC depends on latitude as well as longitude, which also depends on the location of the satellite with respect to the solar eclipse path. The amount of reduction in VTEC decreases as the present obscuration decreases, which is directly related to the electron production by the photoionization process. The analysis of electron density height profile derived from the COSMIC (Constellation Observing System for Meteorology, Ionosphere & Climate) satellite over the Indian region shows significant reduction from 100 km altitude up to 800 km altitude with a maximum of 48% at 360 km altitude. The oscillatory nature in total electron content data at all stations is observed with different wave periods lying between 40 and 120 min, which are attributed to gravity wave effects generated in the lower atmosphere during the total solar eclipse.

Description: Application of nonlinear autoregressive moving average exogenous input models to geospace: advances in understanding and space weather forecasts Annales Geophysicae, 31, 1579-1589, 2013 Author(s): R. J. Boynton, M. A. Balikhin, S. A. Billings, and O. A. Amariutei The nonlinear autoregressive moving average with exogenous inputs (NARMAX) system identification technique is applied to various aspects of the magnetospheres dynamics. It is shown, from an example system, how the inputs to a system can be found from the error reduction ratio (ERR) analysis, a key concept of the NARMAX approach. The application of the NARMAX approach to the Dst (disturbance storm time) index and the electron fluxes at geostationary Earth orbit (GEO) are reviewed, revealing new insight into the physics of the system. The review of studies into the Dst index illustrate how the NARMAX approach is able to find a coupling function for the Dst index from data, which was then analytically justified from first principles. While the review of the electron flux demonstrates how NARMAX is able to reveal new insight into the physics of the acceleration and loss processes within the radiation belt.

Description: Energy exchange and wave action conservation for magnetohydrodynamic (MHD) waves in a general, slowly varying medium Annales Geophysicae, 32, 1495-1510, 2014 Author(s): A. D. M. Walker Magnetohydrodynamic (MHD) waves in the solar wind and magnetosphere are propagated in a medium whose velocity is comparable to or greater than the wave velocity and which varies in both space and time. In the approximation where the scales of the time and space variation are long compared with the period and wavelength, the ray-tracing equations can be generalized and then include an additional first-order differential equation that determines the variation of frequency. In such circumstances the wave can exchange energy with the background: wave energy is not conserved. In such processes the wave action theorem shows that the wave action, defined as the ratio of the wave energy to the frequency in the local rest frame, is conserved. In this paper we discuss ray-tracing techniques and the energy exchange relation for MHD waves. We then provide a unified account of how to deal with energy transport by MHD waves in non-uniform media. The wave action theorem is derived directly from the basic MHD equations for sound waves, transverse Alfvén waves, and the fast and slow magnetosonic waves. The techniques described are applied to a number of illustrative cases. These include a sound wave in a medium undergoing a uniform compression, an isotropic Alfvén wave in a steady-state shear layer, and a transverse Alfvén wave in a simple model of the magnetotail undergoing compression. In each case the nature and magnitude of the energy exchange between wave and background is found.

Description: On the influence of CMEs on the global 3-D coronal electron density Annales Geophysicae, 29, 1019-1028, 2011 Author(s): M. Kramar, J. Davila, H. Xie, and S. Antiochos In order to analize the influence of a Coronal Mass Ejection (CME) on the coronal streamer belt, we made 3-D reconstructions of the electron density in the corona at heliospheric distances from 1.5 to 4 R ⊙ for periods before and after a CME occured. The reconstructions were performed using a tomography technique. We studied two CME cases: (i) a slow CME on 1 June 2008; (ii) two fast CMEs on 31 December 2007 and 2 January 2008. For the first case of slow CME, it was found: (i) the potential magnetic field configuration in the CME initiation region before the CME does not agree with the coronal density structure while after the CME the agreement between the field and density is much better. This could be manifistation of that that the field was non-potential before the CME and after the CME the field relaxes towards a more potential state. (ii) It was shown that the dimming caused by the slow CME is not due to rotation of the corona and a line-of-sight (LOS) effect but a streamer blow out effect took place.

Description: Longitudinal study of the ionospheric response to the geomagnetic storm of 15 May 2005 and manifestation of TADs Annales Geophysicae, 29, 1063-1070, 2011 Author(s): S. Sharma, P. Galav, N. Dashora, and R. Pandey Response of low latitude ionosphere to the geomagnetic storm of 15 May 2005 has been studied using total electron content (TEC) data, obtained from three GPS stations namely, Yibal, Udaipur and Kunming situated near the northern crest of equatorial ionization anomaly at different longitudes. Solar wind parameters, north-south component of the interplanetary magnetic field (IMF B z ) and AE index data have been used to infer the strength of the geomagnetic storm. A large value of eastward interplanetary electric field at 06:15 UT, during the time of maximum southward IMF B z has been used to infer the transmission of an eastward prompt penetration electric field (PPEF) which resulted in a peak in TEC at 07:45 UT due to the local uplift of plasma in the low latitudes near the anomaly crest over a wide range of longitudes. Wave-like modulations superposed over the second enhancement in TEC between 09:15 UT to 10:30 UT have been observed at all the three stations. The second enhancement in TEC along with the modulations of up to 5 TECU have been attributed to the combined effect of super plasma fountain and traveling atmospheric disturbances (TAD). Observed large enhancements in TEC are a cause of concern for satellite based navigation and ground positioning. Increased [O/N 2 ] ratio between 09:15 UT to 10:15 UT when modulations in TEC have been also observed, confirms the presence of TADs over a wide range of longitudes.

Description: Volume cross section of auroral radar backscatter and RMS plasma fluctuations inferred from coherent and incoherent scatter data: a response on backscatter volume parameters Annales Geophysicae, 29, 1081-1092, 2011 Author(s): M. V. Uspensky, P. Janhunen, A. V. Koustov, and K. Kauristie Norway and Finland STARE radar measurements in the eastward auroral electrojet are combined with EISCAT CP-1 measurements of the electron density and electric field vector in the common scattering volume to investigate the variation of the auroral radar volume cross section (VCS) with the flow angle of observations (radar look direction with respect to the E × B electron drift). The data set available consists of ~6000 points for flow angles of 40–85° and electron drifts between 500 and 2000 m s −1 . The EISCAT electron density N(h) -profile data are used to estimate the effective electron density, aspect angle and thickness of the backscattering layer. It is shown that the flow angle variation of the VCS is rather weak, only ~5 dB within the range of the considered flow angles. The VCS values themselves respond almost linearly to the square of both the electron drift velocity magnitude and the effective electron density. By adopting the inferred shape of the VCS variation with the flow angle and the VCS dependence upon wavelength, the relative amplitude of electrostatic electron density fluctuations over all scales is estimated. Inferred values of 2–4 percent react nearly linearly to the electron drift velocity in the range of 500–1000 m s −1 but the rate of increase slows down at electron drifts 〉1000 m s −1 and density fluctuations of ~5.5 percent due to, perhaps, progressively growing nonlinear wave losses.

