67 P/Churyumov-Gerasimenko彗星反磁区附近的电子动力学
Electron Dynamics near Diamagnetic Regions of Comet 67P/Churyumov-Gerasimenko
摘要：
罗塞塔号探测器在67 P/丘留莫夫-格拉西缅科彗星周围探测到短暂和零星的反磁性区域。在本文中，我们提出了体和超热电子动力学的统计分析，以及超热电子俯仰角分布（PAD）的抗磁性区域附近的一个案例研究。体电子密度与当地的中性密度，我们发现一个明显的增强在彗星的南纬度测量的电子密度。能量在几十eV到几百eV之间的超热电子的通量在航天器每次进入抗磁性区域时都会减少。我们提出了一种机制，在这种减少可以解释太阳风电子被绑在磁场和后被运输的衰减磁场环境中，有有限的访问的抗磁性区域。我们的分析表明，超热电子PAD演变从一个几乎各向同性的抗磁腔外的场取向分布附近的边界。当电子回旋半径变得与磁场线曲率的大小相当时，电子输运变得混乱和非绝热，这决定了通量变化的能量上限。这项研究是基于罗塞塔观测在大约200公里的彗星中心距离时，彗星是在1.24天文单位从太阳和南部夏季彗星季节。
英文摘要：
The Rosetta spacecraft detected transient and sporadic diamagnetic regions around comet 67P/Churyumov-Gerasimenko. In this paper we present a statistical analysis of bulk and suprathermal electron dynamics, as well as a case study of suprathermal electron pitch angle distributions (PADs) near a diamagnetic region. Bulk electron densities are correlated with the local neutral density and we find a distinct enhancement in electron densities measured over the southern latitudes of the comet. Flux of suprathermal electrons with energies between tens of eV to a couple of hundred eV decreases each time the spacecraft enters a diamagnetic region. We propose a mechanism in which this reduction can be explained by solar wind electrons that are tied to the magnetic field and after having been transported adiabatically in a decaying magnetic field environment, have limited access to the diamagnetic regions. Our analysis shows that suprathermal electron PADs evolve from an almost isotropic outside the diamagnetic cavity to a field-aligned distribution near the boundary. Electron transport becomes chaotic and non-adiabatic when electron gyroradius becomes comparable to the size of the magnetic field line curvature, which determines the upper energy limit of the flux variation. This study is based on Rosetta observations at around 200 km cometocentric distance when the comet was at 1.24 AU from the Sun and during the southern summer cometary season.
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