Magnetospheric Multiscale (MMS) Observations of Magnetic Reconnection in Foreshock Transients

Abstract

Magnetic reconnection is a fundamental process of energy conversion in plasmas between electromagnetic fields and particles. Magnetic reconnection has been observed directly in a variety of plasmas in the solar wind and Earth's magnetosphere. Most recently, electron magnetic reconnection without ion coupling was observed for the first time in the turbulent magnetosheath and within the transition region of Earth's bow shock. In the ion foreshock upstream of Earth's bow shock, there may also be magnetic reconnection especially around foreshock transients that are very turbulent and dynamic. With observations from the National Aeronautics and Space Administration's Magnetospheric Multiscale mission inside foreshock transients, we report two events of magnetic reconnection with and without a strong guide field, respectively. In both events, a super-ion-Alfvénic electron jet was observed within a current sheet with thickness less than or comparable to one ion inertial length. In both events, energy was converted from the magnetic field to electrons, manifested as an increase in electron temperature. Weak or no ion coupling was observed in either event. Results from particle-in-cell simulations of magnetic reconnection with and without a strong guide field are qualitatively consistent with observations. Our results imply that magnetic reconnection is another electron acceleration/heating process inside foreshock transients and could play an important role in shock dynamics.

Direct Multipoint Observations Capturing the Reformation of a Supercritical Fast Magnetosonic Shock

“…The compressed waves and current-sheet energy transfer at electron scales culminate in the formation of a new shock ramp, with correlated |B| and density, out of the preexisting "foot"-like structure upstream of the most intense, thin current layer. Once formed, the new shock ramp and "foot" region continue converting energy of the incident ion and electron populations via whistler-mode precursor and electrostatic fluctuations within a few d i upstream of the shock ramp, dissipative wave-mode-coupling downstream of the ramp, and along thin current layers that may also be reconnecting (e.g., Gingell et al 2019;Liu et al 2020). As we know from many observations of foreshock transient shocks, the extent of the shocked plasma then must expand rapidly back up to ion-kinetic and ultimately MHD scales.…”

“…There is still much debate over the principal physical mechanisms responsible for the bulk deceleration and heating of plasma across the shock (e.g., Wilson et al 2014a). Recent results from simulations and observations at Earthʼs bow shock have highlighted the importance of energy dissipation and heating via ion-kinetic coupling between the incident plasma and reflected ion populations (Caprioli & Spitkovsky 2014a;Caprioli & Spitkovsky 2014b;Goodrich et al 2019) and via electron-kinetic-scale physics such as energy dissipation in large-amplitude, electron-scale electrostatic waves (Wilson et al 2014b;Goodrich et al 2018), whistler-mode turbulence (Hull et al 2020), and reconnection along thin, intense, electron-scale current sheets (Gingell et al 2019;Liu et al 2020). Upstream of quasi-parallel supercritical shocks, largescale transient structures can form in the ion foreshock due to reflected ions' kinetic interactions with the turbulent and discontinuous incident plasma (e.g., Omidi et al 2010;Turner et al 2018;Schwartz et al 2018;Haggerty & Caprioli 2020).…”

ARTEMIS Observations of Foreshock Transients in the Midtail Foreshock

“…Foreshock transients can not only disturb the bow shock but also accelerate/heat particles (e.g., Liu et al, 2017; Wilson et al, 2016), for example, through Fermi acceleration (Liu et al, 2017; Liu et al, 2018; Turner et al, 2018), shock drift acceleration (Liu et al, 2016), betatron acceleration (Liu et al, 2019), and magnetic reconnection (Liu et al, 2020). Inside the HFA observed at the midtail bow shock, we also see significant electron heating and ion heating which were even stronger than the magnetosheath heating (Figures 1b‐D and 1b‐E).…”

Ion-scale Current Structures in Short Large-amplitude Magnetic Structures

“…The forms of the current structures and their relative importance in the overall shock energy conversion are critical open questions. Recent simulations (Karimabadi et al 2014;Gingell et al 2017;Bessho et al 2019) and observations (Gingell et al 2019a(Gingell et al , 2019bWang et al 2019;Liu et al 2020) showed that some of the current sheets in the shock transition region can be reconnecting. Observations also showed that below 10Hz magnetosonic whistler waves generate a significant fraction of the total current densities (Wilson et al 2014a(Wilson et al , 2014b.…”