Description: Flux quanta, magnetic field lines, merging – some sub-microscale relations of interest in space plasma physics Annales Geophysicae, 29, 1121-1127, 2011 Author(s): R. A. Treumann, R. Nakamura, and W. Baumjohann We clarify the notion of magnetic field lines in plasma by referring to sub-microscale (quantum mechanical) particle dynamics. It is demonstrated that magnetic field lines in a field of strength B carry single magnetic flux quanta Φ 0 = h/e . The radius of a field line in the given magnetic field B is calculated. It is shown that such field lines can merge and annihilate only over the length ℓ ∥ of their strictly anti-parallel sections, for which case we estimate the power generated. The length ℓ ∥ becomes a function of the inclination angle θ of the two merging magnetic flux tubes (field lines). Merging is possible only in the interval 1 2 πθ≤π. This provides a sub-microscopic basis for "component reconnection" in classical macro-scale reconnection. We also find that the magnetic diffusion coefficient in plasma appears in quanta D 0 m = e Φ 0 / m e = h/m e . This lets us conclude that the bulk perpendicular plasma resistivity is limited and cannot be less than η 0⊥ =μ 0 e Φ 0 / m e =μ 0 h/m e ~10 −9 Ohm m. This resistance is an invariant.

Description: Could periodic patterns in human mortality be sensitive to solar activity? Annales Geophysicae, 29, 1113-1120, 2011 Author(s): R. Díaz-Sandoval, R. Erdélyi, and R. Maheswaran Seasonal behaviour of human diseases have been observed and reported in the literature for years. Although the Sun plays an essential role in the origin and evolution of life on Earth, it is barely taken into account in biological processes for the development of a specific disease. Higher mortality rates occur during the winter season in the Northern Hemisphere for several diseases, particularly diseases of the cardiovascular and respiratory systems. This increment has been associated with seasonal and social causes. However, is there more behind these correlations, in particular in terms of solar variability? In this paper we attempt to make a first step towards answering this question. A detailed wavelet analysis of periodicities for diseases from England and Wales seem to reveal that mortality periodicities (3 days to half a year) could be due to the Earth's position around the Sun. Moreover, crosswavelet and wavelet coherence analysis show common features between medical diseases and solar proxies around solar maximum activity suggesting that this relation, if any, has to be searched in times of high solar activity.

Description: Excitation of zero-frequency magnetic field-aligned currents by ionospheric heating Annales Geophysicae, 29, 1147-1152, 2011 Author(s): A. V. Streltsov and T. R. Pedersen Time-dependent, three-dimensional numerical simulations of the reduced MHD model describing shear Alfvén waves in the magnetosphere provide an interesting prediction superficially similar to results of several ionospheric heating experiments conducted at high altitudes. In these experiments, heating of the ionospheric F-region with a constant/zero-frequency beam of HF waves causes luminous structures in the ionosphere in the form of a ring or a solid spot with a characteristic size comparable to the size of the heated spot. Simulations suggest that spots/rings or similar optical appearance might be associated with a magnetic field-aligned current system produced by the ionospheric heating. Two of the most interesting features of this current system are (1) strong localization across the ambient magnetic field and (2) distinctive non-symmetrical luminous signatures (ring/spot) in magnetically conjugate locations in the ionosphere.

Description: Comment on "The PC index: review of methods", by McCreadie and Menvielle (2010) Annales Geophysicae, 29, 1137-1146, 2011 Author(s): P. Stauning The Polar Cap (PC) index is a controversial topic within the IAGA scientific community. Since 1997 discussions of the validity of the index to be endorsed as an official IAGA index have ensued. The article: "The PC index: review of methods", written by two members of the IAGA PC index committee, H. McCreadie and M. Menvielle, holds a critical review of some aspects of the methods used to derive PC index values. However, a number of incorrect statements and factual errors have been found and shall be called attention to and discussed in this commentary. Further critical comments concern the Corrigendum issued by the same authors and published in Ann. Geophys., 29, 813–814, 2011.

Description: The role of the bow shock in solar wind-magnetosphere coupling Annales Geophysicae, 29, 1129-1135, 2011 Author(s): R. E. Lopez, V. G. Merkin, and J. G. Lyon In this paper we examine the role of the bow shock in coupling solar wind energy to the magnetosphere using global magnetohydrodynamic simulations of the solar wind-magnetosphere interaction with southward IMF. During typical solar wind conditions, there are two significant dynamo currents in the magnetospheric system, one in the high-latitude mantle region tailward of the cusp and the other in the bow shock. As the magnitude of the (southward) IMF increases and the solar wind becomes a low Mach number flow, there is a significant change in solar wind-magnetosphere coupling. The high-latitude magnetopause dynamo becomes insignificant compared to the bow shock and a large load appears right outside the magnetopause. This leaves the bow shock current as the only substantial dynamo current in the system, and the only place where a significant amount of mechanical energy is extracted from the solar wind. That energy appears primarily as electromagnetic energy, and the Poynting flux generated at the bow shock feeds energy back into the plasma, reaccelerating it to solar wind speeds. Some small fraction of that Poynting flux is directed into the magnetosphere, supplying the energy needed for magnetospheric dynamics. Thus during periods when the solar wind flow has a low Mach number, the main dynamo in the solar wind-magnetosphere system is the bow shock.

Description: Measurements of aerosol intensive properties over Visakhapatnam, India for 2007 Annales Geophysicae, 29, 973-985, 2011 Author(s): K. Niranjan, B. Spandana, T. Anjana Devi, V. Sreekanth, and B. L. Madhavan Information on the aerosol intensive properties like Single Scattering Albedo (SSA) and asymmetry parameter are very limited, particularly over the peninsular India, though extensive reports are available on the aerosol bulk properties. In view of the importance of these parameters in evaluating the aerosol radiative forcing, we present for the first time the temporal variation in SSA with measurements on aerosol absorption and scattering coefficients over Visakhapatnam (17.72° N, 83.32° E; located on the east coast of India) for the year 2007. The inferred SSA was in the range of 0.65 and 0.9 with an annual mean of 0.76 ± 0.013 and with a probable value of 0.80, indicating a marginal atmospheric warming over the region. The mixed layer contribution to column Aerosol Optical depth is found to be 35 % in summer while it is well above 35 % in winter, indicating the confinement of aerosol within the boundary layer during winter. The asymmetry parameter which represents the angular scattering in radiative forcing estimation is found to be around 0.65 ± 0.1 for the location. The day to day variability in SSA is found to be well correlated with the variations in surface BC mass concentrations and/or the relative dominance of the fine/coarse mode aerosol. The results are discussed in light of the aerosol physical and optical properties and the asymmetry parameter.

