Plasma density is an important factor in determining wave-particle interactions in the magnetosphere. We develop a machine-learning-based electron density (MLED) model in the inner magnetosphere using electron density data from Van Allen Probes between September 25, 2012 and August 30, 2019. This MLED model is a physics-based nonlinear network that employs fundamental physical principles to describe variations of electron density. It predicts the plasmapause location under different geomagnetic conditions, and models separately the electron densities of the plasmasphere and of the trough. We train the model using gradient descent and backpropagation algorithms, which are widely used to deal effectively with nonlinear relationships among physical quantities in space plasma environments. The model gives explicit expressions with few parameters and describes the associations of electron density with geomagnetic activity, solar cycle, and seasonal effects. Under various geomagnetic conditions, the electron densities calculated by this model agree well with empirical observations and provide a good description of plasmapause movement. This MLED model, which can be easily incorporated into previously developed radiation belt models, promises to be very helpful in modeling and improving forecasting of radiation belt electron dynamics.
This study presents signatures of seismo-ionospheric perturbations possibly related to the 14 July 2019
Analysis of Incoherent Scatter Radar (ISR) data collected during an experiment involving alternating O/X mode pumping reveals that the high-frequency enhanced ion line (HFIL) and plasma line (HFPL) did not appear immediately after the onset of pumping, but were delayed by a few seconds. By examining the initial behaviors of the ion line, plasma line, and electron temperature, as well as ionosphere conditions, we find that (1) the HFIL and HFPL were delayed not only in the X mode pumping but also in the O mode pumping and (2) the HFIL was not observed prior to enhancement of the electron temperature. Our analysis suggests that (1) leakage of the X mode to the O mode pumping may not be ignored and (2) spatiotemporal uncertainties and spatiotemporal variations in the profiles of ion mass and electron density may have played important roles in the apparent failure of the Bragg condition to apply; (3) nevertheless, the absence of parametric decay instability (PDI) cannot be ruled out, due to our inability to match conditions caused by the spatiotemporal uncertainties.
The uplift of the Qinghai–Tibet Plateau (TP) strongly influences climate change, both regionally and globally. Surface observation data from this region have limited coverage and are difficult to obtain. Consequently, the vertical crustal deformation velocity (VCDV) distribution of the TP is poorly constrained. In this study, the VCDV from the TP was inverted by using data from the gravity recovery and climate experiment (GRACE). We were able to obtain the vertical crustal movement by deducting the hydrological factors, based on the assumption that the gravity signal detected by GRACE is mainly composed of hydrological factors and vertical crustal movement. From the vertical crustal movement, we inverted the distribution of the VCDV across the TP. The results showed that the VCDV of the southern, eastern, and northern TP is ~1.1 mm/a, ~0.5 mm/a, and −0.1 mm/a, respectively, whereas that of the region between the Qilian Haiyuan Fault and the Kunlun Fault is ~0.0 mm/a. These results are consistent with the distribution of crustal deformation, thrust earthquakes and faults, and regional lithospheric activity. The hydrology, crustal thickness, and topographic factors did not change the overall distribution of the VCDV across the TP. The influence of hydrological factors is marked, with the maximum differences being approximately −0.4 mm/a in the northwest and 1.0 mm/a in the central area. The results of this study are significant for understanding the kinematics of the TP.
Electromagnetic ion cyclotron (EMIC) waves are widely believed to play an important role in influencing the radiation belt and ring current dynamics. Most studies have investigated the effects or characteristics of EMIC waves by assuming their left-handed polarization. However, recent studies have found that the reversal of polarization, which occurs at higher latitudes along the wave propagation path, can change the wave-induced pitch angle diffusion coefficients. Whether such a polarization reversal can influence the global ring current dynamics remains unknown. In this study, we investigate the ring current dynamics and proton precipitation loss in association with polarization-reversed EMIC waves by using the ring current–atmosphere interactions model (RAM). The results indicate that the polarization reversal of H-band EMIC waves can truly decrease the scattering rates of protons of 10 to 50 keV or >100 keV in comparison with the scenario in which the EMIC waves are considered purely left-handed polarized. Additionally, the global ring current intensity and proton precipitation may be slightly affected by the polarization reversal, especially during prestorm time and the recovery phase, but the effects are not large during the main phase. This is probably because the H-band EMIC waves contribute to the proton scattering loss primarily at E < 10 keV, an energy range that is not strongly affected by the polarization reversal.
