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On the estimation of the current density in space plasmas: Multi- versus single-point techniques

机译：关于空间等离子体中电流密度的估计：多点技术与单点技术

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Thanks to multi-spacecraft mission, it has recently been possible to directly estimate the current density in space plasmas, by using magnetic field time series from four satellites flying in a quasi perfect tetrahedron configuration. The technique developed, commonly called "curlometer" permits a good estimation of the current density when the magnetic field time series vary linearly in space. This approximation is generally valid for small spacecraft separation. The recent space missions Cluster and Magnetospheric Multiscale (MMS) have provided high resolution measurements with inter-spacecraft separation up to 100 km and 10 km, respectively. The former scale corresponds to the proton gyroradius/ion skin depth in "typical" solar wind conditions, while the latter to sub-proton scale. However, some works have highlighted an underestimation of the current density via the curlometer technique with respect to the current computed directly from the velocity distribution functions, measured at sub-proton scales resolution with MMS. In this paper we explore the limit of the curlometer technique studying synthetic data sets associated to a cluster of four artificial satellites allowed to fly in a static turbulent field, spanning a wide range of relative separation. This study tries to address the relative importance of measuring plasma moments at very high resolution from a single spacecraft with respect to the multi-spacecraft missions in the current density evaluation.
机译：由于具有多飞船的任务，最近有可能通过使用以准完美四面体配置飞行的四颗卫星的磁场时间序列来直接估算空间等离子体中的电流密度。当磁场时间序列在空间中线性变化时，开发的技术通常称为“旋度计”，可以很好地估计电流密度。该近似值通常适用于小型航天器的分离。最近的航天任务“星团”和“磁层多尺度”（MMS）提供了高分辨率的测量结果，航天器之间的间隔分别达到100 km和10 km。前者比例对应于“典型”太阳风条件下的质子回旋半径/离子趋肤深度，而后者对应于亚质子比例。但是，一些工作强调了通过卷尺技术相对于直接从速度分布函数计算的电流的电流密度的低估，该速度是通过MMS在亚质子尺度分辨率下测量的。在本文中，我们探索了卷尺技术的局限性，研究与合成数据集相关的合成数据集，该合成数据集与允许在静态湍流场中飞行的四颗人造卫星组成，并跨越了较大的相对距离。这项研究试图解决在当前密度评​​估中相对于多航天器任务而言，从单个航天器以高分辨率测量等离子体矩的相对重要性。

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