Description: Corrigendum to "Downward auroral currents from the reconnection Hall-region", published in Ann. Geophys., 29, 679–685, 2011 Annales Geophysicae, 29, 1061-1061, 2011 Author(s): R. A. Treumann, R. Nakamura, and W. Baumjohann No abstract available.

Description: Hybrid models of solar wind plasma heating Annales Geophysicae, 29, 1071-1079, 2011 Author(s): L. Ofman, A.-F. Viñas, and P. S. Moya Remote sensing and in-situ observations show that solar wind ions are often hotter than electrons, and the heavy ions flow faster than the protons by up to an Alfvén speed. Turbulent spectrum of Alfvénic fluctuations and shocks were detected in solar wind plasma. Cross-field inhomogeneities in the corona were observed to extend to several tens of solar radii from the Sun. The acceleration and heating of solar wind plasma is studied via 1-D and 2-D hybrid simulations. The models describe the kinetics of protons and heavy ions, and electrons are treated as neutralizing fluid.The expansion of the solar wind is considered in 1-D hybrid model. A spectrum of Alfvénic fluctuations is injected at the computational boundary, produced by differential streaming instability, or initial ion temperature anisotropy, and the parametric dependence of the perpendicular heating of H + -He ++ solar wind plasma is studied. It is found that He ++ ions are heated efficiently by the Alfvénic wave spectrum below the proton gyroperiod.

Description: On the effect of IMF turning on ion dynamics at Mercury Annales Geophysicae, 29, 987-996, 2011 Author(s): D. C. Delcourt, T. E. Moore, and M.-C. H. Fok We investigate the effect of a rotation of the Interplanetary Magnetic Field (IMF) on the transport of magnetospheric ion populations at Mercury. We focus on ions of planetary origin and investigate their large-scale circulation using three-dimensional single-particle simulations. We show that a nonzero B X component of the IMF leads to a pronounced asymmetry in the overall circulation pattern. In particular, we demonstrate that the centrifugal acceleration due to curvature of the E × B drift paths is more pronounced in one hemisphere than the other, leading to filling of the magnetospheric lobes and plasma sheet with more or less energetic material depending upon the hemisphere of origin. Using a time-varying electric and magnetic field model, we investigate the response of ions to rapid (a few tens of seconds) re-orientation of the IMF. We show that, for ions with gyroperiods comparable to the field variation time scale, the inductive electric field should lead to significant nonadiabatic energization, up to several hundreds of eVs or a few keVs. It thus appears that IMF turning at Mercury should lead to localized loading of the magnetosphere with energetic material of planetary origin (e.g., Na + ).

Description: Instability of combined gravity-inertial-Rossby waves in atmospheres and oceans Annales Geophysicae, 29, 997-1003, 2011 Author(s): J. F. McKenzie The properties of the instability of combined gravity-inertial-Rossby waves on a β-plane are investigated. The wave-energy exchange equation shows that there is an exchange of energy with the background stratified medium. The energy source driving the instability lies in the background enthalpy released by the gravitational buoyancy force. It is shown that if the phase speed of the westward propagating low frequency-long wavelength Rossby wave exceeds the Poincaré-Kelvin (or "equivalent" shallow water) wave speed, instability arises from the merging of Rossby and Poincaré modes. There are two key parameters in this instability condition; namely, the equatorial/rotational Mach (or Froude) number M and the latitude θ 0 of the β-plane. In general waves equatorward of a critical latitude for given M can be driven unstable, with corresponding growth rates of the order of a day or so. Although these conclusions may only be safely drawn for short wavelengths corresponding to a JWKB wave packet propagating internally and located far from boundaries, nevertheless such a local instability may play a significant role in atmosphere-ocean dynamics.

Description: The correlation between solar and geomagnetic activity – Part 3: An integral response model Annales Geophysicae, 29, 1005-1018, 2011 Author(s): Z. L. Du An integral response model is proposed to describe the relationship between geomagnetic activity ( aa index) and solar activity (represented by sunspot number R z ): The aa at a given time t is the integral of R z at past times ( t '≤ t ) multiplied by an exponential decay factor of the time differences ( e −(t−t')/τ ), where τ is the decay time scale (~40 months). The correlation coefficient of aa with the reconstructed series based on this model ( r f =0.85) is much higher than that of aa with R z ( r 0 =0.61). If this model is applied to each solar cycle, the correlation coefficient will be higher ( r f =0.95). This model can naturally explain some phenomena related to aa and R z , such as (i) the significant increase in the aa index (and its baseline) over the twentieth century; (ii) the longer lag times of aa to R z at solar cycle maxima than at minima; and (iii) the variations in the correlations related to solar and Hale cycles. These results demonstrate that aa depends not only on the present R z but also on past values. The profile of aa can be better predicted from R z by this model than by point-point correspondence.

Description: Cluster observations of trapped ions interacting with magnetosheath mirror modes Annales Geophysicae, 29, 1049-1060, 2011 Author(s): J. Soucek and C. P. Escoubet Mirror modes are among the most intense low frequency plasma wave phenomena observed in the magnetosheaths of magnetized planets. They appear as large amplitude non-propagating fluctuations in the magnetic field magnitude and plasma density. These structures are widely accepted to represent a non-linear stage of the mirror instability, dominant in plasmas with large ion beta and a significant ion temperature anisotropy T ⊥ / T ∥ 〉1. It has long been recognized that the mirror instability both in the linear and non-linear stage is a kinetic process and that the behavior of resonant particles at small parallel velocities is crucial for its development and saturation. While the dynamics of the instability and the effect of trapped particles have been studied extensively in theoretical models and numerical simulations, only spurious observations of the trapped ions were published to date. In this work we used data from the Cluster spacecraft to perform the first detailed experimental study of ion velocity distribution associated with mirror mode oscillations. We show a conclusive evidence for the predicted cooling of resonant ions at small parallel velocities and heating of trapped ions at intermediate pitch angles.

Description: MHD waves generated by high-frequency photospheric vortex motions Annales Geophysicae, 29, 1029-1035, 2011 Author(s): V. Fedun, S. Shelyag, G. Verth, M. Mathioudakis, and R. Erdélyi In this paper, we discuss simulations of MHD wave generation and propagation through a three-dimensional open magnetic flux tube in the lower solar atmosphere. By using self-similar analytical solutions for modelling the magnetic field in Cartesian coordinate system, we have constructed a 3-D magnetohydrostatic configuration which is used as the initial condition for non-linear MHD wave simulations. For a driver we have implemented a high-frequency vortex-type motion at the footpoint region of the open magnetic flux tube. It is found that the implemented swirly source is able to excite different types of wave modes, i.e. sausage, kink and torsional Alfvén modes. Analysing these waves by magneto-seismology tools could provide insight into the magnetic structure of the lower solar atmosphere.

Description: Average properties and small-scale variations of the mesospheric Na and Fe layers as observed simultaneously by two closely colocated lidars at 30° N Annales Geophysicae, 29, 1037-1048, 2011 Author(s): L. Chen and F. Yi We report the average properties and small-scale variation features of the mesospheric Na and Fe layers at 30° N from extensive simultaneous and common-volume Na and Fe lidar measurements at Wuhan, China. The annual mean Na and Fe density profiles are derived in terms of an averaging method taken from an early literature. The mean Na and Fe profiles preserve the sharp gradients present in most individual density profiles near the layer bottom. Near the bottommost of the layers the mean Na and Fe scale heights are respectively −0.42 and −0.30 km. The mean layer parameters coincide well with the previous report. The Na and Fe densities in the lowest several kilometers of the layers consistently exhibit nearly the same time variations. A clear-cut distinction between the Na and Fe time variations always appears in an altitude range near 90 km. A relatively weak positive correlation between them persistently occurs also in an altitude range near 100 km. The mean increase and decrease rates for both Na and Fe are altitude dependent and have a single-peak structure. The time constant of the layer variation is ~0.07–2.0 h for Na and ~0.02–1.7 h for Fe, suggesting that the variability is dominated by small-scale processes. However, there is also a slow net increase in each of the annual mean column abundances (Na and Fe) during night.

Description: Polarisation in the auroral red line during coordinated EISCAT Svalbard Radar/optical experiments Annales Geophysicae, 29, 1101-1112, 2011 Author(s): M. Barthélémy, J. Lilensten, F. Pitout, C. Simon Wedlund, R. Thissen, D. Lorentzen, F. Sigernes, J. Moen, G. Gronoff, I. McCrea, H. Rothkael, H. Ménager, and A. Aruliah The polarisation of the atomic oxygen red line in the Earth's thermosphere is observed in different configurations with respect to the magnetic field line at high latitude during several coordinated Incoherent Scatter radar/optical experiment campaigns. When pointing northward with a line-of-sight nearly perpendicular to the magnetic field, we show that, as expected, the polarisation is due to precipitated electrons with characteristic energies of a few hundreds of electron Volts. When pointing toward the zenith or southward with a line-of-sight more parallel to the magnetic field, we show that the polarisation practically disappears. This confirms experimentally the predictions deduced from the recent discovery of the red line polarisation. We show that the polarisation direction is parallel to the magnetic field line during geomagnetic activity intensification and that these results are in agreement with theoretical work.

Description: New insights into the stratospheric and mesosphere-lower thermospheric ozone response to the abrupt changes in solar forcing Annales Geophysicae, 29, 1093-1099, 2011 Author(s): Karanam Kishore Kumar, K. V. Subrahmanyam, and Sherine Rachel John Using a unique set of satellite based observations of the vertical distribution of ozone during the recent annular solar eclipse of 15 January 2010, we demonstrate for the first time, a complete picture of the response of stratospheric ozone to abrupt changes in solar forcing. The stratospheric ozone decreased after the maximum obscuration of the Sun and then gradually increased with time. A dramatic increase in stratospheric ozone of up to 4 ppmv is observed 3 h after the maximum obscuration of the Sun. The present study also reports for the first time the mesosphere-lower thermospheric ozone response to solar eclipse. Thus it is envisaged that the present results will have important implications in understanding the ozone response to abrupt changes in solar forcing and time-scales involved in such response.

Description: Corrigendum to "The PC index: review of methods" published in Ann. Geophys., 28, 1887–1903, 2010 Annales Geophysicae, 29, 813-814, 2011 Author(s): H. McCreadie and M. Menvielle No abstract available.

Description: Pre-monsoon aerosol characteristics over the Indo-Gangetic Basin: implications to climatic impact Annales Geophysicae, 29, 789-804, 2011 Author(s): A. K. Srivastava, S. Tiwari, P. C. S. Devara, D. S. Bisht, Manoj K. Srivastava, S. N. Tripathi, P. Goloub, and B. N. Holben Sun/sky radiometer observations over the Indo-Gangetic Basin (IGB) region during pre-monsoon (from April–June 2009) have been processed to analyze various aerosol characteristics in the central and eastern IGB region, represented by Kanpur and Gandhi College, respectively, and their impacts on climate in terms of radiative forcing. Monthly mean aerosol optical depth (AOD at 500 nm) and corresponding Angstrom Exponent (AE at 440–870 nm, given within the brackets) was observed to be about 0.50 (0.49) and 0.51 (0.65) in April, 0.65 (0.74) and 0.67 (0.91) in May and 0.69 (0.45) and 0.77 (0.71) in June at Kanpur and Gandhi College, respectively. Results show a positive gradient in AOD and AE from central to eastern IGB region with the advancement of the pre-monsoon, which may be caused due to diverse geographical location of the stations having different meteorological conditions and emission sources. Relatively lower SSA was observed at the eastern IGB (0.89) than the central IGB (0.92) region during the period, which suggests relative dominance of absorbing aerosols at the eastern IGB as compared to central IGB region. The absorbing aerosol optical properties over the station suggest that the atmospheric absorption over central IGB region is mainly due to dominance of coarse-mode dust particles; however, absorption over eastern IGB region is mainly due to dominance of fine-particle pollution. The derived properties from sun/sky radiometer during pre-monsoon period are used in a radiative-transfer model to estimate aerosol radiative forcing at the top-of-the atmosphere (TOA) and at the surface over the IGB region. Relatively large TOA and surface cooling was observed at the eastern IGB as compared to the central IGB region. This translates into large heating of the atmosphere ranging from 0.45 to 0.55 K day −1 at Kanpur and from 0.45 to 0.59 K day −1 at Gandhi College.

