Magnetospheric boundary perturbations on MHD and kinetic scales

Abstract

To study the magnetopause on both MHD and kinetic scales, we have analyzed two Time History of Events and Macroscale Interactions during Substorms/Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon' s Interaction with the Sun magnetopause crossings under steady slow-solar wind and minimum magnetic shear conditions. These events approximate a ground state of the magnetospheric boundary with minimum influences from large-solar wind disturbances and magnetic reconnection. Our observations reveal evidence for the Kelvin-Helmholtz instability, the quasi-periodicity of magnetopause surface waves accompanied by highly asymmetrical plasma signatures between the inbound (from magnetosheath to low-latitude boundary layer (LLBL)) and the outbound (from LLBL to magnetosheath) magnetopause crossings. Stronger plasma and magnetic gradients were observed during the outbound crossings but more gradual and volatile variations at higher frequencies during the inbounds. The scale lengths of the magnetic and plasma gradients were comparable or less than the proton gyroradius. Enhancements of lower hybrid waves occurred at the locations of strong gradients or variations. We interpreted the collocations of the lower hybrid waves and plasma gradients and their variations in terms of (1) lower hybrid instabilities that directly convert solar wind flow energy into lower hybrid waves and other wave modes in the LLBL, or (2) Kelvin-Helmholtz instability and magnetic reconnection which produce the conditions for the lower hybrid instabilities to grow. The rate of ion diffusion across the magnetopause caused by the lower hybrid instability is marginally sufficient to populate the LLBL. The diffusion coefficient of O + is~30 times larger than that of H + . The lower hybrid waves could contribute to the energy dissipation at plasma gradients in magnetopause surface wave structures and limit Kelvin-Helmholtz instability growth further downstream.

Properties of the Equatorial Magnetotail Flanks ∼50–200 RE Downtail

“…Another promising yet underutilized approach for further investigation of the magnetotail boundary's response to solar wind conditions is comparison of global MHD results with statistical spacecraft studies (e.g., Sibeck & Lin, ). We consider this study, along with the investigations of Chen et al () and Wang et al (), as a first step in construction of a detailed picture of MHD and kinetic properties of the distant magnetotail boundary layer.…”

“…Therefore, the most promising mechanism responsible for the magnetosheath plasma transport to the plasma sheet side is particle pitch angle scattering. Several plasma wave modes have been proposed to play an important role in such scattering: low‐hybrid drift waves (e.g., Chen et al, ), ion cyclotron waves (so far, observed in the dayside magnetopause, see, e.g., Panov et al, ), and kinetic Alfvén waves (e.g., Chaston et al, ). Effects of all these wave modes have been well studied at the dayside magnetopause, where they heat particles and transport them across the magnetic field (see review by Wing et al, , and references therein).…”

“…However, such separation would take place only in the absence of direct connection between magnetic field lines in the plasma sheet and those in the sides of the magnetosheath sides. Taking into account the fluctuating nature of the magnetosheath field and the significant contribution of the interplanetary magnetic field to boundary layer orientation (e.g., Chen et al, ; Sibeck, Siscoe, Slavin, Smith, & Tsurutani, ; Sibeck, Siscoe, Slavin, Smith, Tsurutani, & Lepping, ), we should assume that magnetic field lines in the plasma sheet are connected to those in the sides of the magnetosheath (at least sometimes). Thus, two types of plasma should be able to cross this boundary layer.…”

“…Strong plasma pressure and magnetic field gradients at the magnetopause serve as a free energy source for various plasma waves that enable stochastic charged particle diffusion across the magnetic surfaces (e.g., Chen et al, 2015;Galeev et al, 1986;Johnson & Cheng, 1997;Treumann, 1999, and references therein). The stochastic transport can be enhanced by large-scale magnetopause perturbations as Kelvin-Helmholtz vortexes (e.g., Chen et al, 2015;Fairfield et al, 2000;Hasegawa et al, 2004;Nakai & Ueno, 2011) or as various surface waves (e.g., De Keyser & Roth, 2003;Plaschke, 2016;Sibeck et al, 1990) usually observed at the flanks of the magnetosphere. Such plasma transport results in formation of a boundary layer with a monotonic, gradual plasma density profile (e.g., De Keyser et al, 2004;Fujimoto et al, 1997Fujimoto et al, , 1998Phan, 1997).…”

“…Modeling of plasma transport mechanisms and their efficiency in the distant magnetopause require investigation of its kinetic structure. Observations at lunar orbit (around −55 Earth radii, R E , downtail) clearly show the presence of a boundary layer separating cold, flowing magnetosheath plasmas from hot, stagnant, plasma sheet plasmas (e.g., Chen et al, 2015;Wang, Xing, et al, 2014). Weak magnetic fields at the low-latitude magnetotail flanks (plasma sheet flanks) suggest that the nightside magnetopause configuration should differ significantly from the dayside configuration (e.g., Panov et al, 2008;Phan & Paschmann, 1996) and from the configuration of the near-Earth flank (e.g., Haaland et al, 2014;Sanchez et al, 1990), where the magnetic field plays the most important role in the pressure balance (De Keyser et al, 2005).…”