In recent decades, global seismic observations have identified increasingly complex anisotropy of the Earth’s inner core. Numerous seismic studies have confirmed hemispherical variations in the inner core’s anisotropy. Here, based on ab initio molecular dynamics calculations, we report how the anisotropy of hexagonal close-packed (hcp)-iron, under inner core conditions, could be altered when alloyed with light elements. We find that light elements in binary allows with iron — hcp-Fe-X (X = C, O, Si, and S) — could have significant effects on density, sound velocities, and anisotropy, compared with the behavior of pure hcp-iron; the anisotropy of these binary alloys depends on combined effects of temperature and the particular alloying light element. Furthermore, the change in anisotropy strength with increasing temperature can be charted for each alloy. Alloying pure iron with some light elements such as C or O actually does not increase but decreases core anisotropy at high temperatures. But the light element S can significantly enhance the elastic anisotropy strength of hcp-Fe.
In this work, we interpreted gravity data to determine the structural characteristics responsible for high-gravity anomalies in Bagodo, North Cameroon. These anomalies had not previously been characterized through a local study. Thus, we undertook a regional–residual separation of the gravity anomalies by using the polynomial method. Geophysical signatures of near-surface small-extent geological structures were revealed. To conduct a quantitative interpretation of the gravity anomalies, one profile was drawn on a residual Bouguer anomaly map and then interpreted by spectral analysis, the ideal body solution, and 2.5-dimensional modeling. Our results showed that the intrusive body in the Bagodo area consists of two trapezoidal blocks. The first and second blocks have roofs approximately 7.5 and 14 km deep, respectively, whereas their bases are approximately 17 km deep. These values are in agreement with those obtained by the ideal body solution, which showed two cells with a density contrast of 0.3 g·cm−3 in comparison with the surrounding rocks. The density of this body was estimated to be approximately 3 g·cm−3. The topography of these rocks showed that they are basaltic rocks that would have cooled in fracture zones as an intrusion.
We present a statistical study of “trunk-like” structures observed in He+ and O+ in the inner magnetosphere. The main characteristic of these structures is that the energy of the peak flux decreases earthward. Using observations from the Helium Oxygen Proton Electron (HOPE) instrument onboard Van Allen Probe A, we identify the trunks observed from November 2012 to June 2019 and obtain the universal time, L shell, magnetic local time (MLT), and energy information of each trunk’s root and tip. We then investigate the behavior of trunks in terms of their frequency of occurrence, temporal evolution, spatial and energy distribution, and trunk dependence on different geomagnetic indices. We find that (1) the trunks are always located at L = 1.5−4.0 and have a preferential location mainly concentrated at MLT = 18−24, (2) the sector within MLT = 14−16 is a forbidden zone without trunk roots, and (3) the energy of He+ trunks is the largest near dusk and gradually decreases in the counterclockwise direction, whereas the energy of O+ trunks is relatively evenly distributed with MLT and L. The differences between He+ and O+ trunks are probably due to the different charge exchange and Coulomb collision lifetime. The dependence on different geomagnetic indices indicates that the trunk structures occur more frequently during relatively quiet periods.
Water is essential for the formation of a magmatic arc by lowering the melting temperature of materials in the mantle wedge. As such, it is logical to attribute the absence of a magmatic arc to insufficient water released from the subducting plate, although a number of other factors may cause volcanic arc quiescence as well, such as a slab window or flat slab subduction. In this contribution, we present a possible but testable correlation between the occurrence of a magmatic arc and seamount subduction in light of bathymetric data obtained near trenches. This correlation, if it holds true, in turn means that a magmatic arc is unlikely to occur when the subducting slabs have not been severely fractured and that one of the main reasons for excluding effects such as the slab window or flat slab subduction may be that the plate is not accompanied by seamounts. Therefore, the role that seamount subduction plays in recycling water back into the mantle deserves more attention from the earth sciences community.