Description: Annual and semiannual variations of vertical total electron content during high solar activity based on GPS observations Annales Geophysicae, 29, 865-873, 2011 Author(s): M. P. Natali and A. Meza Annual, semiannual and seasonal variations of the Vertical Total Electron Content (VTEC) have been investigated during high solar activity in 2000. In this work we use Global IGS VTEC maps and Principal Component Analysis to study spatial and temporal ionospheric variability. The behavior of VTEC variations at two-hour periods, at noon and at night is analyzed. Particular characteristics associated with each period and the geomagnetic regions are highlighted. The variations at night are smaller than those obtained at noon. At noon it is possible to see patterns of the seasonal variation at high latitude, and patterns of the semiannual anomaly at low latitudes with a slow decrease towards mid latitudes. At night there is no evidence of seasonal or annual anomaly for any region, but it was possible to see the semiannual anomaly at low latitudes with a sudden decrease towards mid latitudes. In general, the semiannual behavior shows March–April equinox at least 40 % higher than September one. Similarities and differences are analyzed also with regard to the same analysis done for a period of low solar activity.

Description: Modelling of spacecraft spin period during eclipse Annales Geophysicae, 29, 875-882, 2011 Author(s): E. Georgescu, F. Plaschke, U. Auster, K.-H. Fornaçon, and H. U. Frey The majority of scientific satellites investigating the Earth magnetosphere are spin stabilized. The attitude information comes usually from a sun sensor and is missing in the umbra; hence, the accurate experimental determination of vector quantities is not possible during eclipses. The spin period of the spacecraft is generally not constant during these times because the moment of inertia changes due to heat dissipation. The temperature dependence of the moment of inertia for each spacecraft has a specific signature determined by its design and distribution of mass. We developed an "eclipse-spin" model for the spacecraft spin period behaviour using magnetic field vector measurements close to the Earth, where the magnetic field is dominated by the dipole field, and in the magnetospheric lobes, where the magnetic field direction is mostly constant. The modelled spin periods give us extraordinarily good results with accumulated phase deviations over one hour of less than 10 degrees. Using the eclipse spin model satellite experiments depending on correct spin phase information can deliver science data even during eclipses. Two applications for THEMIS B, one in the lobe and the other in the lunar wake, are presented.

Description: Apparent temperature anisotropies due to wave activity in the solar wind Annales Geophysicae, 29, 909-917, 2011 Author(s): D. Verscharen and E. Marsch The fast solar wind is a collisionless plasma permeated by plasma waves on many different scales. A plasma wave represents the natural interplay between the periodic changes of the electromagnetic field and the associated coherent motions of the plasma particles. In this paper, a model velocity distribution function is derived for a plasma in a single, coherent, large-amplitude wave. This model allows one to study the kinetic effects of wave motions on particle distributions. They are by in-situ spacecraft measured by counting, over a certain sampling time, the particles coming from various directions and having different energies. We compare our results with the measurements by the Helios spacecraft, and thus find that by assuming high wave activity we are able to explain key observed features of the measured distributions within the framework of our model. We also address the recent discussions on nonresonant wave–particle interactions and apparent heating. The applied time-averaging procedure leads to an apparent ion temperature anisotropy which is connected but not identical to the intrinsic temperature of the underlying distribution function.

Description: The numerical simulation on ionospheric perturbations in electric field before large earthquakes Annales Geophysicae, 32, 1487-1493, 2014 Author(s): S. F. Zhao, X. M. Zhang, Z. Y. Zhao, and X. H. Shen Many observational results have shown electromagnetic abnormality in the ionosphere before large earthquakes. The theoretical simulation can help us to understand the internal mechanism of these anomalous electromagnetic signals resulted from seismic regions. In this paper, the horizontal and vertical components of electric and magnetic field at the topside ionosphere are simulated by using the full wave method that is based on an improved transfer matrix method in the lossy anisotropic horizontally stratified ionosphere. Taken account into two earthquakes with electric field perturbations recorded by the DEMETER satellite, the numerical results reveal that the propagation and penetration of ULF (ultra-low-frequency) electromagnetic waves into the ionosphere is related to the spatial distribution of electron and ion densities at different time and locations, in which the ion density has less effect than electron density on the field intensity. Compared with different frequency signals, the minimum values of electric and magnetic field excited by earthquakes can be detected by satellite in current detection capability have also been calculated, and the lower frequency wave can be detected easier.

Description: Nature Geoscience 7, 860 (2014). doi:10.1038/ngeo2301 Author: Jay J. Ague Aqueous subduction-zone fluids contain CO2 and methane. New calculations indicate that these fluids also host a wide array of organic carbon species, in concentrations sufficient to influence the deep carbon cycle.

Description: A comparison between VEGA 1, 2 and Giotto flybys of comet 1P/Halley: implications for Rosetta Annales Geophysicae, 32, 1441-1453, 2014 Author(s): M. Volwerk, K.-H. Glassmeier, M. Delva, D. Schmid, C. Koenders, I. Richter, and K. Szegö Three flybys of comet 1P/Halley, by VEGA 1, 2 and Giotto, are investigated with respect to the occurrence of mirror mode waves in the cometosheath and field line draping in the magnetic pile-up region around the nucleus. The time interval covered by these flybys is approximately 8 days, which is also the approximate length of an orbit or flyby of Rosetta around comet 67P/Churyumov–Gerasimenko. Thus any significant changes observed around Halley are changes that might occur for Rosetta during one pass of 67P/CG. It is found that the occurrence of mirror mode waves in the cometosheath is strongly influenced by the dynamical pressure of the solar wind and the outgassing rate of the comet. Field line draping happens in the magnetic pile-up region. Changes in nested draping regions (i.e. regions with different B x directions) can occur within a few days, possibly influenced by changes in the outgassing rate of the comet and thereby the conductivity of the cometary ionosphere.

Description: Nature Geoscience 7, 925 (2014). doi:10.1038/ngeo2278 Authors: Devin McPhillips, Paul R. Bierman & Dylan H. Rood

Description: Nature Geoscience 7, 920 (2014). doi:10.1038/ngeo2284 Authors: Peter B. Reich, Sarah E. Hobbie & Tali D. Lee

Description: Nature Geoscience 7, 931 (2014). doi:10.1038/ngeo2296 Author: Colin Goldblatt, Tyler D. Robinson, Kevin J. Zahnle & David Crisp

Description: Magnetic curvature driven Rayleigh-Taylor instability revisited Annales Geophysicae, 29, 411-413, 2011 Author(s): O. A. Pokhotelov and O. G. Onishchenko The problem of incomplete finite ion Larmor radius (FLR) stabilization of the magnetic curvature driven Rayleigh-Taylor instability (RTI) in low beta plasma with homogeneous ion temperature is investigated. For this purpose a model hydrodynamic description of nonlinear flute waves with arbitrary spatial scales compared to the ion Larmor radius is developed. It is shown that the RTI is not stabilized by FLR effects in a plasma with cold electrons when the ratio of characteristic spatial scale of the plasma inhomogeneity to local effective radius of curvature of the magnetic field lines is larger than 1/4. The crucial role in the absence of the complete FLR stabilization plays the contribution of the compressibility of the polarization part of the ion velocity.

