Ion gyroradius effects on particle trapping in kinetic Alfvén waves along auroral field lines
Ion gyroradius effects on particle trapping in kinetic Alfvén waves along auroral field lines
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DOI：
10.1002/2016ja022566
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发表时间：
2016-11
期刊：
影响因子：
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通讯作者：
P. Damiano;J. Johnson;C. Chaston
中科院分区：
文献类型：
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作者：
P. Damiano;J. Johnson;C. Chaston
在这项研究中，一个2-D自洽的混合gyrofluid动力学电子模型被用来研究阿尔文波传播沿着偶极磁场线的离子与电子的温度比的范围。调查的重点是了解这些影响的作用，对电子捕获的动力学阿尔文波来源于等离子体片和这种捕获的作用，在电离层的整体电子俘获。这项工作也建立在我们以前的努力，考虑类似的系统在固定的初始平行电流的限制，而不是固定的初始垂直电场。结果表明，粒子俘获效应在冷离子极限下最强，当Alfvén波的相速度增加时，Alfvén波能将俘获电子沿场线沿着带很远，产生相对较大的俘获电子数.然而，随着离子温度的升高，动力学阿尔文波携带和激励被捕获电子的能力被垂直于环境磁场的更显著的波能量色散所降低，这降低了波的振幅。这种波振幅的减小反过来又减少了平行电流和电离层边界处的高能电子群中明显的高能尾部的范围（这可能有助于解释观测中看到的宽带电子束的有限范围）。即使在冷离子极限下，动力学阿尔文波的俘获效应对于所考虑的参数仅导致适度的电子跃迁（在数十eV的量级上），并且电子到keV水平的初级跃迁与波到达电离层边界相一致。
In this study, a 2‐D self‐consistent hybrid gyrofluid‐kinetic electron model is used to investigate Alfvén wave propagation along dipolar magnetic field lines for a range of ion to electron temperature ratios. The focus of the investigation is on understanding the role of these effects on electron trapping in kinetic Alfvén waves sourced in the plasma sheet and the role of this trapping in contributing to the overall electron energization at the ionosphere. This work also builds on our previous effort by considering a similar system in the limit of fixed initial parallel current, rather than fixed initial perpendicular electric field. It is found that the effects of particle trapping are strongest in the cold ion limit and the kinetic Alfvén wave is able to carry trapped electrons a large distance along the field line yielding a relatively large net energization of the trapped electron population as the phase speed of the wave is increased. However, as the ion temperature is increased, the ability of the kinetic Alfvén wave to carry and energize trapped electrons is reduced by more significant wave energy dispersion perpendicular to the ambient magnetic field which reduces the amplitude of the wave. This reduction of wave amplitude in turn reduces both the parallel current and the extent of the high‐energy tails evident in the energized electron populations at the ionospheric boundary (which may serve to explain the limited extent of the broadband electron energization seen in observations). Even in the cold ion limit, trapping effects in kinetic Alfvén waves lead to only modest electron energization for the parameters considered (on the order of tens of eV) and the primary energization of electrons to keV levels coincides with the arrival of the wave at the ionospheric boundary.