Global Empirical Picture of Magnetospheric Substorms Inferred From Multimission Magnetometer Data

Abstract

Magnetospheric substorms represent key explosive processes in the interaction of the Earth's magnetosphere with the solar wind, and their understanding and modeling are critical for space weather forecasting. During substorms, the magnetic field on the nightside is first stretched in the antisunward direction and then it rapidly contracts earthward bringing hot plasmas from the distant space regions into the inner magnetosphere, where they contribute to geomagnetic storms and Joule dissipation in the polar ionosphere, causing impressive splashes of aurora. Here we show for the first time that mining millions of spaceborne magnetometer data records from multiple missions allows one to reconstruct the global 3‐D picture of these stretching and dipolarization processes. Stretching results in the formation of a thin (less than the Earth's radius) and strong current sheet, which is diverted into the ionosphere during dipolarization. In the meantime, the dipolarization signal propagates further into the inner magnetosphere resulting in the accumulation of a longer lived current there, giving rise to a protogeomagnetic storm. The global 3‐D structure of the corresponding substorm currents including the substorm current wedge is reconstructed from data.

Signatures of Nonideal Plasma Evolution During Substorms Obtained by Mining Multimission Magnetometer Data

“…They have found that the B z gradient is directed predominantly earthward, although in the growth phase the B z field may become extremely weak (≤2 nT). That result is also consistent with (Stephens et al, ) and the present study, where the TGRs are found beyond ≈11 R E , with the only exception of a very strong driving case considered in section .…”

“…The comparison of Figure with Figures 6–8 in Stephens et al (), describing typical substorm evolution cases (see also other substorm examples below), shows that the 11 July 2017 substorm was rather unusual, even in contrast to conventional substorms of the similar strength in terms of the AL minimum (Figure 8 in Stephens et al, ). In particular, it did not provide any substantial dipolarization of the B z field in the expansion phase (Figures g and i) or any substantial variations of the field‐aligned currents (Figures c and d), thin and thick CS amplitudes (Figure f).…”

“…Sitnov et al () proposed to use modules similar to the original TS07D FAC basis functions shifted in latitude to fill the possible gap between R1 and R2 distributions. In this study, following Sitnov et al () and Stephens et al (), we describe the FAC system using 16 basis functions with the first two Fourier harmonics ( l =1,2) for R1 and R2, as well as their latitude‐shifted clones.…”

“…Hence it must be lower compared to the ideal/initial regimes of tearing (Merkin & Sitnov, ; Sitnov et al, ). At the same time, the formation of TGRs in the late growth phase is suggested by statistical visualization of substorms (Machida et al, ; Wang et al, ), remote‐sensing studies (Sergeev et al, ), and by first results of a substorm reconstruction with a data‐mining (DM) approach (Stephens et al, ).…”

“…X‐lines, TCSs, and TGRs do not exhaust the range of non‐MHD effects in the magnetosphere of which the most well known is the storm‐time ring current provided by the energy dependent drifts of hot plasma populations in the inner magnetosphere (Wolf, ). While these effects can also be captured by the method described below, many of them were already discussed in earlier investigations of storms (Sitnov et al, , ; Stephens et al, ) and storm‐scale effects of substorms (Stephens et al, ). Thus, the present study largely complements those earlier investigations with the focus on substorm effects in the magnetotail.…”