ALBERT

Description: Madagascar occupies a key position in the assembly and break-up of the supercontinent Gondwana. It has been used in numerous geological studies to reconstruct its original position within Gondwana and to derive plate kinematics. Seismological observations in Madagascar to date have been sparse. Using a temporary, dense seismic profile across southern Madagascar, we present the first published study of seismic anisotropy from shear-wave splitting analyses of teleseismic phases. The splitting parameters obtained show significant small-scale variation of fast polarization directions and delay times across the profile, with fast polarization rotating from NW in the center to NE in the east and west of the profile. The delay times range between 0.4 and 1.5 s. A joint inversion of waveforms at each station is applied to derive hypothetical one-layer splitting parameters. We use finite-difference, full-waveform modelling to test several hypotheses about the origin and extent of seismic anisotropy. Our observations can be explained by asthenospheric anisotropy with a fast polarization direction of 50°, approximately parallel to the absolute plate motion direction, in combination with blocks of crustal anisotropy. Predictions of seismic anisotropy as inferred from global mantle flow models or global anisotropic surface wave tomography are not in agreement with the observations. Small-scale variations of splitting parameters require significant crustal anisotropy. Considering the complex geology of Madagascar, we interpret the change in fast-axis directions as a ~150 km wide zone of ductile deformation in the crust as a result of the intense reworking of lithospheric material during the Pan-African orogeny. This fossil anisotropic pattern is underlain by asthenospheric anisotropy induced by plate motion.

Description: An online coupled regional climate-chemistry model called RegCCMS is used to investigate the interactions between anthropogenic aerosols and the East Asian summer monsoon (EASM) over East Asia. The simulation results show that the mean aerosol loading and optical depth over the region are 17.87 mg/m 2 and 0.25, respectively. Sulfate and black carbon (BC) account for approximately 61.2 % and 7.8 % of the total aerosols, respectively. The regional mean radiative forcing (RF) is approximately −3.64, −0.55 and +0.88 W/m 2 at the TOA for the total aerosol effect, the total aerosol direct effect, and the BC direct effect, respectively. The surface direct RF of BC accounts for approximately 31 % of the total RF of all aerosols. Because of the total aerosol effect, both the energy budgets and air temperature are considerably reduced in the region with high aerosol loadings, leading to decreases in the land-ocean air temperature gradient in summer. The total column absorbed solar radiation (TCASR) and surface air temperature (SAT) decrease by 8.4 W/m 2 and 0.31 K, respectively. This cooling effect weakens horizontal and vertical atmospheric circulations over East Asia. The wind speed at 850 hPa decreases by 0.18 m/s, and the precipitation decreases by 0.29 mm/d. The small responses of solar radiation, air temperature and atmospheric circulations to the BC warming effect are opposite to those of the total aerosol effect. The BC-induced enhancement of atmospheric circulation can increase local floods in South China, while droughts in North China may worsen in response to the BC semi-direct effect. The total aerosol effect is much more significant than the BC direct effect. The East Asian summer monsoon becomes weaker due to the total aerosol effect. However, this weakness could be partially offset by the BC warming effect. Sensitivity analyses further indicate that the influence of aerosols on the EASM might be more substantial in years when the southerlies or southwesterlies at 850 hPa are weak compared with years when the winds are strong. Changes in the EASM can induce variations in the distribution and magnitude of aerosols. Aerosols in the lower troposphere over the region can increase by 3.07 and 1.04 µg/m 3 due to the total aerosol effect and the BC warming effect, respectively.

Description: [1]  We present new seismicity images based on a two-year seismic deployment in the Pamir and SW Tien Shan. 9,532 earthquakes were detected, located and rigorously assessed in a multistage automatic procedure utilizing state-of-the-art picking algorithms, waveform cross-correlation and multi-event relocation. The obtained catalog provides new information on crustal seismicity and reveals the geometry and internal structure of the Pamir-Hindu Kush intermediate-depth seismic zone with improved detail and resolution. The relocated seismicity clearly defines at least two distinct planes, one beneath the Pamir, the other beneath the Hindu Kush, separated by a gap across which strike and dip directions change abruptly. The Pamir seismic zone forms a thin (ca.10 km width), curviplanar arc that strikes east–west and dips south at its eastern end, then progressively turns by 90 degrees to reach a north–south strike and a due eastward dip at its southwestern termination. Pamir deep seismicity outlines several streaks at depths between 70 and 240 km, with the deepest events occurring at its southwestern end. Intermediate-depth earthquakes are clearly separated from shallow crustal seismicity, which is confined to the uppermost 20–25 km. The Hindu Kush seismic zone extends from 40 to 250 km depth and generally strikes east–west, yet bends northeast, towards the Pamir, at its eastern end. It may be divided vertically into an upper and lower part separated by a gap at approx. 150 km depth. In the upper part, events form a plane that is 15–25 km thick in cross-section and dips sub-vertically north to northwest. Seismic activity is more virile in the lower part, where several distinct clusters form a complex pattern of sub-parallel planes. The observed geometry could be reconciled either with a model of two-sided subduction of Eurasian and previously underthrusted Indian continental lithosphere or by a purely Eurasian origin of both Pamir and Hindu Kush seismic zones, which necessitatesa contortion and oversteepening of the latter.

