ALBERT

Description: We evaluate climatologies of upper tropospheric ozone and nitric acid retrieved from two satellite instruments (ACE-FTS and OSIRIS) with long-term in situ measurements from aircraft (MOZAIC, CR-AVE, PRE-AVE, PEM Tropics, and TC4) and ozonesondes. A global chemical transport model (GEOS-Chem) is used to guide the evaluation and to relate sparse in situ measurements with the satellite retrievals. Both satellite retrievals generally reproduce broad ozone features in the upper troposphere such as summer enhancements in the northern subtropics and larger concentrations over the tropical Atlantic versus the tropical Pacific. These comparisons indicate biases in annual, tropical mean ozone concentrations from both ACE-FTS (10–13%) and OSIRIS (5%) relative to aircraft and ozonesonde observations. More uncertain evidence suggests that nitric acid from ACE-FTS has a positive mean bias of 15%. We demonstrate that an upper limit on the ozone production efficiency in the upper troposphere can be determined using ACE-FTS satellite measurements of O3 and HNO3. The resulting value of 196 (+34, −61) mol/mol is in broad agreement with model simulations. Higher OPE values inferred from ACE-FTS over the tropical Pacific (249 (+21, −68) mol/mol) than the tropical Atlantic (146 (+16, −41) mol/mol) reflect increasing ozone production efficiency with decreasing pollution. This analysis indicates a new capability of satellite observations to provide insight into ozone production in the tropical troposphere.

Description: In the laboratory, we have investigated the growth and composition of frost flowers. Their ionic composition has shown little difference from those of field measurements. Young frost flowers grown on sea ice are saline, leading us to speculate that wicking occurs continually during their growth on sea ice. The surface area of frost flowers is only a little larger than the area of ice underneath, consistent with recent field measurements from the Arctic. Time-lapse photography has allowed us to observe the extreme mobility of freshly forming sea ice, at the stage at which the mush has become rather solid, and continuing while the flowers grow. This mobility results in new brine being expelled to the surface, which therefore remains wet. During various stages of frost flower growth, we observed their freshly formed dendritic parts rapidly diminishing in size after contacting the surface, consistent with repeated wicking. Frost flowers proved to be very stable in the presence of wind, such that no aerosol was observed when wind was blown across them in the laboratory chamber. This is consistent with recent field observations of frost flowers coexisting with wind-blown snow.

Description: A widely accepted explanation of the location of the inner edge of the electron plasma sheet and its dependence on electron energy is based on drift motions of individual particles. The boundary is identified as the separatrix between drift trajectories linking the tail to the dayside magnetopause (open paths) and trajectories closed around the Earth. A statistical study of the inner edge of the electron plasma sheet using THEMIS Electrostatic Analyzer plasma data from November 2007 to April 2009 enabled us to examine this model. Using a dipole magnetic field and a Volland-Stern electric field with shielding, we find that a steady state drift boundary model represents the average location of the electron plasma sheet boundary and reflects its variation with the solar wind electric field in the local time region between 21:00 and 06:00, except at high activity levels. However, the model does not reproduce the observed energy dispersion of the boundaries. We have also used the location of the inner edge of the electron plasma sheet to parameterize the potential drop of the tail convection electric field as a function of solar wind electric field (Esw) and geomagnetic activity. The range of Esw examined is small because the data were acquired near solar minimum. For the range of values tested (meaningful statistics only for Esw 〈 2 mV/m), reasonably good agreement is found between the potential drop of the tail convection electric field inferred from the location of the inner edge and the polar cap potential drop calculated from the model of Boyle et al. (1997).