Description: Forecasting relativistic electron flux using dynamic multiple regression models Annales Geophysicae, 29, 415-420, 2011 Author(s): H.-L. Wei, S. A. Billings, A. Surjalal Sharma, S. Wing, R. J. Boynton, and S. N. Walker The forecast of high energy electron fluxes in the radiation belts is important because the exposure of modern spacecraft to high energy particles can result in significant damage to onboard systems. A comprehensive physical model of processes related to electron energisation that can be used for such a forecast has not yet been developed. In the present paper a systems identification approach is exploited to deduce a dynamic multiple regression model that can be used to predict the daily maximum of high energy electron fluxes at geosynchronous orbit from data. It is shown that the model developed provides reliable predictions.

Description: Airglow observations over the equatorial ionization anomaly zone in Taiwan Annales Geophysicae, 29, 749-757, 2011 Author(s): J. Y. Liu, P. K. Rajesh, I. T. Lee, and T. C. Chow Airglow imaging at mid-latitude stations often show intensity modulations associated with medium scale travelling ionospheric disturbances (MSTID), while those carried out near the equatorial regions reveal depletions caused by equatorial plasma bubbles (EPB). Two all sky cameras are used to observe plasma depletions in the 630.0 nm emission over the equatorial ionization anomaly (EIA) region, Taiwan (23° N, 121° E; 13.5° N Magnetic) during 1998–2002 and 2006–2007. The results show EPB and MSTID depletions in different solar activity conditions. Several new features of the EPB depletions such as bifurcation, secondary structure on the walls, westward tilt, etc., are discussed in this paper. Evidence of tilted depletions with secondary structures developing on the eastern wall that later evolve to appear as bifurcations, are presented for the first time. Moreover, detail investigations are carried out using International Reference Ionosphere (IRI) model as well as the electron density from Ionosonde and Global Positioning System (GPS) Occultation Experiment (GOX) onboard FORMOSAT-3/COSMIC satellite, to understand the conditions that favor the propagation of MSTID to the latitude of Taiwan.

Description: 3-D mesoscale MHD simulations of a cusp-like magnetic configuration: method and first results Annales Geophysicae, 29, 759-770, 2011 Author(s): E. Adamson and K. Nykyri We present a local mesoscale model of the magnetospheric cusp region with high resolution (up to 300 km). We discuss the construction and implementation of the initial configuration and give a detailed description of the numerical simulation. An overview of simulation results for the case of strongly northward interplanetary magnetic field (IMF) is then presented and compared with data from Cluster 2 spacecraft from 14 February 2003. Results show a cusp diamagnetic cavity (CDC) with depth normal to the magnetospheric boundary on the order of 1–2 R E and a much larger extent of ~5–9 R E tangential to the boundary, bounded by a gradual inner boundary with the magnetospheric lobe and a more distinct exterior boundary with the magnetosheath. These results are qualitatively consistent with observational data.

Description: Polar cap ion beams during periods of northward IMF: Cluster statistical results Annales Geophysicae, 29, 771-787, 2011 Author(s): R. Maggiolo, M. Echim, J. De Keyser, D. Fontaine, C. Jacquey, and I. Dandouras Above the polar caps and during prolonged periods of northward IMF, the Cluster satellites detect upward accelerated ion beams with energies up to a few keV. They are associated with converging electric field structures indicating that the acceleration is caused by a quasi-static field-aligned electric field that can extend to altitudes higher than 7 R E (Maggiolo et al., 2006; Teste et al., 2007). Using the AMDA science analysis service provided by the Centre de Données de la Physique des Plasmas, we have been able to extract about 200 events of accelerated upgoing ion beams above the polar caps from the Cluster database. Most of these observations are taken at altitudes lower than 7 R E and in the Northern Hemisphere. We investigate the statistical properties of these ion beams. We analyze their geometry, the properties of the plasma populations and of the electric field inside and around the beams, as well as their dependence on solar wind and IMF conditions. We show that ~40 % of the ion beams are collocated with a relatively hot and isotropic plasma population. The density and temperature of the isotropic population are highly variable but suggest that this plasma originates from the plasma sheet. The ion beam properties do not change significantly when the isotropic, hot background population is present. Furthermore, during one single polar cap crossing by Cluster it is possible to detect upgoing ion beams both with and without an accompanying isotropic component. The analysis of the variation of the IMF B Z component prior to the detection of the beams indicates that the delay between a northward/southward turning of IMF and the appearance/disappearance of the beams is respectively ~2 h and 20 min. The observed electrodynamic characteristics of high altitude polar cap ion beams suggest that they are closely connected to polar cap auroral arcs. We discuss the implications of these Cluster observations above the polar cap on the magnetospheric dynamics and configuration during prolonged periods of northward IMF.

Description: Tweets on Earth Nature Geoscience 4, 209 (2011). doi:10.1038/ngeo1137 Nature Geoscience has joined Twitter. We share our take on exciting developments in the Earth and planetary sciences as they happen.

Description: Closed ranks in oceanography Nature Geoscience 4, 211 (2011). doi:10.1038/ngeo1113 Authors: LuAnne Thompson, Renellys C. Perez & Amelia E. Shevenell

Description: Not the earthquake's fault Nature Geoscience 4, 210 (2011). doi:10.1038/ngeo1116 Authors: Brian G. McAdoo & Lisabeth Paravisini-Gebert

Description: Coal's true cost Nature Geoscience 4, 209 (2011). doi:10.1038/ngeo1136 The deaths of birds have become a rallying point against the proliferation of wind farms. Yet the loss of human life in mines is rarely linked with coal as an energy source.

Description: Our choice from the recent literature Nature Geoscience 4, 215 (2011). doi:10.1038/ngeo1131

Description: Science-based insurance Nature Geoscience 4, 213 (2011). doi:10.1038/ngeo1117 Authors: Molly E. Brown, Daniel E. Osgood & Miguel A. Carriquiry Crops are at risk in a changing climate. Farmers in the developing world will be able to insure against harvest failure if robust insurance packages, based on a geophysical index rather than individual loss, become widely available.

Description: Earth's core: A new twist on inner-core spin Nature Geoscience 4, 216 (2011). doi:10.1038/ngeo1091 Author: Mathieu Dumberry An observed hemispheric structure in the Earth's inner core has been hard to reconcile with evidence that it rotates faster than the mantle. Detection of a shift of the hemisphere boundary that occurred over geological timescales removes the contradiction.

