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Evidence of the Nonstationarity of the Terrestrial Bow Shock from Multi-Spacecraft Observations: Methodology, Results and Quantitative Comparison with PIC Simulations
Annales Geophysicae ( IF 1.9 ) Pub Date : 2020-09-25 , DOI: 10.5194/angeo-2020-54
Christian Mazelle , Bertrand Lembege

Abstract. The nonstationarity of the terrestrial bow shock is analyzed in detail from in situ magnetic field measurements issued from the FGM experiment on board of Cluster mission. Attention is focused on statistical analysis of quasiperpendicular supercritical shock crossings. The present analysis stresses for the first time the importance of a careful and accurate methodology in the data processing which can be a source of confusion/misunderstanding if not treated properly. The analysis performed using 96 shock front crossings shows evidence of a strong variability of the microstructures of the shock front (foot and ramp) which are analyzed in deep details. Main results are: (i) most statistics clearly evidence that the ramp thickness is very narrow and can be as low as a few c/ωpe (electron inertia length), (ii) the width is narrower when the angle θBn (between the shock normal and the upstream magnetic field) approaches 90°, (iii) the foot thickness strongly varies but its variation has an upper limit provided by theoretical estimates given in previous studies (e.g., Schwartz et al., 1983; Gosling and Thomsen, 1985; Gosling and Robson, 1985); (iv) the presence of foot and overshoot, as shown in all front profiles confirms the importance of dissipative effects. Present results indicate that these features can be signatures of the shock front self-reformation among a few mechanisms of nonstationarity identified from numerical simulation/theoretical works. A comparison 2D PIC simulation for a perpendicular supercritical shock (used as reference), has been performed and it shows that: (a) the ramp thickness varies only slightly in time over a large fraction of the reformation cycle and reaches a lower bound value of the order of a few electron inertial length, (ii) in contrast, the foot width strongly varies during a self-reformation cycle but always stays lower than an upper bound value in agreement with the value given by Woods (1971), and (iii) as a consequence, the time variability of the whole shock front is depending on both ramp and foot variations. Moreover, a detailed comparative analysis shows that much elements of analysis were missing in previous reported works concerning both (i) the important criteria used in the data selection and (ii) the different and careful steps of the methodology used in the data processing itself. This absence of these precise elements of analysis makes the comparison with present work difficult, worse, it makes some final results and conclusive statements quite questionable at present time. A least, looking for a precise estimate of the shock transition thickness presents nowadays a restricted interest, since recent results show that the terrestrial shock is rather nonstationary and one unique typical spatial scaling of the microstructures of the front (ramp, foot) must be replaced by some variation ranges (with lower bound/upper bound values) within which the spatial scales of the fine structures can extend.

中文翻译:

多航天器观测显示的地面弓激波非平稳性的证据:方法,结果以及与PIC模拟的定量比较

摘要。从机群任务的FGM实验发出的原位磁场测量结果中,详细分析了地面弓激波的非平稳性。注意力集中在准肾盂超临界休克穿越的统计分析上。本分析首次强调了谨慎而准确的方法在数据处理中的重要性,如果处理不当,可能会造成混淆/误解。使用96个冲击波前交叉点进行的分析表明,冲击波前孔的微结构(脚和坡道)的强烈变异性的证据得到了深入的详细分析。主要结果是:(i)大多数统计数据清楚地表明,坡道厚度非常窄,可以低至几c / ωPE(电子惯性长度),(ii)所述宽度更窄,当角度θ BN(在冲击法线和上游磁场之间)接近90°,(iii)脚的厚度变化很大,但其变化有一个上限,该上限由先前研究提供的理论估计值确定(例如Schwartz等,1983; Gosling和汤姆森,1985年;戈斯林和罗布森,1985年)。(iv)脚部和超调的存在(如所有前部轮廓所示)确认了耗散效应的重要性。目前的结果表明,这些特征可能是从数值模拟/理论工作确定的一些非平稳性机制中,冲击波前部自我变形的特征。对垂直超临界冲击(用作参考)进行了比较2D PIC仿真,结果表明:(a)在整个重整周期的大部分时间内,坡道厚度仅在时间上略有变化,并且达到了一些电子惯性长度的下限值,(ii)相反,在自重过程中,脚的宽度变化很大重新形成周期,但始终保持低于上限值,与伍兹(1971)给出的值一致,并且(iii)结果,整个冲击波前部的时间变化性取决于坡度和脚部变化。此外,详细的比较分析表明,以前的报告工作缺少许多分析要素,涉及到(i)数据选择中使用的重要标准以及(ii)数据处理本身中使用的方法的不同而谨慎的步骤。由于缺乏这些精确的分析要素,因此很难与当前的工作进行比较,更糟的是,这使得某些最终结果和结论性陈述在目前非常可疑。至少,对于冲击过渡厚度的精确估计如今受到关注,因为最近的结果表明,地面冲击相当不稳定,必须替换前部(斜坡,脚)的微观结构的一种独特的典型空间比例由一些精细结构的空间尺度可以在其中变化的变化范围(具有下限/上限)。
更新日期:2020-09-25
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