An Extended MHD Study of the 16 October 2015 MMS Diffusion Region Crossing

Abstract

The Magnetospheric Multiscale (MMS) mission has given us unprecedented access to high cadence particle and field data of magnetic reconnection at Earth's magnetopause. MMS first passed very near an X‐line on 16 October 2015, the Burch event, and has since observed multiple X‐line crossings. Subsequent 3‐D particle‐in‐cell (PIC) modeling efforts of and comparison with the Burch event have revealed a host of novel physical insights concerning magnetic reconnection, turbulence‐induced particle mixing, and secondary instabilities. In this study, we employ the Gkeyll simulation framework to study the Burch event with different classes of extended, multifluid magnetohydrodynamics (MHD), including models that incorporate important kinetic effects, such as the electron pressure tensor, with physics‐based closure relations designed to capture linear Landau damping. Such fluid modeling approaches are able to capture different levels of kinetic physics in global simulations and are generally less costly than fully kinetic PIC. We focus on the additional physics one can capture with increasing levels of fluid closure refinement via comparison with MMS data and existing PIC simulations. In particular, we find that the ten‐moment model well captures the agyrotropic structure of the pressure tensor in the vicinity of the X‐line and the magnitude of anisotropic electron heating observed in MMS and PIC simulations. However, the ten‐moment model is found to have difficulty resolving the lower hybrid drift instability, which plays a fundamental role in heating and mixing electrons in the current layer.

An improved ten-moment closure for reconnection and instabilities

“…[32][33][34] In the magnetosphere, the LHDI may drive turbulence during magnetopause reconnection, 35,36 and can quicken the onset of reconnection in regimes relevant to magnetotail reconnection. 37,38 Prior studies of the LHDI using the ten-moment model show that it is important to retain the ion kinetic effects using the nonlocal closure, 19,20 and we find the heat flux closure in this work is able to reproduce the growth of the LHDI in a current sheet.…”

“…We have also been able to simulate the lower-hybrid drift instability in a current sheet with the ten-moment model, which was previously only possible when using a nonlocal closure or tuning the model parameters carefully. 19 To date, the only large-scale simulations performed using the ten-moment model have used local relaxation of the pressure tensor, 20,22,23 with some success in comparison to observations. The closure used in this paper, while having larger computational costs, shows better agreement with kinetic simulations, particularly for the reconnection and instability physics being investigated.…”

“…These results show that it is possible to simulate both reconnection and the LHDI using the same parameters (k s % 1=d s ), which was not possible with the local relaxation of the pressure tensor. 19,20 The role of the LHDI during the reconnection process is beyond the scope of this paper and will be the subject of future work. In kinetic simulations, it has been shown that the LHDI can increase the tearing mode growth rate prior to reconnection, 37,59 or lead to particle mixing 36 and cause apparent anomalous resistivity and viscosity.…”

“…12,13 In this paper, we consider the multi-fluid moment model, in which moments of the kinetic equation up to the pressure tensor are evolved. This has been used in local studies of reconnection and instabilities [14][15][16][17][18][19][20][21] as well as global simulations of Ganymede, 22 Mercury, 23 and Uranus. 24 While these studies have had some success in terms of both theoretical results and observational comparisons, the issue of closure (in this case, the form of the heat flux) still remains, both in terms of physical modeling and computational cost.…”

Fluid simulations of three-dimensional reconnection that capture the lower-hybrid drift instability

“…Recent studies showed agreement with MMS data for kinetic particle-in-cell (PIC) simulations (e.g. Nakamura et al 2018a) and for 10-moment fluid simulations (TenBarge et al 2019). Both models retain the anisotropic pressure tensor which is one mechanism to break the frozen-in condition in collisionless reconnection (Egedal et al 2019).…”