Statistical Study of Small-scale Magnetic Holes in the Upstream Regime of the Martian Bow Shock
Statistical Study of Small-scale Magnetic Holes in the Upstream Regime of the Martian Bow Shock
复制标题
火星弓形激波上游流域小尺度磁洞的统计研究
DOI：
10.3847/1538-4357/ac1c07
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发表时间：
2021
期刊：
影响因子：
--
通讯作者：
T. L. Zhang
中科院分区：
文献类型：
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作者：
G. Q. Wang;M. Volwerk;A. M. Du;S. D. Xiao;M. Y. Wu;Y. Q. Chen;T. L. Zhang
小尺度磁洞（Small-scale magnetic holes，SSMH）在1 Au太阳风、地球电流片和磁鞘中经常被观测到。这些动力学尺寸结构在能量耗散和颗粒输运中起着重要作用。在这里，我们报告的存在SSMHs的上游制度的火星弓冲击和统计调查这些SSMHs的基础上5年的观测火星大气和挥发性EvolutioN航天器。共发现549个SSMH，其持续时间和大小服从对数正态分布。中位持续时间为0.46秒，中位尺寸为0.22离子惯性长度。我们把一个孤立的SSMH或一列SSMH看作是一个SSMH事件。SSMH事件的平均发生率为每天10.6起事件。在离子前震区，平均发生率较大，表明离子前震是SSMH的重要震源。当离子数密度和行星际磁场（IMF）强度较大时，SSMH事件的发生往往较大，表明SSMH的产生可能与离子和IMF强度有关。虽然SSMH的产生机制还不清楚，但SSMH事件发生率与离子数密度以及IMF强度之间的联系的发现可能为进一步揭示太阳风或行星前震中SSMH的产生机制提供线索。
Small-scale magnetic holes (SSMHs) are frequently observed in the solar wind at 1 au, as well as the terrestrial current sheet and magnetosheath. These kinetic-size structures play an important role in energy dissipation and particle transportation. Here, we report the existence of SSMHs in the upstream regime of the Martian bow shock and statistically investigate these SSMHs based on 5 yr observations of the Mars Atmosphere and Volatile EvolutioN spacecraft. A total of 549 SSMHs are found, and their durations and sizes obey the lognormal distribution. The median duration is ∼0.46 s, and the median size is ∼2 ion inertial lengths. We regard an isolated SSMH or a train of SSMHs as a SSMH event. The average occurrence rate of the SSMH events is ∼0.6 event per day. The occurrence rate is much larger in the region belonging to the ion foreshock on average, suggesting that the ion foreshock is an important source of SSMHs. The occurrence of the SSMH events tends to be larger when the ion number density and the interplanetary magnetic field (IMF) strength are larger, indicating that the generation of SSMHs might be associated with ions and the IMF strength. Although their generation mechanism is still unclear, the finding of the link between the occurrence rate of the SSMH events and ion number density, as well as the IMF strength, might provide a clue to further reveal the generation mechanism of SSMHs in the solar wind or planetary foreshock.