Description: Four years of trace gas measurements have been acquired using the Bruker 125HR Fourier Transform Infrared (FTIR) spectrometer installed at the Polar Environment Atmospheric Research Laboratory (PEARL) in the Canadian high Arctic. These have been compared with data from three models, namely the Canadian Middle Atmosphere Model Data Assimilation System (CMAM-DAS), the Global Environmental Multiscale stratospheric model with the online Belgium Atmospheric CHemistry package (GEM-BACH), and the off-line 3D chemical transport model SLIMCAT to assess the total reactive nitrogen, NOy, budget above Eureka, Nunavut (80.05°N, 86.42°W). The FTIR data have been also compared with satellite measurements by the Atmospheric Chemistry Experiment-Fourier Transform Spectrometer (ACE-FTS). The FTIR is able to measure four of the five primary species that form NOy: NO, NO2, HNO3, and ClONO2, while the fifth, N2O5, was obtained using the N2O5/(NO + NO2) ratio derived from the models and ACE-FTS. Combining these results, a four-year time series of NOy 15–40 km partial columns was calculated. Comparisons with each model were made, revealing mean differences (± standard error of the mean) relative to the FTIR of (−16.0 ± 0.6)%, (5.5 ± 1.0)%, and (−5.8 ± 0.4)% for CMAM-DAS, GEM-BACH, and SLIMCAT, respectively. The mean difference between the ACE-FTS and FTIR NOy partial columns was (5.6 ± 2.3)%. While we found no significant seasonal and interannual differences in the FTIR NOy stratospheric columns, the partial columns display nearly twice as much variability during the spring compared to the summer period.

Description: The explosive phase of the eruption of the Eyjafjallajökull volcano in Iceland beginning on 14 April 2010 caused extensive disruption to aviation in Europe with serious social and economic consequences. Despite its impact, the explosive phase was modest in size and the amount of sulphur dioxide (SO2) released was low. The potential of hyperspectral thermal infrared measurements to discriminate emissions from similar events by measuring SO2 is examined using the Infrared Atmospheric Sounding Interferometer (IASI) on board MetOp-A. The transported plume in the initial stages of the explosive phase contained low amounts of SO2 at low altitude which placed it at the detection limit of space-based sensors used to monitor the volcanic threat to aviation using current methods. A recently developed technique for the fast retrieval of SO2 from IASI is applied in the context of the Eyjafjallajökull eruption to show that IASI is easily capable of sensing the SO2 in the plume at this stage where existing methods fail. The fast SO2 retrieval is calibrated against a fully quantitative optimal estimation retrieval of SO2 total column amount and plume altitude to derive the detection limit for the plume on 15 April 2010. An estimate of the general detection limit for the instrument is placed conservatively at 0.3 Dobson Units (DU) which is an order of magnitude lower than previously thought.

Description: Recent observations in the inner magnetotail have shown rapid and significant flux increases (usually an order of magnitude of increase within seconds) of suprathermal electrons (tens of keV to hundreds of keV) associated with earthward moving dipolarization fronts. To explain where and how these suprathermal electrons are produced during substorm intervals, two types of acceleration models have been suggested by previous studies: acceleration that localizes near the reconnection site and acceleration that occurs during earthward transport. We perform an analytical analysis of adiabatic acceleration to show that the slope of source differential fluxes is critical for understanding adiabatic flux enhancements during earthward transport. Observationally, two earthward propagating dipolarization fronts accompanied by energetic electron flux enhancements observed by the THEMIS spacecraft have been analyzed; in each event the properties of dipolarization fronts in the inner magnetosphere (XGSM ≈ −10RE) were well correlated with those further down the tail (XGSM ≈ −15RE or XGSM ≈ −20RE). Coupled with theoretical analysis, this enables us to estimate the relative acceleration that occurred as the electrons propagated earthward between the two spacecraft. During the two events studied, the differential fluxes of supra thermal electrons had steep energy spectra with power law indices of −4 to −6.These spectra were much steeper than those at lower energy, as well as those of the supra thermal electrons observed before the fronts arrived. A compression factor of 1.5 as the electrons propagated earthward induced a flux increase of suprathermal electrons by a factor of 7 to 17. Provided these steep spectra, we demonstrate that adiabatic acceleration from the betatron and Fermi mechanisms simultaneously operating can account for these flux increases. Since both analytical analysis and data from the two events show that adiabatic acceleration during earthward transport does not significantly change the power law indices, the steep spectra were likely to be traced back to their source region, presumably near the reconnection site.

Description: [1]  The Methodology based on the Error Reduction Ratio (ERR) determines the causal relationship between the input and output for a wide class of nonlinear systems. In the present study, ERR is used to identify the most important solar wind parameters, which control the fluxes of energetic electrons at geosynchronous orbit. The results show that for lower energies, the fluxes are indeed controlled by the solar wind velocity, as was assumed before. For the lowest energy range studied here (24.1 keV), the solar wind velocity of the current day is the most important control parameter for the current day's electron flux. As the energy increases, the solar wind velocity of the previous day becomes the most important factor. For the higher energy electrons (around 1 MeV), the solar wind velocity registered two days in the past is the most important controlling parameter. Such a dependence can, perhaps, be explained by either local acceleration processes due to the interaction with plasma waves or by radial diffusion if lower energy electrons possess higher mobility. However, in the case of even higher energies (2.0 MeV), the solar wind density replaces the velocity as the key control parameter. Such a dependence could be a result of solar wind density influence on the dynamics of various waves and pulsations that affect acceleration and loss of relativistic electrons. The study also shows that statistically the variations of daily high energy electron fluxes show little dependence on the daily averaged B z , daily time duration of the southward IMF and daily integral ∫  B s dt (where B s is the southward component of IMF).

Description: [1]  Fast plasma flows are believed to play important roles in transporting mass, momentum, and energy in the magnetotail during active periods, such as the magnetospheric substorms. In this paper, we present Cluster observations of a plasma-depleted flux tube, i.e., a plasma bubble associated with fast plasma flow before the onset of a substorm in the near-Earth tail around X  = −18  R E . The bubble is bounded by both sharp leading (∂ b z /∂ x  〈 0) and trailing (∂ b z /∂ x  〉 0) edges. The two edges are thin current layers (approximately ion inertial length) that carry not only intense perpendicular current but also field-aligned current. The leading edge is a dipolarization front (DF) within a slow plasma flow, while the trailing edge is embedded in a super-Alfvénic convective ion jet. The electron jet speed exceeds the ion flow speed thus producing a large tangential current at the trailing edge. The electron drift is primarily given by the E × B drift. Interestingly, the trailing edge moves faster than the leading edge, which causes shrinking of the bubble and local flux pileup inside the bubble. This resulted in a further intensification of B z , or a secondary dipolarization. Both the leading and trailing edges are tangential discontinuities that confine the electrons inside the bubble. Strong electron acceleration occurred corresponding to the secondary dipolarization, with perpendicular fluxes dominating the field-aligned fluxes. We suggest that betatron acceleration is responsible for the electron energization. Whistler waves and lower hybrid drift waves were identified inside the bubble. Their generation mechanisms and potential roles in electron dynamics are discussed.

Description: [1]  The cross polar cap potential is considered an instantaneous monitor of the rate at which magnetic flux couples the solar wind to the Earth's magnetosphere-ionosphere system. Studies have shown that the cross polar cap potential responds linearly to the solar wind electric field under nominal solar wind conditions, but asymptotes to the order of 200 kV for large electric field. Saturation of the cross polar cap potential is also found to occur in MHD simulations. Several mechanisms have been proposed to explain this phenomenon. Two well-developed models are those of Siscoe et al . (2002), herein referred to as the Siscoe-Hill model, and of Kivelson and Ridley (2008), herein referred to as the Kivelson-Ridley model. In this study, we compare the mathematical formulas as well as the predictions of the two models with data. We find that the two models predict similar saturation limits. Their difference can be expressed in terms of a factor, which is close to unity during a saturation interval. A survey of the differences in the model predictions show that, on average, the potential of the Kivelson-Ridley model is smaller than that of the Siscoe-Hill model by 10 kV. Measurements of AMIE, DMSP, PC index, and SuperDARN are used to differentiate between the two models. However, given the uncertainties of the measurements, it is impossible to conclude that one model does a better job than the other of predicting the observed cross polar cap potentials.

Description: Magnetic holes filled with isotropic energetic electrons (up to a few 105 eV) have been observed by THEMIS in the vicinity of dipolarization fronts. These structures can partially contribute to the initial seed population of energetic electrons within the magnetosphere; therefore finding their nature is important for understanding of the population of high energy electrons within the magnetosphere. Previously, these structures have been interpreted as the result of the mirror instability due to the similarity in their appearance with mirror dips observed in the terrestrial magnetosheath and solar wind. The THEMIS data shown here prove that the measured properties of these structures contradict to the interpretation as mirror waves. In the present study it is shown that these waves do not exhibit the effects on the ion population that are expected due to mirror wave structures. However, they do have a pronounced effect on the high energy electron population. The evolution of the high energy electron population within these structures is investigated. It is then argued that the tearing instability can be responsible for their generation.