Spatial Scales and Plasma Properties of the Distant Magnetopause: Evidence for Selective Ion and Electron Transport

Abstract

One of the most important sources of magnetospheric plasma is particle entry through the distant magnetotail boundary, the nightside magnetopause. This entry mechanism depends on the magnetopause configuration. Off the equator, the strong lobe magnetic field renders the magnetopause a tangential or a rotational discontinuity, and thus the magnetosheath field orientation predominantly controls particle entry through magnetic reconnection. At the equatorial, distant tail magnetopause, however, the magnetic field's control of particle entry is diminished because the plasma beta there is large on both sides of the boundary. Thus, transport there can be significantly different from that at the dayside and off‐equatorial magnetopauses. Using observations from two Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun probes, we investigate plasma transport mechanisms around the distant equatorial magnetopause. We find that transport occurs as a series of abrupt transitions in density, ion and electron temperatures, and ion kinetic energy of spatial scales as small as a typical plasma sheet ion gyroradius. Analysis of the particle phase space density reveals that an energy‐selection mechanism controls electron transport across the magnetopause, whereas ion transport is likely controlled by spatial diffusion driven by low‐frequency magnetic field fluctuations. We discuss the importance of these fluctuations for the magnetopause structure (e.g., the thickness of the transitions in plasma density, ion and electron temperatures, and ion kinetic energy).

Comparison of the Flank Magnetopause at Near‐Earth and Lunar Distances: MMS and ARTEMIS Observations

“…The thermal plasma pressure is often comparable or even exceeds the magnetic pressure on both sides of the near‐Earth (Phan & Paschmann, 1996) and distant (Artemyev et al, 2017; Hasegawa et al, 2000;Lukin et al, 2019) equatorial magnetopauses (but often this is not the case for high‐latitude magnetopause; see Haaland et al, 2014; Panov et al, 2008). The magnetosheath ram pressure is considerably more important at the subsolar magnetopause than at the flanks due to different angle of the magnetosheath plasma flows with respect to the magnetopause surface.…”

“…Most of magnetopause crossings in our data set are characterized by the magnetosheath plasma flow parallel to the magnetopause surface, and the plasma flow along the normal to the surface almost coincides with the magnetopause velocity (see Figure S1). Thus, there is almost no ram pressure contribution to the pressure balance (see also Artemyev et al, 2017; Hasegawa et al, 2000; Lukin et al, 2019).…”

“…The magnetosheath ram pressure is considerably more important at the subsolar magnetopause than at the flanks due to different angle of the magnetosheath plasma flows with respect to the magnetopause surface. It is the thermal plasma pressure that plays the dominant role in shaping the magnetopause configuration on the nightside (Artemyev et al, 2017; Dimmock & Nykyri, 2013; Hasegawa et al, 2000; Lukin et al, 2019).…”

“…For MMS crossings the pressure balance is clearer, than for ARTEMIS crossings, where the magnetosheath pressure (magnetic plus plasma) is generally greater than the magnetospheric pressure. This is likely due to strong Alfvenic fluctuations of the magnetic field (Lukin et al, 2019) on the magnetosheath side in a weak background magnetic field at the lunar orbit: Such strong fluctuations complicate the accurate evaluation of the magnetic pressure, which should be contributed only by two magnetic field components tangential to the magnetopause surface (hence the total field is an overestimate).…”

“…To perform such an investigation, we examine observations of the four MMS (Burch et al, 2016) and two ARTEMIS (Angelopoulos, 2011) spacecraft. Although there are already several data sets of the magnetopause current sheet characteristics separately using MMS (e.g., Haaland et al, 2020; Paschmann et al, 2018) and ARTEMIS (e.g., Lukin et al, 2019) measurements, there is no comparison of magnetopause observations at different radial distances performed within same interval including many crossings.…”