Cluster observations of bidirectional beams caused by electron trapping during antiparallel reconnection
Cluster observations of bidirectional beams caused by electron trapping during antiparallel reconnection
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DOI：
10.1029/2009ja014650
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发表时间：
2010-03
影响因子：
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通讯作者：
J. Egedal;A. Le;N. Katz;L. Chen;B. Lefebvre;W. Daughton;A. Fazakerley
中科院分区：
文献类型：
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作者：
J. Egedal;A. Le;N. Katz;L. Chen;B. Lefebvre;W. Daughton;A. Fazakerley
[1]一些航天器飞行任务主要在重新连接区域的流入处观察到了所谓的双向电子束。在这里，我们证明了电子分布函数中的这些束状特征可以用电子陷阱来解释。俘获主要由正加速势Φ∥控制，它与重联区附近平行电场的结构有关。在动力学模拟结果的指导下，我们推广了最新的适用于反平行重联流入区的电子分布函数的解析模型。该模型被成功地与四星团航天器在活跃的重联区内观测到的数据进行了比较。在电子分布中记录的各向异性与Φ∥对电子的主要电子俘获一致。在分析中，我们确定了Φ∥在与重联区相遇期间沿着集群航天器的路径的轮廓。EΦ∥的典型值超过1keV(远高于环瓣等离子体中的电子温度)，Φ∥捕获所有热电子。这对于与离子扩散区相关的霍尔电流系统的内部结构是重要的，因为扩展的陷阱显著改变了电子流体的电学和运动学性质。最后，在进气区和排气区的交界处，进气区的eΦ∥值平稳地接近重联排气中观察到的所谓平顶分布的肩部能量(最高可达15keV)。这表明Φ∥可能对平顶分布的形成也很重要。
[1] So-called bidirectional electron beams have been observed by a number of spacecraft missions mainly in the inflow of reconnection regions. Here we show that these beam-like features in the electron distribution function are explained by electron trapping. The trapping is mainly controlled by a positive acceleration potential, Φ∥, which is related to the structure of the parallel electric fields in the vicinity of the reconnection region. Guided by the results of a kinetic simulation, we extend a recent analytical model for the electron distribution function applicable to the inflow region in antiparallel reconnection. The model is successfully compared to data observed by the four Cluster spacecraft inside an active reconnection region. The anisotropy recorded in the electron distributions is consistent with mainly electric trapping of electrons by Φ∥. In the analysis we determine the profiles of Φ∥ along the paths of the Cluster spacecraft during their encounter with a reconnection region. Typical values of eΦ∥ are in excess of 1 keV (much higher than the electron temperature in the ambient lobe plasma) and Φ∥ traps all thermal electrons. This is important for the internal structure of the Hall current system associated with the ion diffusion region because extended trapping significantly alters the electrical and kinematic properties of the electron fluid. Finally, at the boundary between the inflow and exhaust regions the values of eΦ∥ in the inflow region smoothly approach the shoulder energies (up to 15 keV) of the so-called flat-top distribution observed in the reconnection exhaust. This suggests that Φ∥ may be important also to the formation of the flat-top distributions.