Global View of Current Sheet Thinning: Plasma Pressure Gradients and Large‐Scale Currents

Abstract

The formation of a thin magnetotail current sheet is a key process in magnetic energy accumulation before magnetic reconnection during substorms. Although the 3‐D configuration of a thinning current sheet has been well reproduced in many numerical simulations, observational details of this configuration have been less thoroughly studied. To investigate the global and local 3‐D structure of a thinning current sheet, we use a data set collected by three spacecraft of the Time History of Events and Macroscale Interactions during Substorms mission, two spacecraft of the Acceleration, Reconnection, Turbulence and Electrodynamics of Moon's Interaction with the Sun mission, and the Geostationary Operational Environmental Satellite 15. We demonstrate that near‐Earth current sheet thinning is accompanied by the formation of equatorial plasma pressure gradients directed from the flanks toward midnight. These gradients are associated with an intensification of the dawn‐dusk current (during current sheet thinning) and field‐aligned currents of opposite polarity at the dawn and dusk flanks. Simultaneous Time History of Events and Macroscale Interactions during Substorms and Acceleration, Reconnection, Turbulence and Electrodynamics of Moon's Interaction with the Sun observations show that current sheet thinning occupies a large portion of the tail, from the near‐Earth region to lunar orbit. An increase in the equatorial plasma pressure (and the lobe magnetic field pressure) is provided by a cold plasma density increase in the near‐Earth tail and likely by a plasma temperature increase in the distant tail. We discuss plasma convection patterns that are consistent with observed properties of the current sheet thinning.

Thermodynamics of the Magnetotail Current Sheet Thinning

“…Indeed, plasma sheet lobe observations show such enhanced outflows for the near‐Earth CS thinning (Artemyev et al., 2020; Kistler et al., 2005). Moreover, the analysis of the ion(Artemyev, Angelopoulos, Runov, & Petrukovich, 2019) and electron (Artemyev, Angelopoulos, Liu, & Runov, 2017) energy spectrum evolution in the thinning CS shows that temperature decrease is generally associated with the decrease of high‐energy fluxes and growth of low‐energy (subthermal) population.…”

“…Therefore, there is plasma isotropization in thinning CS, and the pre‐reconnection CS is rather isotropic. But even being isotropic in average, CSs can contain significantly anisotropic plasma populations (Artemyev, Angelopoulos, Vasko, et al., 2019), that is pre‐reconnection CS configuration still can be quite different from the purely isotropic model.…”

“…The main observational problem of tracing the plasma characteristics' evolution in the thinning CSs is that observations combine effects of temporal evolution and spacecraft effect motion (due to | B x | increase) away from the equatorial plane in the highly inhomogeneous plasma sheet. This problem does not allow ambiguous interpretation of observed trends for the near‐Earth CS thinning probed by THEMIS spacecraft (see discussion in Artemyev, Angelopoulos, Runov, & Petrukovich, 2019). For example, the observed temperature decrease may be explained as a result of spacecraft motion to plasma sheet boundary (to higher | B x |) that is conjugated to the more distant tail with colder plasma (Artemyev, Angelopoulos, Hietala, et al., 2017; Lu et al., 2017).…”

“…The last panel of Figure 3 shows the evolution of ion and electron anisotropy during the CS thinning in the middle tail (Cluster observations). This event is characterized by the initially quite strong anisotropy of both ions ( T ion,∥ / T ion,⊥ ∼ 0.8) and electrons ( T elc,∥ / T elc,⊥ ∼ 1.2) (note a finite electron anisotropy is usually observed in the magnetotail (Artemyev et al., 2014; Walsh et al., 2011; Yushkov et al., 2017), whereas an ion anisotropy is usually very small (Artemyev, Angelopoulos, Vasko, et al., 2019; Stiles et al., 1978; Walsh et al., 2011). The ion isotropization during the CS thinning may be explained by the ion scattering around the equatorial plane (Büchner & Zelenyi, 1989; Zelenyi et al., 2013).…”

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

“…The global nature of CS thinning and stretching prior to the substorm onset is also seen from a superposed epoch analysis of substorms (Miyashita et al, 2009, Figures 2 and 7) as a decrease of the northward magnetic field B z and increase of the total pressure in the premidnight sector (2R E < y < 12R E and −20R E < x < −5R E ). An even more audacious hypothesis that TCSs may occupy the entire tail from near-Earth region to lunar orbit has recently been proposed by Artemyev et al (2019) based on GOES-15, THEMIS, and ARTEMIS observations. At the same time, analysis of self-consistent tail current sheet equilibria suggests that in an isotropic CS with realistic values of the normal magnetic field B z ≥ 0.03B 0 , the current sheet thickness L z (x) at distances L x ∼ 10 2 L, where L is the CS thickness near the Earth, becomes too large L z ∼ L exp(L x ∕L)(B z ∕B 0 ) > 20L for it to maintain a kinetic scale.…”