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

Abstract

One of the main sources of magnetospheric particles is solar wind penetration through the magnetopause-a current sheet separating the cold, dense magnetosheath from the hot, rarified magnetospheric plasmas. The mechanism responsible for magnetosheath particle transport across the magnetopause has been better investigated for the near-Earth dayside magnetopause (contrary to the nightside), where it was found that such a transport is controlled by the current sheet thickness, structure, and dynamics. Because plasma properties and magnetic field intensity in the magnetosheath significantly change as a function of radial distance from the Earth, the current sheet characteristics are different near Earth and at distant nightside magnetopause. Comparative investigations between near-Earth and distant magnetopauses, however, require statistical observations at the two locations during the same time interval, that enable control for the same upstream solar wind driving conditions. In this paper, we perform such a study using the four Magnetosheric Multiscale (MMS) and two Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) probes to compare the characteristics of magnetic field and plasma populations during magnetopause crossings, which are separated by about 50 R E. We find that the current sheet profiles is similar at two locations. We also show that the magnetopause current sheet thickness scales with the local magnetosheath ion gyroradius. A weaker magnetic field on both sides of the current sheet is correlated with smaller current density at lunar distances.

Particle‐In‐Cell Simulations of Electrostatic Solitary Waves in Asymmetric Magnetic Reconnection

“…In this study, we use a model where the magnetic field and plasma density across the current sheet are asymmetric while the temperature is symmetric. However, statistical studies have shown that there is typically strong temperature asymmetry in the magnetopause current sheet (Lukin et al, 2020). Using particle-in-cell simulations, Sang et al (2019) indicated that the influence of temperature asymmetry on the in-plane current (dominated by electron flow) is much weaker than that of magnetic field and density asymmetry.…”

“…In this study, we use a model where the magnetic field and plasma density across the current sheet are asymmetric while the temperature is symmetric. However, statistical studies have shown that there is typically strong temperature asymmetry in the magnetopause current sheet (Lukin et al., 2020). Using particle‐in‐cell simulations, Sang et al.…”

Magnetic Field Gradient Across the Flank Magnetopause

“…A study of Artemyev et al (2017) showed that plasma and magnetic field characteristics are very similar for boundary layers observed at the lunar orbit ( ≈ 55 R E ) and farther downtail as far as ≈ 200 R E and that the dynamical magnetosheath pressure does not contribute to the pressure balance across the boundary layer at these distances. Furthermore, Lukin et al (2020) compared the characteristics of magnetic field and plasma populations during simultaneous magnetopause crossings, which are separated by about 50 R E (dayside vs night sides), and found that the magnetosheath current sheet profiles are similar at these two locations. Nevertheless, a flank magnetopause configuration and dynamics are critical for understanding the transport of magnetosheath plasma toward the magnetotail (Wing et al, 2014;Haaland et al, 2019).…”

On the Role of Kinetic Alfven Waves in the Magnetosheath Ion Thermalization Around the Night‐Side Magnetopause

“…where B 0 = 20 nT is the magnitude of ambient magnetic field, L = 2000 km is the magnetopause scale length, and x is along the magnetopause normal (x = 0 is the center position of the magnetopause current sheet, x < 0 and x > 0 correspond to the magnetosheath and magnetosphere, respectively). We further assume the plasma number density profile based on observations (e.g., Lukin et al, 2019Lukin et al, , 2020 as follows:…”

“…The above two examples demonstrate that the magnetic field profiles cannot be well described by the simplest Harris current sheet model, because of the significant magnetic field shear contributing to the pressure balance and making the current sheet almost force‐free. Indeed, several statistical works (Haaland et al., 2014, 2019, 2020; Lukin et al., 2019, 2020) have demonstrated that the magnetopause current sheet often has a force‐free configuration with | B | ∼ const across the sheet. Thus we use a simplified version of such force‐free current sheet model (see details and possible generalization of this model in, e.g., Allanson et al., 2015; Harrison & Neukirch, 2009b; Panov et al., 2011): $$${B}_{0x}(x)=0,\,{B}_{0y}(x)=\frac{{B}_{0}}{\mathrm{cosh}(x/L)},\,{B}_{0z}(x)={B}_{0}\,\mathrm{tanh}(x/L)$$$ where B 0 = 20 nT is the magnitude of ambient magnetic field, L = 2000 km is the magnetopause scale length, and x is along the magnetopause normal ( x = 0 is the center position of the magnetopause current sheet, x < 0 and x > 0 correspond to the magnetosheath and magnetosphere, respectively).…”