The search for potential signs of Martian life has been planned and implemented by worldwide countries for decades. Due to the high expenditure on Mars exploration, more easily accessible Mars-like regions on Earth are invaluable targets for astrobiology research to understand the detection of potential Martian biosignatures. In order to detect potential life signals beyond Earth, questions on how to define life and biosignatures need to be carefully inspected. This review summarizes scientific instrumental techniques, our “eyes” and “hands”, that facilitate identifying and quantifying biosignatures on Mars. Scientific devices that can be applied in astrobiology include electromagnetic spectrum-based spectrometers, mass spectrometers, redox potential indicators, circular dichroism polarimeters, in situ nucleic acid sequencers, life isolation/cultivation systems, and imagers. These techniques should be first tested in Mars analog extreme environments on Earth to validate their practicality on Mars. To better understand the instrumental detectability of biosignatures on Mars through its evolutionary history, terrestrial Mars analogs are divided into four major categories according to their similarities to different geological ages of Mars (the Early-Middle Noachian Period, the Late Noachian-Early Hesperian Period, the Late Hesperian-Early Amazonian Period, and the Middle-Late Amazonian Period). Future missions are suggested to explore more on the early terrains in Mars’ Southern Hemisphere once the landing issue is solved engineeringly, since these explorations permit investigating a continuum of the habitability shift through Mars geological history. Moreover, practical applications of scientific instruments listed above are briefly reviewed based on the four categories of Mars analogs, and instruments applied for autonomous robotic rover tests in Mars analogs are further discussed. For engineering efficiency, a Mars rover ought to be equipped with as few assemblable instruments as possible. Therefore, once candidate landing regions on Mars are defined, the portable suites of instruments should be smartly devised on the basis of the geological, geochemical, geomorphological, and chronological characteristics of the landing regions. Laboratory-based experiments without engineering restrictions are further encouraging and appealing if Mars sample-return missions are successfully completed. To exclude false positive and false negative conclusions in life discovery, the combined use and replication studies of multiple analytical techniques must be performed to confirm the observational results.
Kinetic Alfvén waves (KAWs), with a strong parallel disturbed electric field, play an important role in the energy transport and particle acceleration in the magnetotail. Based on the high-resolution observations of the Magnetospheric Multiscale (MMS) mission, we present the detailed acceleration process of electrons by KAWs in the plasma sheet boundary layer (PSBL). MMS observed strong electromagnetic disturbances carrying parallel disturbed electric field with an amplitude up to 8 mV/m. The measured ratio of the electric to magnetic field perturbations is larger than the local Alfvén speed and enhances as the frequency increases, in consistent with the theoretical predictions for KAWs. These evidences indicate that the electromagnetic disturbances should be identified as KAWs. During the KAWs, the energy flux of electrons at energies above 1 keV in the parallel and anti-parallel direction significantly enhance, implying occurrences of electron beams at higher energies. Meanwhile, the KAWs become more electrostatic-like and filled with high frequency ion acoustic waves. The energy enhancement of electron beams accords to the derived work done by the observed parallel disturbed electric field of KAWs, indicating electron acceleration caused by KAWs. Therefore, the paper provides a direct evidence of electron acceleration by KAWs embodying electrostatic ion acoustic waves in the PSBL.
A meteor radar chain located along the 120°E meridian in the Northern Hemisphere from low to middle latitudes provides long-term horizontal wind observations of the mesosphere and lower thermosphere (MLT) region. In this study, we report a seasonal variation and its latitudinal feature in the horizontal mean wind in the MLT region observed by 6 meteor radars located at Mohe (53.5°N, 122.3°E), Beijing (40.3°N, 116.2°E), Mengcheng (33.4°N, 116.5°E), Wuhan (30.6°N, 114.4°E), Kunming (25.6°N, 108.3°E) and Fuke (19.5°N, 109.1°E) stations. In addition, we compare the MLT wind measured by the meteor radars and the simulated by Whole Atmosphere Community Climate Model (WACCM). In general, the WACCM appears to well capture the seasonal and latitudinal variations in the zonal wind component. Especially, the temporal evolution of the eastward zonal wind maximum shifts from July to May as the latitude decreases. However, the WACCM meridional wind show differences with the meteor radar observations.
Active-source surface wave exploration is advantageous because it has high imaging accuracy, is not affected by high-speed layers and has a low cost. It has unique advantages in the investigation of shallow surface structures. For the development and utilization of urban underground space, there are two important parameters in the shallow surface, namely, shear wave velocity (vs.) and site predominant period, which determine the elevation and aseismic grade of the building design. The traditional method mainly obtains the above two parameters through the testing and measurement of drilling samples. However, this method is extremely expensive and time consuming. Therefore, this paper uses the multichannel surface wave acquisition method to extract the fundamental dispersion curve of the single shot data using the phase shift method and obtains the vs. characteristics in the uppermost 40 m by inversion. According to the vs. profile, the following two conclusions were obtained. First, the study area can be roughly divided into 5 layers, among which the layers 0-8 m, 14-20 m, and 20-30 m are low-velocity layers, corresponding to miscellaneous fill, a water-bearing sand layer and a sand layer. Therefore, the vs. is relatively low, and the layers at 8-14 m and 30-40 m are high-velocity layers that are mainly composed of clay, with a relatively better compactness and relatively high vs. values. In addition, the low-speed anomaly suddenly appears in the high-speed area at 20-40 m. Combining this with geological data suggests that it is an ancient river channel. Second, according to the vs. value, the V_se(equivalent shear wave velocity) was calculated. The construction site soil is classified as Category III, with good engineering geological conditions. In addition, according to vs., we calculate the site predominant period to be 0.56-0.77 s. Therefore, in the overall structural design of foundation engineering, the natural vibration period of the structure should be strictly controlled to avoid the predominant period of the site.
Water budget closure is a method used to study the balance of basin water storage and the dynamics of relevant hydrological components (e.g., precipitation, evapotranspiration, and runoff). When water budget closure is connected with terrestrial water storage change (TWSC) estimated from Gravity Recovery and Climate Experiment (GRACE) data, variations in basin runoff can be understood comprehensively. In this study, total runoff variations in the Yangtze River Basin (YRB) and its sub-basins are examined in detail based on the water budget closure equation. We compare and combine mainstream precipitation and evapotranspiration models to determine the best estimate of precipitation minus evapotranspiration. In addition, we consider human water consumption, which has been neglected in earlier studies, and discuss its impact. To evaluate the effectiveness and accuracy of the combined hydrological models in estimating subsurface runoff, we collect discharge variations derived from in situ observations in the YRB and its sub-basins and compare these data with the models’ final estimated runoff variations. The estimated runoff variations suggest that runoff over the YRB has been increasing, especially in the lower sub-basins and in the post-monsoon season, and is accompanied by apparent terrestrial water loss.
The tidal Love numbers of the Moon are a set of nondimensional parameters that describe the deformation responses of the Moon to the tidal forces of external celestial bodies. They play an important role in the theoretical calculation of the Moon’s tidal deformation and the inversion of its internal structure. In this study, we introduce the basic theory for the theoretical calculation of the tidal Love numbers and propose a new method of solving the tidal Love numbers: the spectral element method. Moreover, we explain the mathematical theory and advantages of this method. On the basis of this new method, using 10 published lunar internal structure reference models, the lunar surface and lunar internal tidal Love numbers were calculated, and the influence of different lunar models on the calculated Love numbers was analyzed. Results of the calculation showed that the difference in the second-degree lunar surface Love numbers among different lunar models was within 8.5%, the influence on the maximum vertical displacement on the lunar surface could reach ±8.5 mm, and the influence on the maximum gravity change could reach ±6 μGal. Regarding the influence on the Love numbers inside the Moon, different lunar models had a greater impact on the Love numbers h2 and l2 than on k2 in the lower lunar mantle and core.
The Neogene Terror Rift in the Antarctic Victoria Land Basin (VLB) of the Ross Sea, Antarctica, is composed of the Discovery Graben and the Lee Arch. Many Neogene volcanoes are aligned in the north-south direction in the southern VLB, belonging to the McMurdo Volcanic Group. However, due to multiple glaciations and limited seismic data, the volcanic processes are still unclear in the northern VLB, especially in the Terror Rift. Multichannel seismic profiles were collected at the VLB from the 32nd Chinese National Antarctic Research Expedition (CHINARE). We utilized four seismic profiles from the CHINARE and additional historical profiles, along with gravity and magnetic anomalies, to analyze faults and stratigraphic characteristics in the northern Terror Rift and volcanism in the VLB. Negative flower structures found in the northern Terror Rift suggest that the Terror Rift was affected by dextral strike-slip faults extending from the northern Victoria Land (NVL). After the initial orthogonal tension, the rift transited into an oblique extension, forming a set of downward concaving normal faults and accommodation zones in the Terror Rift. On the Lee Arch, several imbricated normal faults formed and converged into a detachment fault. Under gravitational forces, the strata bent upward and formed a rollover anticline. Many deep faults and thin strata subjected to erosion facilitated volcanic activity. A brittle volcanic region in the VLB was affected by dextral strike-slip movements and east-west extension, resulting in two Neogene volcanic chains that connect three igneous provinces in the VLB: the Hallett, Melbourne, and Erebus Provinces. These two chains contain mud volcanoes with magnetic nuclei, volcanic intrusions, and late-stage volcanic eruptions. Volcanisms have brought about opposite polarities of magnetic anomalies in Antarctica, indicating the occurrence of multiple volcanic activities.
