The Occurrence and Prevalence of Time Domain Structures in the Kelvin-Helmholtz Instability at Different Positions Along the Earth’s Magnetospheric Flanks

F D Wilder,D M Malaspina,R E Ergun,B L Giles,P Hansel,R J Strangeway,J L Burch,D Gove,S Eriksson,N Ahmadi,R B Torbert

doi:10.3389/fspas.2021.756563

Abstract

The Kelvin-Helmholtz instability (KHI) is thought to be an important driver for mass, momentum, and energy transfer between the solar wind and magnetosphere. This can occur through global-scale “viscous-like” interactions, as well as through local kinetic processes such as magnetic reconnection and turbulence. An important aspect of these kinetic processes for the dynamics of particles is the electric field parallel to the background magnetic field. Parallel electric field structures that can occur in the KHI include the reconnection electric field of high guide field reconnection, large amplitude ion acoustic waves, as well as time domain structures (TDS) such as double layers and electrostatic solitary waves. In this study, we present a survey of parallel electric field structures observed during three Kelvin Helmholtz events observed by NASA’s Magnetospheric Multiscale (MMS), each at different positions along the magnetosphere’s dusk flank. Using data from MMS’s on-board solitary wave detector (SWD) algorithm, we statistically investigate the occurrence of TDS within the KHI events. We find that early in the KHI development, TDS typically occur in regions with strong field-aligned currents (FACs) on the magnetospheric side of the vortices. Further down the flanks, as the vortices become more rolled up, the prevalence of large electric currents decreases, as well as the prevalence of SWDs. These results suggest that as the instability develops and vortices grow in size along the flanks, kinetic-scale activity becomes less prevalent.

Highlights

In addition to magnetic reconnection at the dayside magnetopause, viscous-like interactions at magnetospheric flanks are thought to transfer energy and momentum from the solar wind to the magnetosphere (Axford and Hines, 1961)
Electron phase space holes and double layers have been observed in the bursty bulk flow braking region, where magnetic field spectra consistent with a turbulent cascade were observed (Ergun et al, 2014; Stawarz et al, 2015) The present study shows that time domain structures (TDS) tend to occur on the magnetospheric side of the vortices near strong fluctuations in the magnetic field and field-aligned currents, which suggests that the processes that lead to their occurrence are likely to happen there
Future studies should investigate what these processes and instabilities that lead to the presence of TDS are, and what role they play in the overall mass/ energy transfer between the solar wind and magnetosphere, as well as the dissipation of turbulence on the flanks

Summary

INTRODUCTION

In addition to magnetic reconnection at the dayside magnetopause, viscous-like interactions at magnetospheric flanks are thought to transfer energy and momentum from the solar wind to the magnetosphere (Axford and Hines, 1961). A similar picture is shown in Figure 4C for Ni, with the mean being 9 cm−3 for high SWD count intervals, and 12.8 cm−3 for the overall event All of this suggests that while TDS are seen throughout the KHI, there is a bias towards the portion of each period where magnetosphere-like plasma is present. The data from the down-tail event, shown, shows an even more nonlinear relation, with a significant portion of the lowest density plasma moving at the same anti-sunward speed as the highdensity population This suggests an increasing state of roll-up for the instability as it is observed at different positions down the flanks, with the post-noon event not having rolled up vortices, and the down-tail exhibiting the most non-linear “rollup” (Hasegawa et al, 2004).

SUMMARY AND CONCLUSION

DATA AVAILABILITY STATEMENT