Velocity shear instability and plasma billows at the Earth's magnetic boundary

Abstract

The Kelvin-Helmoltz instability (KH) with formation of vortices appears in a wide variety of terrestrial, interplanetary, and astrophysical contexts. We study a series of iterated rolled-up coherent plasma structures (15) that flow in the equatorial Earth's boundary layer (BL), observed on October 24, 2001. The data were recorded during a 1.5 hour-long Wind crossing of the BL at the dawn magnetospheric flank, tailward of the terminator (X-13 R E ). The interplanetary magnetic field (IMF) was radially directed, almost antiparallel to the magnetosheath (MS) flow. This configuration is expected to be adverse to the KH instability because of the collinearity of field and flow, and the high compressibility of the MS. We analyze the BL stability with compressible MHD theory using continuous profiles for the physical quantities. Upstream, at near Earth sites, we input parameters derived from an exact MHD solution for collinear flows. Further downtail at Wind position we input measured parameters. The BL is found KH unstable in spite of unfavorable features of the external flow. On the experimental side, the passage of vortices is inferred from the presence of low densityhot plasma being accelerated to speeds higher than that of the contiguous MS. It is further supported by the peculiar correlation of relative motions (in the bulk velocity frame): colddense plasma drifts sunward, while hot-tenuous plasma moves tailward. This event differs from many other studies that reported BL vortices under strongly northward IMF orientations. This is a case of KH vortices observed under an almost radial IMF, with implicit significance for the more common Parker's spiral fields, and the problem of plasma entry in the magnetosphere.

Steepening of waves at the duskside magnetopause

“…The dusk flank, where MMS was observing MP waves, was, however, behind the quasi‐perpendicular shock in those 13 cases. Another 6 of the 23 groups are associated with strong IMF | B x |>2.8| B y |, i.e., radial IMF that should also be less favorable for KHI development at the equatorial flank MP, although KH waves have been observed under such conditions [ Gratton et al , ; Farrugia et al , ]. In the four remaining cases/groups, B x and B y are of equal sign and comparable; hence, the dayside dusk flank MP should have been situated below the quasi‐parallel shock.…”

Magnetopause Waves Controlling the Dynamics of Earth's Magnetosphere

“…The magnetopause expansion causes the magnetopause to become unstable to Rayleigh-Taylor instability, and the Kelvin-Helmholtz instability combined with Rayleigh-Taylor instability drives the entire dayside magnetopause to become Kelvin-Helmholtz-unstable (Farrugia et al 1998). Gratton et al (2012) and Farrugia et al (2014) reported rolled-up Kelvin-Helmholtz vortices observed during a radial IMF. These studies indicate that Kelvin-Helmholtz waves are commonly observed over the entire surface of the dayside magnetopause for a wider range of IMF orientations than was previously thought, possibly providing a constant, although less efficient than reconnection, mechanism of solar wind transport into the magnetosphere.…”