Earth and Planetary Physics最新文献

The ion-to-electron temperature ratio is a good indicator of the processes involved in solar wind plasma entering and being transported inside Earth's plasma sheet. In this study, we have demonstrated that patchy magnetic reconnection has the potential to preserve the ion-to-electron temperature ratio under certain conditions. If the charged particles are non-adiabatically accelerated no more than once in a single reconnection, the temperature ratio would be preserved; on the other hand, this ratio would not be preserved if they are accelerated multiple times. Consequently, under a northward interplanetary magnetic field (IMF) condition, the reconnection in the nonlinear phase of the Kelvin-Helmholtz instability is the dominant process for solar-originated plasma entering the Earth's magnetosphere, and the ion-to-electron temperature ratio is preserved inside the plasma sheet. When the direction of the IMF is southward, the reflection of electrons from the magnetic mirror point, and subsequent multiple non-adiabatic accelerations at the reconnection site, are the primary reasons for the observed low ion-to-electron temperature ratio close to the Earth at midnight. While reconnections that occur in the night-side far tail might preserve the ratio, turbulence on the boundaries of the bursty bulk flows (BBFs) could change the ratio in the far tail through the violation of the frozen-in condition of the ions. The plateau in the contour of the calculated ion-to-electron temperature ratio in the down tail distance between 40 and 60 Earth radii may explain the strong correlation between the ion and electron temperatures in the outer central plasma sheet, which has not been clearly understood till date.

Responses of atmospheric carbon dioxide (CO₂) density to geomagnetic secular variation are investigated using the Whole Atmosphere Community Climate Model-eXtended (WACCM-X). Our ensemble simulations show that CO₂ volume mixing ratios (VMRs) increase at high latitudes and decrease at mid and low latitudes by several ppmv in response to a 50% weakening of the geomagnetic field. Statistically significant changes in CO₂ are mainly found above ~90 km altitude and primarily redetermine the energy budget at ~100-110 km. Our analysis of transformed Eulerian mean (TEM) circulation found that CO₂ change is caused by enhanced upwelling at high latitudes and downwelling at mid and low latitudes as a result of increased Joule heating. We further analyzed the atmospheric CO₂ response to realistic geomagnetic weakening between 1978 and 2013, and found increasing (decreasing) CO₂ VMRs at high latitudes (mid and low latitudes) accordingly. For the first time, our simulation results demonstrate that the impact of geomagnetic variation on atmospheric CO₂ distribution is noticeable on a time scale of decades.

Previous studies indicate that, in the Jovian magnetosphere, the long-term trend of the radial profile of relativistic electron intensities is primarily shaped by slow radial diffusion. However, measurements by the Galileo spacecraft reveal the existence of transient increases in MeV electron intensities well above the ambient distribution. It is unclear how common such transient enhancements are, and to which dynamic processes in Jupiter's magnetosphere their occurrence is linked. We investigate the radial distributions of 11 MeV and 1 MeV electron intensities from to ( denotes the Jovian radius), measured by the Galileo spacecraft from 1996 to 2002. We find transient enhancements of MeV electrons during seven Galileo crossings, mostly occurring around ∼20R_J. An apparent dawn-dusk asymmetry of their occurrence is resolved, with a majority of events discovered at dawn. This dawn-dusk asymmetry, as well as the average recurrence time scale of a few days, implies a potential relationship between the MeV electron transients and the storm-like dynamics in the middle and outer magnetosphere detected using a variety of Galileo, Juno and remote sensing aurora observations. We suggest that the observations of some of these transients in the inner magnetosphere may result from a synergy between the convective transport by a large-scale dawn-dusk electric field and the sources provided by injections in the middle magnetosphere.

The nearly analytic discrete (NAD) method is a kind of finite difference method with advantages of high accuracy and stability. Previous studies have investigated the NAD method for simulating wave propagation in the time-domain. This study applies the NAD method to solving three-dimensional (3D) acoustic wave equations in the frequency-domain. This forward modeling approach is then used as the “engine” for implementing 3D frequency-domain full waveform inversion (FWI). In the numerical modeling experiments, synthetic examples are first given to show the superiority of the NAD method in forward modeling compared with traditional finite difference methods. Synthetic 3D frequency-domain FWI experiments are then carried out to examine the effectiveness of the proposed methods. The inversion results show that the NAD method is more suitable than traditional methods, in terms of computational cost and stability, for 3D frequency-domain FWI, and represents an effective approach for inversion of subsurface model structures.

For the first time, the effect of ions on complex conductivity and permittivity of dusty plasma at Polar Mesosphere Summer Echoes (PMSE) altitude is analyzed. Because of ions higher mass and smaller thermal velocity, generally, their effects are not considered in the study of electromagnetic properties of dusty plasmas. In this study, we modified the equations of conductivity and permittivity by adding the effect of ions. In the PMSE altitude region between 80 and 90 km, a local reduction in electron density (i.e., an electron bite-out), is produced by electron absorption onto dust particles. The bite-out condition contains high dust density and smaller electron density. From simulation results in comparatively strong bite-out conditions, we found that the ion effects on conductivity become significant with smaller dust size, lower electron temperature, and lower neutral density. For comparatively weak bite-out conditions, the ion effects on conductivity become significant with larger dust size, higher electron temperature, and higher neutral density. On the other hand, for different dust sizes, electron temperatures and neutral density, the ion effects on complex permittivity become significant only in very strong bite-out conditions. Based on these simulation results, we conclude that, in the absence of electron bite-out conditions, the effect of ions on complex conductivity and permittivity is not significant and can be ignored. However, during bite-out conditions, the effect of ions becomes significant and cannot be ignored because it significantly changes the conductivity and permittivity of dusty plasmas.

A particle-in-cell simulation of symmetric reconnection with zero guide field is carried out to understand the dynamics of ions along the separatrices. Through the investigation of ion velocity distributions at different moments and locations along the separatrices, a typical distribution is found: two counter-streaming populations in the perpendicular direction, with another two populations accelerated into distinct energy levels in the parallel direction. Backward tracing of ions reveals that the counter-streaming cores are mostly composed of ions initially located at the same side of the separatrix, while the other two accelerated populations in the parallel direction are composed of ions crossing through the neutral sheet. Through analysis of energy conversion of these populations, it is found that the ion energization along the separatrix is attributable primarily to the Hall electric field, while that in the region between the two separatrices is caused primarily by the induced reconnection electric field. For the counter-streaming population, the low-energy ions that cross the separatrix twice are affected by both Hall and reconnection electric fields, while the high-energy ions that directly enter the separatrix from the unperturbed plasma are energized mainly by the Hall electric field. For the two energized populations in the parallel direction, the ions with lower-energy are accelerated mainly by the in-plane electric field and the Hall electric field on the opposite side of the separatrix, whereas the ions with higher-energy not only experience the same energization process but also are constantly accelerated by the reconnection electric field.

Polar mesosphere summer echoes (PMSE) are observed simultaneously with Digisonde and EISCAT VHF radar. The phenomenon of irregular Es layers is called PMSE-like or PMSE-Es (Polar Mesosphere Summer Echoes-Es) and has some relationship with real PMSE. In this paper, the characteristics of irregular Es layers at 80–100 km were observed by Digisonde at Tromsø during 2003–2014 are statistically analyzed with ionograms. The diurnal, day-to-day and year-to-year variations and discrepancies of occurrence rate between PMSE and PMSE-Es are compared with the statistical results observed by Esrange MST radar (ESRAD), and the reasons are discussed. The results show that the trends in the occurrence rate of PMSE-Es are similar to the trends in the occurrence rate of PMSE, but there are some notable differences. The occurrence rate of PMSE-Es is much lower than the occurrence rate of PMSE. The minimum value of PMSE-Es appears 1–2 hours earlier than the minimum value of the PMSE occurrence rate, while PMSE-Es appear earlier than PMSE in the year. In addition, there is a significant positive correlation between the annual average occurrence rates of PMSE and PMSE-Es. PMSE-Es is a relatively important occurrence in the polar mesopause. Analysis of its characteristics can provide new ideas and methods for studying the formation mechanism of PMSE.

Energetic electron measurements and spacecraft charging are of great significance for theoretical research in space physics and space weather applications. In this paper, the energetic electron detection package (EEDP) deployed on three Chinese navigation satellites in medium Earth orbit (MEO) is reviewed. The instrument was developed by the space science payload team led by Peking University. The EEDP includes a pinhole medium-energy electron spectrometer (MES), a high-energy electron detector (HED) based on ΔE-E telescope technology, and a deep dielectric charging monitor (DDCM). The MES measures the energy spectra of 50−600 keV electrons from nine directions with a 180°×30° field of view (FOV). The HED measures the energy spectrum of 0.5−3.0 MeV electrons from one direction with a 30° cone-angle FOV. The ground test and calibration results indicate that these three sensors exhibit excellent performance. Preliminary observations show that the electron spectra measured by the MES and HED are in good agreement with the results from the magnetic electron-ion spectrometer (MagEIS) of the Van Allen Probes spacecraft, with an average relative deviation of 27.3% for the energy spectra. The charging currents and voltages measured by the DDCM during storms are consistent with the high-energy electron observations of the HED, demonstrating the effectiveness of the DDCM. The observations of the EEDP on board the three MEO satellites can provide important support for theoretical research on the radiation belts and the applications related to space weather.

The theory of plate tectonics came together in the 1960s, achieving wide acceptance after 1968. Since then it has been the most successful framework for investigations of Earth's evolution. Subduction of the oceanic lithosphere, as the engine that drives plate tectonics, has played a key role in the theory. However, one of the biggest unanswered questions in Earth science is how the first subduction was initiated, and hence how plate tectonics began. The main challenge is how the strong lithosphere could break and bend if plate tectonics-related weakness and slab-pull force were both absent. In this work we review state-of-the-art subduction initiation (SI) models with a focus on their prerequisites and related driving mechanisms. We note that the plume-lithosphere-interaction and mantle-convection models do not rely on the operation of existing plate tectonics and thus may be capable of explaining the first SI. Re-investigation of plate-driving mechanisms reveals that mantle drag may be the missing driving force for surface plates, capable of triggering initiation of the first subduction. We propose a composite driving mechanism, suggesting that plate tectonics may be driven by both subducting slabs and convection currents in the mantle. We also discuss and try to answer the following question: Why has plate tectonics been observed only on Earth?