Description: Structural geology: Natural fault lubricants Nature Geoscience 4, 217 (2011). doi:10.1038/ngeo1126 Author: Ben van der Pluijm Motion along faults can occur in sudden earthquakes or through steady, aseismic creep. Rock samples retrieved by drilling deep into a creeping section of the San Andreas Fault show that clay minerals in fault rock promote creep behaviour.

Description: Transformation of tectonic and climatic signals from source to sedimentary archive Nature Geoscience 4, 231 (2011). doi:10.1038/ngeo1087 Authors: John J. Armitage, Robert A. Duller, Alex C. Whittaker & Philip A. Allen The Earth’s sedimentary successions are an archive of past tectonic and climate events. The physical characteristics of the sediment record are controlled by three main factors: the sediment supply from the eroding source region, the grain size distribution of that sediment supply, and the area available for sediment accumulation in the downstream regions. The interplay of these factors can make the interpretation of a climatic or tectonic signal complex, particularly as these processes are nonlinear. Here we assess the evolution of a tectonically active landscape undergoing erosional and depositional processes, using a numerical model that incorporates variations in grain size and the volumetric sediment budget. Our simulations indicate that changes in precipitation and tectonic uplift both generate characteristic patterns of grain size and stratigraphic geometry. An increase in catchment precipitation results in the deposition of a laterally extensive sheet of coarse gravel. The responses to a changing tectonic regime are more diverse: a large increase in uplift rate results first in the deposition of sediments of larger grain size at proximal sites, followed by a reduction in grain size at distal locations. We conclude that the stratigraphic record is strongly controlled by the grain size of sediments released from catchments undergoing tectonic or climatic change.

Description: Early Solar System: Gathering dust Nature Geoscience 4, 219 (2011). doi:10.1038/ngeo1112 Author: Jeff Cuzzi Chondritic meteorites are remnants of the ancient Solar System. Analysis of the dust rims often found on their constituent particles shows that the rims were swept up while the particles wafted about and collided in a weakly turbulent protoplanetary nebula.

Description: Oceanography: Chemical twins, separated Nature Geoscience 4, 220 (2011). doi:10.1038/ngeo1125 Author: Martin Frank How the chemical composition of sea water changes on its journey through the world's oceans is poorly understood. Systematic measurements of dissolved trace metals across the Pacific Ocean suggest that these metals may help track sources and mixing of water masses.

Description: Weakening of the equatorial Atlantic cold tongue over the past six decades Nature Geoscience 4, 222 (2011). doi:10.1038/ngeo1078 Authors: Hiroki Tokinaga & Shang-Ping Xie Seasonal and interannual variations of the equatorial cold tongue are defining features of the tropical Atlantic Ocean, with significant climatic and biogeochemical effects. However, its long-term changes are poorly understood owing to biases in observations and climate models. Here we use a suite of bias-corrected observations, and find that cold-tongue variability has weakened during the past six decades. We find that sea surface temperature has increased across the basin, with a local enhancement over the eastern equatorial Atlantic. This warming pattern of the sea surface is most pronounced during boreal summer, reducing the annual cycle through a positive ocean–atmosphere feedback. Specifically, the eastward-intensified warming leads to enhanced atmospheric convection in the equatorial eastern Atlantic region, as well as to less vigorous trade winds. These in turn deepen the thermocline in the east, and reinforce the sea surface warming pattern. The flattened thermocline and reduced thermocline feedback weaken interannual variability of equatorial sea surface temperatures and Guinea coast precipitation associated with the Atlantic Niño. We suggest that the observed changes could be associated with cooling by anthropogenic aerosols, an effect that is stronger in the Northern than in the Southern Hemisphere. If the aerosol emissions decrease in the next decades, the tropical Atlantic may experience yet another shift as the greenhouse gas forcing increases.

Description: Strong elemental fractionation of Zr–Hf and Nb–Ta across the Pacific Ocean Nature Geoscience 4, 227 (2011). doi:10.1038/ngeo1114 Authors: M. Lutfi Firdaus, Tomoharu Minami, Kazuhiro Norisuye & Yoshiki Sohrin Understanding the circulation of water masses in the world’s oceans is critical to our knowledge of the Earth’s climate system. Trace elements and their isotopes have been explored as tracers for the movement of water masses . One type of candidate elements are the high-field-strength elements zirconium (Zr), hafnium (Hf), niobium (Nb) and tantalum (Ta). Here we measure the distributions of dissolved Zr, Hf, Nb and Ta along two meridional sections in the Pacific Ocean that extend from 65° to 10° S and from 10° to 50° N. We find that all four elements tend to be depleted in surface water. In the deep oceans, their concentrations rise along our transects from the Southern Ocean to the North Pacific Ocean, and show strong correlations with the concentration of silicate. These results indicate that terrigenous sources are important to the budget of Zr, Hf, Nb and Ta in sea water, compared with hydrothermal input. Unexpectedly, the weight ratios for Zr/Hf fall between 45 and 350 and those for Nb/Ta between 14 and 85 in Pacific sea water, higher than the ratios observed in fresh water, in the silicate Earth or in chondritic meteorites. We conclude that the fractionation of Zr/Hf and Nb/Ta ratios will be useful for tracing water masses in the ocean.

Description: Earliest rock fabric formed in the Solar System preserved in a chondrule rim Nature Geoscience 4, 244 (2011). doi:10.1038/ngeo1120 Authors: Philip A. Bland, Lauren E. Howard, David J. Prior, John Wheeler, Robert M. Hough & Kathryn A. Dyl Rock fabrics—the preferred orientation of grains—provide a window into the history of rock formation, deformation and compaction. Chondritic meteorites are among the oldest materials in the Solar System and their fabrics should record a range of processes occurring in the nebula and in asteroids. However, owing to abundant fine-grained material, chondrites have largely resisted traditional in situ fabric analysis. Here we use high-resolution electron backscatter diffraction to map the orientation of submicrometre grains in the Allende CV carbonaceous chondrite. We look at the fine-grained rims surrounding the chondrules—spherical grains cooled from molten droplets before accretion in the meteorite—as well as the matrix material between the chondrules. Although the matrix exhibits a bulk uniaxial fabric indicative of a compressive event in the parent asteroid, we find that the chondrule rims preserve a spherically symmetric fabric centred on the chondrule. We define a method to quantitatively relate fabric intensity to net compression, and reconstruct an initial rim porosity of 70–80%. Our calculations provide meteoritic evidence that the first solids formed in the Solar System accreted with high porosity, similar to modelling and laboratory estimates. We conclude that the chondrule rim textures formed in a nebula setting and may therefore represent the first rock fabric in the Solar System.

