O+ ion beams reflected below the Martian bow shock: MAVEN observations
O+ ion beams reflected below the Martian bow shock: MAVEN observations
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DOI：
10.1002/2016ja022465
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发表时间：
2016-03
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通讯作者：
K. Masunaga;K. Seki;D. Brain;X. Fang;Y. Dong;B. Jakosky;J. Mcfadden;J. Halekas;J. Connerney
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作者：
K. Masunaga;K. Seki;D. Brain;X. Fang;Y. Dong;B. Jakosky;J. Mcfadden;J. Halekas;J. Connerney
通过分析火星大气和挥发物演化（MAVEN）航天器上的超热和热离子成分仪测量的离子速度分布函数（VDFs），研究了在火星弓形激波上方观测到的O+离子束的产生机制。在火星附近的太阳风中，MAVEN经常观测到高能O+离子束（~10 keV或更高）。伴随着O+离子束事件，我们有时会在磁鞘中观察到特征离子VDF：部分环形分布。部分环形分布对应于具有有限初始速度的拾取离子（即，而非新生拾取离子），其相空间密度远小于磁鞘局部拾取O+离子的相空间密度。因此，部分环形分布最有可能产生的反射皮卡O+离子沉淀从上游太阳风下面的弓形激波。O+离子从太阳风注入磁鞘后，受到磁鞘内明显增强的磁场作用，开始围绕磁鞘内等离子体框架的导向中心旋转。因此，一部分沉淀的O+离子被反射回太阳风，在太阳风中产生O+束。光束在太阳下区域附近准向太阳，但在高太阳天顶角（>50°）时相对于向太阳方向具有大角度。反射的O+离子束被太阳风的对流电场加速，可能会逃离火星。
We investigate a generation mechanism of O+ ion beams observed above the Martian bow shock by analyzing ion velocity distribution functions (VDFs) measured by the Suprathermal and Thermal Ion Composition instrument on the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft. In the solar wind near Mars, MAVEN often observes energetic O+ ion beams (~10 keV or higher). Accompanied with the O+ ion beam events, we sometimes observe characteristic ion VDFs in the magnetosheath: a partial ring distribution. The partial ring distribution corresponds to pickup ions with a finite initial velocity (i.e., not newborn pickup ions), and its phase space density is much smaller than that of local pickup O+ ions of the magnetosheath. Thus, the partial ring distribution is most likely produced by the reflection of pickup O+ ions precipitating from the upstream solar wind below the bow shock. After being injected into the magnetosheath from the solar wind, the precipitating O+ ions are subject to the significantly enhanced magnetic field in this region and start to gyrate around the guiding center of the plasma frame in the magnetosheath. Consequently, a part of precipitating O+ ions are reflected back to the solar wind, generating O+ beams in the solar wind. The beams direct quasi‐sunward near the subsolar region but have large angle with respect to the sunward direction at high solar zenith angles (>50°). The reflected O+ beams are accelerated by the convection electric field of the solar wind and may escape Mars.