Geotail observations of the Kelvin‐Helmholtz instability at the equatorial magnetotail boundary for parallel northward fields

Abstract

Abstract. For several hours on March 24, 1995, the Geotail spacecraft remained near the duskside magnetotail boundary some 15 Re behind the Earth while the solar wind remained very quiet (V=330 km s -•, n=14-21 cm -3) with a very steady 11-nT northward magnetic field. Geotail experienced multiple crossings of a boundary between a dense (n=19 cm-3), cool (Tp=40 eV), rapidly flowing (V=310 km s -1) magnetosheath plasma and an interior region characterized by slower tailward velocities (V=100 km s-l), lower but substantial densities (n=3 cm -3) and somewhat hotter ions (220 eV). The crossings recurred with a roughly 3-min periodicity, and all quantities were highly variable in the boundary region. The magnetic field, in fact, exhibited some of the largest fluctuations seen anywhere in space, despite the fact that the exterior magnetosheath field and the interior magnetosphere field were both very northward and nearly parallel. On the basis of an MHD simulation of this event, we argue that the multiple crossings are due to a Kelvin-Helmholtz instability at the boundary that generates vortices which move past the spacecraft. A determination of boundary normals supports Kelvin-Helmholtz theory in that the nonlinear steepening of the waves is seen on the leading edge of the waves rather than on the trailing edge, as has sometimes been seen in the past. It is concluded that the Kelvin-Helmholtz instability is an important process for transferring energy, momentum and particles to the magnetotail during times of very northward interplanetary magnetic field.

Waves on the dusk flank boundary layer during very northward interplanetary magnetic field conditions: Observations and simulation

“…Vertical dashed blue lines delineate sharp increases in B which correspond to increases in density and decreases in temperature, all of which are characteristic of sudden encounters with magnetosheath plasma. These sudden changes followed by slow recoveries create a sawtooth appearance often reported in previous measurements [e.g., Fairfield et al, 2000]. A periodicity of approximately 3 min is readily apparent.…”

“…[2] Waves on flanks of the magnetosphere have been reported by many observers [e.g., Ogilvie and Fitzenreiter, 1989;Kivelson and Chen, 1995;Takahashi et al, 1991;Kokubun et al, 1994;Seon et al, 1995;Fairfield et al, 2000Fairfield et al, , 2003Farrugia et al, 2000;Owen et al, 2004;Hasegawa et al, 2004]. These waves have frequently been attributed to the Kelvin-Helmholtz (KH) instability caused by shear between the high-velocity magnetosheath plasma and the nearly stagnant magnetosphere plasma.…”

Dawn–dusk asymmetry in the Kelvin–Helmholtz instability at Mercury

“…This configuration where the IMF is parallel to the planetary magnetic field in the MP is favourable for the KHI along the equatorial magnetospheric flanks 16 . The 29-September and subsequent observations show a clear asymmetry in the KHI in favour of dusk 11 , which will be explained through consideration of asymmetries in the forces acting on the ions at dusk and dawn.…”

Simulations of the Kelvin–helmholtz Instability Driven by Coronal Mass Ejections in the Turbulent Corona

“…When a coronal mass ejection (CME) expands supersonically upwards from the solar surface, a bow shock is formed ahead of the CME and a strong shear flow develops across the contact discontinuity, separating the shocked ambient plasma from the ejected material. A similar configuration arises at the flanks of the Earth's magnetopause, where the KH instability has also been observed and studied (Fujimoto & Teresawa 1995;Fairfield et al 2000;Nykyri & Otto 2001). More recently it was observed in connection to the magnetopause of other planets, such as Saturn (Masters et al 2010) and Mercury (Sundberg et al 2011).…”