# EPP

ISSN  2096-3955

CN  10-1502/P

## 2021 Vol.5(4)

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PLANETARY SCIENCES
2021, 5(4): 305-313. doi: 10.26464/epp2021041
Abstract:
Jupiter’s magnetic field is thought to be generated in its deep metallic hydrogen region through dynamo action, yet the detailed dynamic process remains poorly understood. Numerical simulations have produced Jupiter-like magnetic fields, albeit using different control parameters and reference state models. In this study, we investigate the influence of different reference state models, based on ab initio calculations and based on the polytropic equation of state. In doing so, we perform five anelastic convection dynamo simulations that can be divided into two groups. In each group, different reference states are used while other control parameters and conditions are set to be identical. We find the reference state model can be very influential for the simulations in which buoyancy force is dominant over the Lorentz force. In this regime, different dynamical outcomes can be attributed to the effective buoyancy force resulting from different reference states. For simulations in which the Lorentz force is dominant over the buoyancy force, however, dynamo actions tend to be insensitive to different reference state models. If Jupiter’s dynamo is in a strong field regime, i.e., the Lorentz force plays a dominant role, our numerical results suggest that Jupiter’s internal reference state, which remains poorly constrained, may play a minor role in the dynamo process of the planet.
PLANETARY SCIENCES
2021, 5(4): 314-326. doi: 10.26464/epp2021037
Abstract:
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 \begin{document}$>$\end{document}11 MeV and \begin{document}$>$\end{document}1 MeV electron intensities from \begin{document}$9R_{J}$\end{document} to \begin{document}$40R_{J}$\end{document} (\begin{document}$R_{J}=71492\;{\rm{km}}$\end{document} 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 ~20RJ. 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.
SPACE PHYSICS: IONOSPHERIC PHYSICS
2021, 5(4): 327-336. doi: 10.26464/epp2021040
Abstract:
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.
SPACE PHYSICS: MAGNETOSPHERIC PHYSICS
2021, 5(4): 337-347. doi: 10.26464/epp2021035
Abstract:
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.
SOLID EARTH: SEISMOLOGY
2021, 5(4): 348-361. doi: 10.26464/epp2021038
Abstract:
On August 8, 2017, a magnitude 7.0 earthquake occurred in Jiuzhaigou County, Sichuan Province, China. The deep seismogenic environment and potential seismic risk in the eastern margin of Tibetan Plateau have once again attracted the close attention of seismologists and scholars at home and abroad. The post-earthquake scientific investigation could not identify noticeable surface rupture zones in the affected area; the complex tectonic background and the reason(s) for the frequent seismicity in the Jiuzhaigou earthquake region are unclear. In order to reveal the characteristics of the deep medium and the seismogenic environment of the M7.0 Jiuzhaigou earthquake region, and to interpret the tectonic background and genesis of the seismicity comprehensively, in this paper, we have reviewed all available observation data recorded by the regional digital seismic networks and large-scale, dense mobile seismic array (China Array) for the northern section of the North–South Seismic Belt around Jiuzhaigou earthquake region. Using double-difference seismic tomography method to invert the three-dimensional P-wave velocity structure characteristics of the upper crust around the Jiuzhaigou earthquake region, we have analyzed and discussed such scientific questions as the relationship between the velocity structure characteristics and seismicity in the Jiuzhaigou earthquake region, its deep tectonic environment, and the ongoing seismic risk in this region. We report that: the P-wave velocity structure of the upper crust around the Jiuzhaigoug earthquake region exhibits obvious lateral inhomogeneity; the distribution characteristics of the shallow P-wave velocity structure are closely related to surface geological structure and formation lithology; the M7.0 Jiuzhaigou earthquake sequence is closely related to the velocity structure of the upper crust; the mainshock of the M7.0 earthquake occurred in the upper crust; the inhomogeneous variation of the velocity structure of the Jiuzhaigou earthquake area and its surrounding medium appears to be the deep structural factor controlling the spatial distribution of the mainshock and its sequence. The 3D P-wave velocity structure also suggests that the crustal low-velocity layer of northeastern SGB (Songpan–Garzê Block) stretches into MSM (Minshan Mountain), and migrates to the northeast, but the tendency to emerge as a shallow layer is impeded by the high-velocity zone of Nanping Nappe tectonics and the Bikou Block. Our results reveal an uneven distribution of high- and low-velocity structures around the Tazang segment of the East Kunlun fault zone. Given that the rupture caused by the Jiuzhaigou earthquake has enhanced the stress fields at both ends of the seismogenic fault, it is very important to stay vigilant to possible seismic hazards in the large seismic gap at the Maqu–Maqên segment of the East Kunlun fault zone.
SOLID EARTH: SEISMOLOGY
2021, 5(4): 362-364. doi: 10.26464/epp2021036
Abstract: