In Which Magnetotail Hemisphere is a Satellite? Problems Using in Situ Magnetic Field Data

A. De Spiegeleer,H. Gunell,M. Hamrin,S. Chong,T. Pitkänen

doi:10.1029/2020ja028923

A. De Spiegeleer, H. Gunell + Show 3 more

Open Access

https://doi.org/10.1029/2020ja028923

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Abstract

AbstractIn Earth's magnetotail plasma sheet, the sunward‐tailward Bx component of the magnetic field is often used to separate the region above and below the cross‐tail current sheet. Using a three‐dimensional magneto‐hydrodynamic simulation, we show that high‐speed flows do not only affect the north‐south magnetic field component (causing dipolarization fronts), but also the sunward‐tailward component via the formation of a magnetic dent. This dent is such that, in the Northern Hemisphere, the magnetic field is tailward while in the Southern Hemisphere, it is earthward. This is opposite to the expected signatures where Bx > 0 (Bx < 0) above (below) the neutral sheet. Therefore, the direction of the magnetic field cannot always be used to identify in which hemisphere an in situ spacecraft is located. In addition, the cross‐tail currents associated with the dent is different from the currents in a tail without a dent. From the simulation, we suggest that the observation of a dawnward current and a tailward magnetic tension force, possibly together with an increase in the plasma beta, may indicate the presence of a magnetic dent. To exemplify, we also present data of a high‐speed flow observed by the Cluster mission, and we show that the changing sign of Bx is likely due to such a dent, and not to the spacecraft moving across the neutral sheet.

Highlights

Since the dawn of observational space physics, particle and magnetic field data have been used to distinguish between different plasma regions
We investigate the signatures of a magnetic dent associated with a dipolarization fronts (DFs) at a high-speed earthward flow off the equatorial plane
We study the transverse signatures of the DF and its associated high-speed flow at y = −2.875RE by showing the velocity along x, Vx (Figure 1b, color plot) and some magnetic field lines uniformly separated in x at z = 0 (Figure 1b, black lines)

Summary

Introduction

Since the dawn of observational space physics, particle and magnetic field data have been used to distinguish between different plasma regions. The sign of the dawn-dusk component, By, can in the simplified textbook picture give information in which local time sector (i.e., toward dusk or dawn) a spacecraft is located It is well-known that high-speed flows in the magnetotail (Baumjohann et al, 1989) deform the magnetic field lines, for example causing dipolarization fronts (DFs; e.g., Angelopoulos et al, 1992; Nakamura et al, 2002; Ohtani et al, 2004). We present data from the multi-spacecraft Cluster mission of a high-speed flow showing evidence of a magnetic dent We use both a three-dimensional MHD simulation and in situ spacecraft observations for this study. We use the Cluster Ion Spectrometry - COmposition and DIstribution Function analyser (CIS-CODIF) instrument for the proton H+ data which have a 4 s resolution (Rème et al, 2001) and the FluxGate Magnetometer instrument for the magnetic field data with a resolution of five vectors per second (Balogh et al, 2001)

Overview

Artificial Spacecraft Crossings

Cluster Observations

Summary and Discussion

Data Availability Statement