Ion thermalization in quasi-perpendicular shocks involving reflected ions
Ion thermalization in quasi-perpendicular shocks involving reflected ions
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DOI：
10.1029/ja095ia05p06337
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发表时间：
1990-05
影响因子：
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通讯作者：
N. Sckopke;G. Paschmann;A. Brinca;C. Carlson;H. Lühr
中科院分区：
文献类型：
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作者：
N. Sckopke;G. Paschmann;A. Brinca;C. Carlson;H. Lühr
通过来自 AMPTE/IRM 航天器的等离子体和磁场数据来检查准垂直地球弓形激波下游的离子热化机制，其中包括三维离子分布、每 4.3 秒导出的等离子体流体参数以及高达 16 Hz 的横向和平行磁波动谱。详细研究的对象是低马赫数、低β激波，其中存在反射旋转离子并有助于下游离子温度，但超出斜坡的过程发生缓慢，因此基本现象变得明显。从 MHD 的角度来看，大多数冲击都具有轻微的严重性。在斜坡的下游，最初分离的核心离子和环离子慢慢合并成一个联合的、各向异性较小的分布，具有高能尾部。离子温度比 T⊥/T∥ 不仅在激波脚和斜坡处很高，而且在下游一定距离内也很高；随着β的增加，其下降速度加快，残留水平减少。离子的能量和螺距角扩散速度大致相同。当激波稍微倾斜时，会出现相对于场方向的分布不对称。它仅缓慢衰减，这可能表明零桨距角附近的桨距角扩散速率降低。低频电磁波存在于质子陀螺频率以下；它们的特点是强烈的左手极化发射和低水平的平行波动，除非非常接近激波。左侧发射通常集中在较窄的频带内，但有时会呈现双峰结构。波和离子分布在激波下游一定距离处接近缓慢变化的平衡。将分析扩展到代表大多数弓形激波遭遇的一种超临界激波后，我们得出结论，我们的推论更普遍有效，尽管热化通常更快，并且似乎涉及非线性过程，这往往会掩盖大多数所指出的特征。
Ion thermalization mechanisms downstream of the quasi-perpendicular Earth's bow shock are examined by means of plasma and magnetic field data from the AMPTE/IRM spacecraft which include three-dimensional ion distributions, plasma fluid parameters derived every ∼4.3 s, and spectra of transverse and parallel magnetic fluctuations up to 16 Hz. The objects studied in detail are low-Mach number, low-β shocks in which reflected-gyrating ions are present and contribute to the downstream ion temperature but where processes beyond the ramp take place slowly, so that the basic phenomenology becomes apparent. In MHD terms, most of these shocks qualify as marginally critical. Downstream of the ramp, the initially separated core and ring ions slowly merge into a joint, less anisotropic distribution possessing a high-energy tail. The ion temperature ratio, T⊥/T∥, is high not only in the shock foot and ramp but also within some distance downstream; its speed of decline rises and the residual level lessens with increasing β. The ions diffuse about equally fast in energy and in pitch angle. An asymmetry of the distributions with respect to the field direction is present when the shock is slightly oblique. It decays only slowly, which might indicate that the pitch angle diffusion rate near zero pitch angle is reduced. Low-frequency electromagnetic waves are present below the proton gyrofrequency; they are characterized by strong left-hand-polarized emissions and a low level of parallel fluctuations except very close to the shock. The left-hand emissions are often concentrated into a narrow frequency band but sometimes they exhibit a double-humped structure. Waves and ion distributions approach a slowly varying equilibrium some distance downstream of the shock. After extending the analysis to one supercritical shock representing the majority of bow shock encounters, we conclude that our deductions are more generally valid, although the thermalization is faster, usually, and appears to involve nonlinear processes which tend to obscure most of the features noted.