Auroral arcs as sites of magnetic stress release
Auroral arcs as sites of magnetic stress release
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DOI：
10.1029/2007ja012378
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发表时间：
2007-09
影响因子：
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通讯作者：
G. Haerendel
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文献类型：
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作者：
G. Haerendel
[1]一个分析模型，极光弧作为一个快速释放的磁剪切应力的结果。剪切应力是由纵向对流建立的，纵向对流是由外磁层中的压力驱动的，而外磁层中的压力与低电离层中施加的摩擦力相反。一个扭曲的偶极子几何形状，允许高等离子体β赤道附近。径向压力分布中沿对流方向沿着延伸的陡峭壁架被认为是极光流片的来源。由于极光加速区存在着与磁场一致的电位降，磁层等离子体和磁场与电离层解耦，在几个阿尔文渡越时间内，这些窄电流片的差分磁能含量被释放出来，并转化为初级极光粒子的动能。一个众所周知的电流-电压关系的能量转换过程的制定。这种情况有两个重要的后果。(1)磁能的损失会导致发生器等离子体的内能随之减少，并导致压力架和极光电流片进入应力更高的磁场区域。这就是观测到极光弧相对于等离子体框架自行的原因。(2)等离子体和场沿着电弧进行快速的应力释放运动，具有大的但大多是可逆的位移。净位移，相当于一个小的S形的贡献，基本上是U形的电位分布以上的极光弧，是一致的通过进行电流片的场线的过境。这种情况被转化为一组简单的关系，表达极光弧的关键参数，例如宽度、能通量、势降和自行。这里的主要成分是一个辅助磁扰动场，其中凝聚了大尺度电流系统的主要性质。它相当于电弧附近横向磁场扰动场的两倍，从而相当于总剪应力。两个自由参数是在源等离子体和等离子体β中的突出部分处的压力跃变的相对大小。将电弧性质的关系的定量结果与观察值相匹配表明，压力跃变的数量级为10%，β值在1和5之间。
[1] An analytical model is presented for auroral arcs as the result of a fast release of magnetic shear stresses. The shear stresses are set up by a longitudinal convection that is driven by pressure forces in the outer magnetosphere against the frictional forces exerted in the lower ionosphere. A distorted-dipole geometry is employed allowing for high plasma beta near the equator. Steep ledges in the radial pressure distribution, extending along the direction of convection, are invoked as the sources of the auroral current sheets. The differential magnetic energy content of these narrow current sheets is released within a few Alfven transit times by the decoupling of the magnetospheric plasma and field from the ionosphere, owing to the existence of field-aligned potential drops in the auroral acceleration region, and converted into kinetic energy of the primary auroral particles. A well-known current-voltage relation is employed for the formulation of the energy conversion process. This scenario has two important consequences. (1) The loss of magnetic energy creates a concomitant decrease of internal energy of the generator plasma and results in a progression of pressure ledge and auroral current sheet into the more highly stressed magnetic field region. This is the reason for the observed proper motion of auroral arcs with respect to the plasma frame. (2) Plasma and field undergo a rapid stress relief motion along the arc with large but mostly reversible displacements. The net displacement, equivalent to a small S-shaped contribution to the essentially U-shaped potential distribution above the auroral arc, is consistent with the transit of the field lines through the progressing current sheet. This scenario is cast into a set of simple relations expressing the key parameters of auroral arcs, such as width, energy flux, potential drop, and proper motion. The main ingredient herein is an auxiliary magnetic perturbation field into which the main properties of the large-scale current system are condensed. It corresponds to about twice the transverse magnetic perturbation field near the arc and thus to the total shear stresses. Two free parameters are the relative magnitude of the pressure jump at the ledge in the source plasma and the plasma beta. Matching the quantitative results of the relations for the arc properties with observed values suggests pressure jumps of order 10% and beta values between 1 and 5.