Because of the viscoelasticity of the subsurface medium, seismic waves will inherently attenuate during propagation, which lowers the resolution of the acquired seismic records. Inverse-Q filtering, as a typical approach to compensating for seismic attenuation, can efficiently recover high-resolution seismic data from attenuation. Whereas most efforts are focused on compensating for high-frequency energy and improving the stability of amplitude compensation by inverse-Q filtering, low-frequency leakage may occur as the high-frequency component is boosted. In this article, we propose a compensation scheme that promotes the preservation of low-frequency energy in the seismic data. We constructed an adaptive shaping operator based on spectral-shaping regularization by tailoring the frequency spectra of the seismic data. We then performed inverse-Q filtering in an inversion scheme. This data-driven shaping operator can regularize and balance the spectral-energy distribution for the compensated records and can maintain the low-frequency ratio by constraining the overcompensation for high-frequency energy. Synthetic tests and applications on prestack common-reflection-point gathers indicated that the proposed method can preserve the relative energy of low-frequency components while fulfilling stable high-frequency compensation.
In a recent paper (Luo H et al., 2022), we found that the peak amplitudes of diurnal magnetic variations, measured during martian days (sols) at the InSight landing site, exhibited quasi Carrington-Rotation (qCR) periods at higher eigenmodes of the natural orthogonal components (NOC); these results were based on ~664 sols of magnetic field measurements. However, the source of these periodic variations is still unknown. In this paper we introduce the neutral-wind driven ionospheric dynamo current model (e.g., Lillis et al., 2019) to investigate the source. Four candidates — the draped IMF, electron density/plasma density, the neutral densities, and the electron temperature in the ionosphere with artificial qCR periodicity, are applied in the modeling to find the main factor likely to be causing the observed surface magnetic field variations that exhibit the same qCR periods. Results show that the electron density/plasma density, which controls the total conductivity in the dynamo region, appears to account for the greatest part of the surface qCR variations; its contribution reaches about 67.6%. The draped IMF, the neutral densities, and the electron temperature account, respectively, for only about 12.9%, 10.3%, and 9.2% of the variations. Our study implies that the qCR magnetic variations on the Martian surface are due primarily to variations of the dynamo currents caused by the electron density variations. We suggest also that the time-varying fields with the qCR period could be used to probe the Martian interior's electrical conductivity structure to a depth of at least 700 km.
Numerous linear grooves have long been recognized as covering the surface of Phobos, but the mechanisms of their formation are still unclear. One possible mechanism is related to the largest crater on Phobos, the Stickney crater, whose impact ejecta may slide, roll, bounce, and engrave groove-like features on Phobos. When the launch velocity is higher than the escape velocity, the impact ejecta can escape Phobos. A portion of these high-velocity ejecta are dragged by the gravitational force of Mars, fall back, and reimpact Phobos. In this research, we numerically test the hypothesis that the orbital ejecta of the Stickney crater that reimpact Phobos could be responsible for a particular subset of the observed grooves on Phobos. We adopt impact hydrocode iSALE-2D (impact-Simplified Arbitrary Lagrangian Eulerian, two-dimensional) to simulate the formation of the Stickney crater and track its impact ejecta, with a focus on orbital ejecta with launch velocities greater than the escape velocity of Phobos. The launch velocity distribution of the ejecta particles is then used to calculate their trajectories in space and determine their fates. For orbital ejecta reimpacting Phobos, we then apply the sliding boulder model to calculate the ejecta paths, which are compared with the observed groove distribution and length to search for causal relationships. Our ejecta trajectory calculations suggest that only ~1% of the orbital ejecta from the Stickney crater can reimpact Phobos. Applying the sliding boulder model, we predict ejecta sliding paths of 9−20 km in a westward direction to the east of the zone of avoidance, closely matching the observed grooves in that region. The best-fit model assumes an ejecta radius of ~150 m and a speed restitution coefficient of 0.3, consistent with the expected ejecta and regolith properties. Our calculations thus suggest the groove class located to the east of the zone of avoidance may have been caused by reimpact orbital ejecta from the Stickney crater.