EPP

地球与行星物理

ISSN  2096-3955

CN  10-1502/P

2019 Vol.3(5)

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RESEARCH ARTICLE
PLANETARY SCIENCES
Photoelectron balance in the dayside Martian upper atmosphere
XiaoShu Wu, Jun Cui, Jiang Yu, LiJuan Liu, ZhenJun Zhou
2019, 3(5): 373-379. doi: 10.26464/epp2019038
Abstract:
Photoelectrons are produced by solar Extreme Ultraviolet radiation and contribute significantly to the local ionization and heat balances in planetary upper atmospheres. When the effect of transport is negligible, the photoelectron energy distribution is controlled by a balance between local production and loss, a condition usually referred to as local energy degradation. In this study, we examine such a condition for photoelectrons near Mars, with the aid of a multi-instrument Mars Atmosphere and Volatile Evolution data set gathered over the inbound portions of a representative dayside MAVEN orbit. Various photoelectron production and loss processes considered here include primary and secondary ionization, inelastic collisions with atmospheric neutrals associated with both excitation and ionization, as well as Coulomb collisions with ionospheric thermal electrons. Our calculations indicate that photoelectron production occurs mainly via primary ionization and degradation from higher energy states during inelastic collisions; photoelectron loss appears to occur almost exclusively via degradation towards lower energy states via inelastic collisions above 10 eV, but the effect of Coulomb collisions becomes important at lower energies. Over the energy range of 30–55 eV (chosen to reduce the influence of the uncertainty in spacecraft charging), we find that the condition of local energy degradation is very well satisfied for dayside photoelectrons from 160 to 250 km. No evidence of photoelectron transport is present over this energy range.
SPACE PHYSICS: MAGNETOSPHERIC PHYSICS
Statistical study on interplanetary drivers behind intense geomagnetic storms and substorms
Tian Tian, Zheng Chang, LingFeng Sun, JunShui Bai, XiaoMing Sha, Ze Gao
2019, 3(5): 380-390. doi: 10.26464/epp2019039
Abstract:
Geomagnetic storms and substorms play a central role in both the daily life of mankind and in academic space physics. The profiles of storms, especially their initial phase morphology and the intensity of their substorms under different interplanetary conditions, have usually been ignored in previous studies. In this study, 97 intense geomagnetic storms (Dstmin ≤ –100 nT) between 1998 and 2018 were studied statistically using the double superposed epoch analysis (DSEA) and normalized superposed epoch analysis (NSEA) methods. These storms are categorized into two types according to different interplanetary magnetic field (IMF) Bz orientations: geomagnetic storms whose IMF is northward, both upstream and downstream relative to the interplanetary shock, and geomagnetic storms whose upstream and downstream IMF is consistently southward. We further divide these two types into two subsets, by different geomagnetic storm profiles: Type I/Type II — one/two-step geomagnetic storms with northward IMF both upstream and downstream of the interplanetary shock; Type III/TypeIV — one/two-step geomagnetic storms with southward IMF both upstream and downstream of the interplanetary shock. The results show that: (1) geomagnetic storms with northward IMF both upstream and downstream of the interplanetary shock have a clear initial phase; geomagnetic storms with southward IMF in both upstream and downstream of the interplanetary shock do not; (2) the IMF is an important controlling factor in affecting the intensity characteristics of substorms. When Bz is positive before and after the interplanetary shock arrival, the Auroral Electrojet (AE) index changes gently during the initial phase of geomagnetic storms, the median value of AE index is maintained at 500–1000 nT; (3) when Bz is negative before and after the interplanetary shock arrival, the AE index rises rapidly and reaches its maxmum value about one hour after storm sudden commencements (SSC), although the time is scaled between reference points and the maximum value of AE is usually greater than 1,000 nT, representing intense substorms; (4) for most cases, the Dst0 usually reaches its minimum at least one hour after Bz. These results are useful in improving contemporary space weather models, especially for those that address geomagnetic storms and substorms.
SPACE PHYSICS: IONOSPHERIC PHYSICS
Stimulated electromagnetic emissions spectrum observed during an X-mode heating experiment at the European Incoherent Scatter Scientific Association
Xiang Wang, Chen Zhou, Tong Xu, Farideh Honary, Michael Rietveld, Vladimir Frolov
2019, 3(5): 391-399. doi: 10.26464/epp2019042
Abstract:
An extraordinary (X-mode) electromagnetic wave, injected into the ionosphere by the ground-based heating facility at Tromsø, Norway, was utilized to modify the ionosphere on November 6, 2017. The high-power high-frequency transmitter facility located at Tromsø belongs to the European Incoherent Scatter Scientific Association. In the experiment, stimulated electromagnetic emission (SEE) spectra were observed. A narrow continuum occurred under cold-start conditions and showed an overshoot effect lasting several seconds. Cascading peaks occurred on both sides of the heating frequency only in the preconditioned ionosphere and also showed an overshoot effect. These SEE features are probably related to the ponderomotive process in the X-mode heating experiment and are helpful for understanding the physical mechanism that generated them during the X-mode heating experiment. The features observed in the X-mode heating experiments are novel and require further investigation.
ATMOSPHERIC PHYSICS
The tropical Pacific cold tongue mode and its associated main ocean dynamical process in CMIP5 models
Yang Li, QuanLiang Chen, JianPing Li, WenJun Zhang, MinHong Song, Wei Hua, HongKe Cai, XiaoFei Wu
2019, 3(5): 400-413. doi: 10.26464/epp2019041
Abstract:
The cold tongue mode (CTM), which represents the out-of-phase relationship in sea surface temperature anomaly (SSTA) variability between the Pacific cold tongue region and elsewhere in the tropical Pacific, shows a long-term cooling trend in the eastern equatorial Pacific. In this study, we investigate how well the CTM is reproduced in historical simulations generated by the 20 models considered in Phase 5 of the Coupled Model Intercomparison Project (CMIP5). Qualitatively, all 20 models roughly capture the cooling SSTA associated with the CTM. However, a quantitative assessment (i.e., Taylor diagrams and the ratio of the trend between the simulations and observations) shows that only five of these 20 models (i.e., CESM1-CAM5, CMCC-CM, FGOALS-g2, IPSL-CM5B-LR, and NorESM1-M) can reproduce with useful accuracy the spatial pattern and long-term trend of the CTM. We find that these five models generally simulate the main ocean dynamical process associated with the CTM. That is, these models adequately capture the long-term cooling trend in the vertical advection of the anomalous temperature by the mean upwelling. We conclude that the performance of these CMIP5 models, with respect to simulations of the long-term cooling trend associated with the vertical advection, and the related long-term decreasing trend of the vertical gradient of the oceanic temperature anomaly, can play an important role in successful reproduction of the CTM.
ATMOSPHERIC PHYSICS