# EPP

ISSN  2096-3955

CN  10-1502/P

## 2022 Vol.6(2)

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PLANETARY SCIENCES
2022, 6(2): 135-148. doi: 10.26464/epp2022005
Abstract:
This paper studies inter-annual variations of 6.5-Day Waves (6.5DWs) observed at altitudes 20−110 km between 52°S−52°N latitudes during March 2002−January 2021, and how these variations were related to the equatorial stratospheric Quasi-Biennial Oscillation (QBO). Temperature amplitudes of the 6.5DWs are calculated based on SABER/TIMED observations. QBO zonal winds are obtained from an ERA5 reanalysis dataset. QBO phases are derived using an Empirical Orthogonal Functions (EOF) method. Wavelet analysis of the observed 6.5DW variations demonstrates obvious spectral maximums around 28−38 months at 32°N−52°N, and around 26−30 months at 32°S−52°S. In the Northern Hemisphere, peak periods lengthened poleward; in the Southern Hemisphere, however, they were unchanged with latitude. Residual 6.5DWs amplitudes have been determined by removing composite amplitudes from 6.5DWs amplitudes. Comparisons between QBO and monthly maximum residual 6.5DWs amplitudes (\begin{document}${A}_{\mathrm{M}\mathrm{m}\mathrm{a}\mathrm{x}}$\end{document}) show clear correlations between the QBO and 6.5DWs in both hemispheres, but the observed relationship is stronger in the NH. When \begin{document}${A}_{\mathrm{M}\mathrm{m}\mathrm{a}\mathrm{x}}$\end{document} were large in the NH, the mean QBO profile was easterly at all levels from 70 to 5 hPa; when the \begin{document}${A}_{\mathrm{M}\mathrm{m}\mathrm{a}\mathrm{x}}$\end{document} were weak, the mean QBO wind was weak westerly below 30 hPa. Linear Pearson correlation coefficients between QBO phases and \begin{document}${A}_{\mathrm{M}\mathrm{m}\mathrm{a}\mathrm{x}}$\end{document} show large positive values at 60−110 km between 20°N−52°N in April and around 64 km at 24°S in February, and large negative values from 80 to 110 km between 20°N−50°N in August and at 96−106 km between 20°S−44°S in February. These results indicate quantitative correlations between QBO and 6.5DWs and provide credible evidences for further studies of QBO modulations on long-term variations of 6.5DWs.
SPACE PHYSICS: MAGNETOSPHERIC PHYSICS
2022, 6(2): 149-160. doi: 10.26464/epp2022012
Abstract:
Relativistic electron injections are one of the mechanisms of relativistic (≥0.5 MeV) electron enhancements in the Earth’s outer radiation belt. In this study, we present a statistical observation of 600 keV electron injections in the outer radiation belt by using data from the Van Allen Probes. On the basis of the characteristics of different injections, 600 keV electron injections in the outer radiation belt were divided into pulsed electron injections and nonpulsed electron injections. The 600 keV electron injections were observed at 4.5 < L < 6.4 under the geomagnetic conditions of 450 nT < AE < 1,450 nT. An L of ~4.5 is an inward limit for 600 keV electron injections. Before the electron injections, a flux negative L shell gradient for ≤0.6 MeV electrons or low electron fluxes in the injected region were observed. For 600 keV electron injections at different L shells, the source populations from the Earth’s plasma sheet were different. For 600 keV electron injections at higher L shells, the source populations were higher energy electrons (~200 keV at X ~ –9 RE), whereas the source populations for 600 keV electron injections at lower L shells were lower energy electrons (~80 keV at X ~ –9 RE). These results are important to further our understanding of electron injections and rapid enhancements of 600 keV electrons in the Earth’s outer radiation belt.
SPACE PHYSICS: MAGNETOSPHERIC PHYSICS
2022, 6(2): 161-176. doi: 10.26464/epp2022020
Abstract:
The cause of substorm onset is not yet understood. Chen CX (2016) proposed an entropy switch model, in which substorm onset results from the development of interchange instability. In this study, we sought observational evidence for this model by using Time History of Events and Macroscale Interactions during Substorms (THEMIS) data. We examined two events, one with and the other without a streamer before substorm onset. In contrast to the stable magnetosphere, where the total magnetic field strength is a decreasing function and entropy is an increasing function of the downtail distance, in both events the total magnetic field strength and entropy were reversed before substorm onset. After onset, the total magnetic field strength, entropy, and other plasma quantities fluctuated. In addition, a statistical study was performed. By confining the events with THEMIS satellites located in the downtail region between ~8 and ~12 Earth radii, and 3 hours before and after midnight, we found the occurrence rate of the total magnetic field strength reversal to be 69% and the occurrence rate of entropy reversal to be 77% of the total 205 events.
ATMOSPHERIC PHYSICS
2022, 6(2): 177-190. doi: 10.26464/epp2022015
Abstract:
The tropospheric impact of Arctic ozone loss events is still debatable. In this study we investigate that question, using the ERA5 reanalysis and long-term integration by a climate-chemistry coupled model (CESM2-WACCM). We begin with the frequency of Arctic ozone loss events. On average, such events occur once in early spring every 14−15 years in ERA5 data and in the model, both of which estimate that roughly 40% of the strong polar vortex events in March are coupled with Arctic ozone loss, the remaining 60% being uncoupled. The composite difference between the two samples might be attributed to the pure impact of the Arctic ozone loss — that is, to ozone loss alone, without the concurrent impact of strong polar vortices. Arctic ozone loss is accompanied by an increase in total ozone in midlatitudes, with the maximum centered in the Central North Pacific. Contrasting Arctic ozone loss events with pure strong polar vortex events that are uncoupled with ozone loss, observations confirm that the stratospheric Northern Annular Mode reverses earlier for the former. For pure strong vortex events in early spring (without Arctic ozone loss), the cold anomalies can extend from the stratosphere to the middle troposphere; when such events are strong, the near surface warm anomalies are biased toward the continents. In contrast, during the other 40% of strong early-spring polar vortex events, those coupled with ozone loss, a concurrent and delayed warming of the near surface over the Arctic and its neighboring areas is observed, due to vertical redistribution of solar radiation by the change in the ozone.
SOLID EARTH: GEODYNAMICS
2022, 6(2): 191-203. doi: 10.26464/epp2022016
Abstract:
Widespread magmatism, metamorphic core complexes (MCCs), and significant lithospheric thinning occurred during the Mesozoic in the North China Craton (NCC). It has been suggested that the coeval exhumation of MCCs with uniform northwest-southeast shear senses and magmatism probably resulted from a decratonization event during the retreat of the paleo-Pacific Plate. Here we used two-dimensional finite element thermomechanical numerical models to investigate critical parameters controlling the formation of MCCs under far-field extensional stress. We observed three end-member deformation modes: the MCC mode, the symmetric-dome mode, and the pure-shear mode. The MCC mode requires a Moho temperature of ≥700 °C and an extensional strain rate of ≥5 × 10−16 s−1, implying that the lithosphere had already thinned when the MCC was formed in the Mesozoic. Considering that the widespread MCCs have the same northwest-southeast extension direction in the NCC, we suggest that the MCCs are surface expressions of both large-scale extension and craton destruction and that rollback of the paleo-Pacific slab might be the common driving force.
SOLID EARTH: SEISMOLOGY
2022, 6(2): 204-212. doi: 10.26464/epp2022010
Abstract:
The Anninghe fault is a large left-lateral strike-slip fault in southwestern China. It has controlled deposition and magmatic activities since the Proterozoic, and seismic activity occurs frequently. The Mianning−Xichang segment of the Anninghe fault is a seismic gap that has been locked by high stress. Many studies suggest that this segment has great potential for large earthquakes (magnitude >7). We obtained three vertical velocity profiles of the Anninghe fault (between Mianning and Xichang) based on the inversion of P-wave first arrival times. The travel time data were picked from seismograms generated by methane gaseous sources and recorded by three linearly distributed across-fault dense arrays. The inversion results show that the P-wave velocity structures at depths of 0−2 km corresponds well with the local lithology. The Quaternary sediments have low seismic velocities, whereas the igneous rocks, metamorphic rocks, and bedrock have high seismic velocities. We then further discuss the fault activities of the two fault branches of the Anninghe fault in the study region based on small earthquakes (magnitudes between \begin{document}${M}_{L}$\end{document} 0.5 and \begin{document}${M}_{L}$\end{document} 2.5) detected by the Xichang array. The eastern fault branch is more active than the western branch and that the fault activities in the eastern branch are different in the northern and southern segments at the border of 28°21′N. The high-resolution models obtained are essential for future earthquake rupture simulations and hazard assessments of the Anninghe fault zone. Future studies of velocity models at greater depths may further explain the complex fault activities in the study region.
PLANETARY SCIENCES
2022, 6(2): 213-217. doi: 10.26464/epp2022011
Abstract:
Recently, kilometer-scale Martian ionospheric irregularities have been measured by the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission (Fowler et al., 2020). In this study, we carried out a simulation of these irregularities, assuming a uniform Martian zonal neutral wind and a cosinusoidal perturbation of the plasma density as the seeding source. Results show that a vertical electric field shear could be induced by such a plasma density perturbation. We find that the vertical electric field shear causes a velocity shear of the plasma between the topside and bottomside ionosphere, which in turn is able to produce kilometer-scale ionospheric irregularities — irregularities of a smaller scale than were seen in previous simulations (Jiang CH et al., 2021). These kilometer-scale variations with altitude, in plasma density and magnetic field profiles, are comparable to the MAVEN observations.