Description: The stream power law model has been widely used to represent erosion by rivers but does not take into account the role played by sediment in modulating erosion and deposition rates. Davy and Lague (2009, https://doi.org/10.1029/2008JF001146) provide an approach to address this issue, but it is computationally demanding because the local balance between erosion and deposition depends on sediment flux resulting from net upstream erosion. Here, we propose an efficient (i.e., O(N) and implicit) method to solve their equation. This means that, unlike other methods used to study the complete dynamics of fluvial systems (e.g., including the transition from detachment‐limited to transport‐limited behavior), our method is unconditionally stable even when large time steps are used. We demonstrate its applicability by performing a range of simulations based on a simple setup composed of an uplifting region adjacent to a stable foreland basin. As uplift and erosion progress, the mean elevations of the uplifting relief and the foreland increase, together with the average slope in the foreland. Sediments aggrade in the foreland and prograde to reach the base level where sediments are allowed to leave the system. We show how the topography of the uplifting relief and the stratigraphy of the foreland basin are controlled by the efficiency of river erosion and the efficiency of sediment transport by rivers. We observe the formation of a steady‐state geometry in the uplifting region, and a dynamic steady state (i.e., autocyclic aggradation and incision) in the foreland, with aggradation and incision thicknesses up to tens of meters.

Description: Kinetic-size magnetic holes (KSMHs) in the turbulent magnetosheath are statistically investigated using high time resolution data from the MMS mission. The KSMHs with short duration (i.e., 〈 0.5 s) have their cross section smaller than the ion gyro-radius. Superposed epoch analysis of all events reveals that an increase in the electron density and total temperature, significantly increase (resp. decrease) the electron perpendicular (resp. parallel) temperature, and an electron vortex inside KSMHs. Electron fluxes at ~ 90° pitch angles with selective energies increase in the KSMHs, are trapped inside KSMHs and form the electron vortex due to their collective motion. All these features are consistent with the electron vortex magnetic holes obtained in 2D and 3D particle-in-cell simulations, indicating that the observed KSMHs seem to be best explained as electron vortex magnetic holes. It is furthermore shown that KSMHs are likely to heat and accelerate the electrons.

Description: The Na density variations in the E region have been studied over the past few decades. Although considerable progress in understanding and in modeling the metal layer observations has been made, Na density features above 100 km have yet to be explained. Various studies have linked them to the Na + variations, a major reservoir for Na in E region. But the lack of comprehensive modeling investigations and of wind and temperature observations prevents further understanding on this important ion-neutral coupling topic. In this study, we conduct a numerical simulation on the summer time Na density behavior in the mid-latitude E region, where both the ion density and the neutral atmosphere are modulated by tidal and gravity waves. Simulation results show good agreement with Na lidar measurements and reveal that atmospheric waves can transport Na upward to generate Na layers and variations in E region considerably. The vertical wind component of the large amplitude tidal wave can extend the Na layer above 120 km into the thermosphere. The simulation also demonstrates that the modulation of large amplitude gravity (GW) wave can generate small scale sporadic Na layers (Na s ) in the E region. Finally, eddy diffusion enhancement in the GW saturation process can significantly alter the Na s spatial and temporal structures.

Description: Abstract The extension of the neutral sodium (Na) layer into the thermosphere (up to 170 km) has recently been observed at low and high latitudes using a Na lidar. However, the geophysical mechanisms and implications of its formation are currently unknown. In this study, we conduct an advanced 2D numerical simulation of the Na and Na+ variations in the E and F regions at low latitudes. The numerical simulations are used to investigate the contributions of the electromagnetic force, neutral wind, diffusion, and gravity. The simulations lead to three major findings. First, Na+ in the subtropical region of the geomagnetic equator acts as the major reservoir of the neutral sodium, and its distribution during nighttime is mostly below 200 km due to the combined effect of the vertical component of the drift and Coulomb‐induced drift. Second, we find that the fountain effect has little influence on the behavior of Na in the nighttime. Third, the probable explanation for the frequent generation of the thermospheric sodium layer during spring equinox at Cerro Pachón, Chile is attributed to the large vertical neutral transport generated by large vertical wind perturbations of unknown origin, with a magnitude exceeding 10 m/s that is closely associated with the semi‐diurnal tide.

Description: Abstract Recent studies reveal that the Secondary sodium layer (SeSL) occurs more frequently in summer than in winter at midlatitudes. However, the physical mechanism underlying such seasonal difference is still a mystery due to the complexity of the process that involves both chemistry and neutral dynamics. In this paper, we undertake a statistical study based on seven‐year Na lidar observations at Utah State University in Logan, Utah (41.7°N, 112°W), and 12‐year Na lidar observations at Arecibo Observatory, Puerto Rico (18.3°N, 67°W), along with a two‐dimensional numerical model simulations, to investigate the mechanism that drives such summer‐winter difference of the SeSL occurrence rate in the middle latitude. The numerical simulations reveal that the Na number density, width of the mesospheric Na main layer, and the behavior of Na+ all strongly influence the winter‐summer occurrence of the nocturnal SeSL at Utah State University. And this mechanism is also able to explain the winter‐summer SeSL difference over other midlatitude locations in China, and that observed in the mid‐low‐latitude station at Arecibo Observatory.