Description: The protracted development of the continent–ocean transition in Afar Nature Geoscience 4, 248 (2011). doi:10.1038/ngeo1095 Authors: Ian D. Bastow & Derek Keir Continental breakup and the transition to seafloor spreading is characterized by extensional faulting, thinning of the lithosphere and, at magmatic margins, voluminous intrusive and extrusive magmatism. It is difficult to discriminate between different mechanisms of extension and magmatism at ancient continental margins because the continent–ocean transition is buried beneath thick layers of volcanic and sedimentary rocks and the tectonic activity that characterized breakup has ceased. Instead, the timing of these mechanisms is inferred from theoretical models or from the geological record preserved at the fully developed, ancient rifted margins. Ongoing rifting in Ethiopia offers a unique opportunity to address these problems because it exposes subaerially the transition between continental rifting towards the south and seafloor spreading further northward. Here we synthesize constraints on the spatial and temporal evolution of magmatism and extension in Ethiopia. We show that although intrusion of magma maintains crustal thickness during the early stages of the continent–ocean transition, subsidence of the margin below sea level, and eruption of voluminous basalt flows, is initiated by late-stage thinning of the heavily intruded, weakened plate just before the onset of seafloor spreading. We thus conclude that faulting, stretching and magma intrusion are each important, but at different times during breakup.

Description: Explosive eruptions at mid-ocean ridges driven by CO2-rich magmas Nature Geoscience 4, 260 (2011). doi:10.1038/ngeo1104 Authors: Christoph Helo, Marc-Antoine Longpré, Nobumichi Shimizu, David A. Clague & John Stix The abundance of volatile compounds, and particularly CO2, in the upper oceanic mantle affects the style of volcanic eruptions. At mid-ocean ridges, eruptions are generally dominated by the gentle effusion of basaltic lavas with a low volatile content. However explosive volcanism has been documented at some ocean spreading centres, indicative of abundant volatile compounds. Estimates of the initial CO2 concentration of primary magmas can be used to constrain the CO2 content of the upper oceanic mantle, but these estimates vary greatly. Here we present ion microprobe measurements of the CO2 content of basaltic melt trapped in plagioclase crystals. The crystals are derived from volcanic ash deposits erupted explosively at Axial Seamount, Juan de Fuca Ridge, in the northeast Pacific Ocean. We report unusually high CO2 concentrations of up to 9,160 ppm, which indicate that the upper oceanic mantle is more enriched in carbon than previously thought. We furthermore suggest that CO2 fluxes along mid-ocean ridges vary significantly. Our results demonstrate that elevated fluxes of CO2 from the upper oceanic mantle can drive explosive eruptions at mid-ocean ridges.

Description: Fan-delta uplift and mountain subsidence during the Haiti 2010 earthquake Nature Geoscience 4, 255 (2011). doi:10.1038/ngeo1115 Authors: Manabu Hashimoto, Yo Fukushima & Yukitoshi Fukahata The relative motion between the Caribbean and North American plates is accommodated by several active faults around Hispaniola Island. The Enriquillo–Plantain Garden fault in southern Haiti is one of these structures. Strain equivalent to a magnitude 7.2 earthquake is estimated to have accumulated along this fault since its last significant activity. The Haiti earthquake of 12 January 2010 was initially reported to have occurred along this fault, but more recent studies proposed slips on previously unrecognized, neighbouring faults. Here we use interferometric synthetic aperture radar data to show that surface deformation caused by the earthquake does not correspond to the present topography. Alluvial fan deltas were uplifted on the north side of the Enriquillo–Plantain Garden fault, whereas mountains located on the south side of the fault subsided, implying that faults other than the Enriquillo–Plantain Garden fault were responsible for the deformation. To determine fault structure, we fit the satellite surface deformation data to a fault model. We show that slip occurred on a fault dipping northward at 42°, with large thrust components. The maximum displacement on the fault was about 4 m at 10–20 km depth, offshore from the Tiburon peninsula. We confirm that the earthquake ruptured a blind thrust fault and show that the fault could not be identified from large-scale present-day topography.

Description: Pleistocene water intrusions from the Mediterranean and Caspian seas into the Black Sea Nature Geoscience 4, 236 (2011). doi:10.1038/ngeo1106 Authors: S. Badertscher, D. Fleitmann, H. Cheng, R. L. Edwards, O. M. Göktürk, A. Zumbühl, M. Leuenberger & O. Tüysüz The hydrological balance of the Black Sea is governed by riverine input and by the exchange with the Mediterranean Sea through the shallow Bosporus Strait. These sources have distinctly different oxygen isotope (δ18O) signatures. Therefore, the δ18O of Black Sea water directly reflects the presence or absence of a connection with the Mediterranean Sea, as well as hydrological changes in the vast watersheds of the Black and Caspian seas. However, the timing of late to middle Pleistocene water intrusions to the Black Sea is poorly constrained in sedimentary sequences. Here we present a stacked speleothem δ18O record from Sofular Cave in northern Turkey that tracks the isotopic signature of Black Sea surface water, and thus allows a reconstruction of the precise timing of hydrological shifts of the Black Sea. Our record, which extends discontinuously over the last 670,000 years, suggests that the connection between the Black Sea and Mediterranean Sea has been open for a significant period at least twelve times since 670,000 yr ago, more often than previously suggested. Distinct minima in the Sofular δ18O record indicate at least seven intervals when isotopically depleted freshwater from the Caspian Sea entered the Black Sea. Our data provide precisely dated evidence for a highly dynamic hydrological history of the Black Sea.

Description: Haematite pseudomicrofossils present in the 3.5-billion-year-old Apex Chert Nature Geoscience 4, 240 (2011). doi:10.1038/ngeo1084 Authors: Craig P. Marshall, Julienne R. Emry & Alison Olcott Marshall Microstructures in the ∼3.5 Gyr Apex Chert Formation were initially described as the oldest bacterial fossils on Earth over 20 years ago. However, the identification of the structures (which resemble cyanobacteria) as biological in origin remains controversial. Here we determine the petrology and geochemistry of similar structures from the original Apex Chert locality using thin sections and Raman spectroscopy. Based on the microscopic examination of thin sections, we identify features similar to those previously identified as microfossils as a series of quartz and haematite-filled fractures. Raman spectroscopy of the fractures shows that carbonaceous material is not, as previously reported, associated with the structures, but is instead disseminated in the surrounding quartz matrix. We suggest that although the microstuctures analysed are not microfossils, the presence of carbonaceous material in the surrounding matrix is consistent with the existence of microbial life at this time, and with evidence of early Archaean life found at other sites. Furthermore, we caution against identifying microstructures as biological in origin without a full morphological and geochemical assessment.