Observing the prevalence of thin current sheets downstream of Earth's bow shock

Abstract

Actively reconnecting, thin current sheets have been observed both within the transition region of Earth's bow shock and far downstream into the magnetosheath. Irrespective of whether these structures arise due to shock processes or turbulent dissipation, they are expected to contribute to particle heating and acceleration within their respective regions. In order to assess the prevalence of thin current sheets in the magnetosheath, we examine shock crossings and extended magnetosheath intervals recorded by the magnetospheric multiscale mission (MMS). For each magnetosheath interval, we quantify the prevalence of current sheets in that region of space using: a one-dimensional measure of structures per unit length of observed plasma, a packing factor corresponding to the fraction of time the spacecraft are within current structures, and a three-dimensional measure requiring an estimate of the number of current sheets within an associated volume. We estimate that volume by considering the three-dimensional cone over which Alfvén and magnetoacoustic waves can propagate during each interval. Using 25 extended magnetosheath intervals observed by MMS, we perform our analysis for different locations in the magnetosheath and for different solar wind conditions. We find that the number density of current sheets is higher toward the magnetosheath flanks, that it reduces as a power law with distance from the bow shock, and that it is not strongly influenced by the properties of the upstream bow shock.

Strong reconnection electric fields in shock-driven turbulence

“…To see this point, let us see the inflow speed v in1 in Eqs. (3), (5), and (6). From these equations, we have the following relations:…”

“…Later, MMS observed electron-only reconnection in the shock transition region, [2][3][4][5] the magnetosheath, 6,7 and the foreshock region 8,9 of the Earth's bow shock. In addition, possible signatures of electron-only reconnection were found in the magnetic spectrum in turbulence in the magnetosheath.…”

Electron‐Scale Reconnection in Three‐Dimensional Shock Turbulence

“…This differs from the standard picture of collisionless reconnection (Ishizawa et al., 2004; Malyshkin, 2008; Yamada, 2011), where the reconnection region consists of an electron‐scale layer within a wider ion‐scale diffusion region. These electron‐only reconnection regions have been observed in the Earth's magnetosheath and shock transition region (Gingell et al., 2019, 2020, 2021; Phan et al., 2018; Stawarz et al., 2022; Wang et al., 2019), foreshock (Liu et al., 2020; Wang et al., 2020), magnetotail (S. Lu et al., 2020) and laboratory experiments (Shi et al., 2022).…”

“…A recent development in the study of reconnection has been the observation of purely electron‐scale reconnection regions, in which ions do not participate in the reconnection process (Gingell et al., 2019, 2020, 2021; Phan et al., 2018; Stawarz et al., 2022; Wang et al., 2019). This differs from the standard picture of collisionless reconnection (Ishizawa et al., 2004; Malyshkin, 2008; Yamada, 2011), where the reconnection region consists of an electron‐scale layer within a wider ion‐scale diffusion region.…”

“…In this work we focus on reconnection in the turbulent environment downstream and in the transition region of quasi‐parallel shocks. Numerous electron‐scale current sheets are observed in these regions, leading to a favorable environment for electron‐scale reconnection (Gingell et al., 2017, 2019, 2020, 2021; Phan et al., 2018; Sharma Pyakurel et al., 2019; Stawarz et al., 2022; Wang et al., 2019). While existing kinetic simulation studies in the shock environment have shown reconnection at both electron and ion scales, the simulations have been two‐dimensional (Bessho et al., 2019; Bessho et al., 2020, 2022; Q. Lu et al., 2021), restricting reconnection to a single plane.…”