A vortical dawn flank boundary layer for near‐radial IMF: Wind observations on 24 October 2001

Abstract

We present an example of a boundary layer tailward of the dawn terminator which is entirely populated by rolled-up flow vortices. Observations were made by Wind on 24 October 2001 as the spacecraft moved across the region at X ∼−13 R E . Interplanetary conditions were steady with a near-radial interplanetary magnetic field (IMF). Approximately 15 vortices were observed over the 1.5 h duration of Wind's crossing, each lasting ∼5 min. The rolling up is inferred from the presence of a hot tenuous plasma being accelerated to speeds higher than in the adjoining magnetosheath, a circumstance which has been shown to be a reliable signature of this in single-spacecraft observations. A blob of cold dense plasma was entrained in each vortex, at whose leading edge abrupt polarity changes of field and velocity components at current sheets were regularly observed. In the frame of the average boundary layer velocity, the dense blobs were moving predominantly sunward and their scale size along X was ∼7.4 R E . Inquiring into the generation mechanism of the vortices, we analyze the stability of the boundary layer to sheared flows using compressible magnetohydrodynamic Kelvin-Helmholtz theory with continuous profiles for the physical quantities. We input parameters from (i) the exact theory of magnetosheath flow under aligned solar wind field and flow vectors near the terminator and (ii) the Wind data. It is shown that the configuration is indeed Kelvin-Helmholtz (KH) unstable. This is the first reported example of KH-unstable waves at the magnetopause under a radial IMF.

Magnetized Kelvin–Helmholtz instability: theory and simulations in the Earth’s magnetosphere context

“…As suggested by the authors this percentage must be considered as a lower limit as all the events with rolled-up vortices but a satellite staying in the inner region with respect to the unperturbed magnetopause are not taken into account. In any case the existence of rolled-up vortices at the magnetospheric flanks has been finally confirmed by several satellite missions and has been proved to be far from rare (Hasegawa et al 2004;Taylor et al 2008Taylor et al , 2012Hasegawa et al 2009;Bavassano Cattaneo et al 2010;Hwang et al 2011;Nishino et al 2011;Faganello et al 2014;Farrugia et al 2014;Lin et al 2014).…”

“…With a simple analysis, Hasegawa (2012) related the radius of the vortex to the jump of the total pressure observed by a satellite crossing. In such a way Nishino et al (2011), Faganello et al (2014) and Farrugia et al (2014) recognized rolled-up structures in the single-spacecraft data of the Geotail, THEMIS and WIND mission respectively. A second useful method is the one developed by Takagi et al (2006) based on the fact that the tenuous magnetospheric arm of a vortex rotates faster than the surrounding dense magnetosheath plasma, as observed in numerical simulations by Matsumoto & Hoshino (2004) and Nakamura et al (2004).…”

“…(2014) and Farrugia et al. (2014) recognized rolled-up structures in the single-spacecraft data of the Geotail, THEMIS and WIND mission respectively. A second useful method is the one developed by Takagi et al.…”

IMF Control of Alfvénic Energy Transport and Deposition at High Latitudes

“…The occurrence of the K‐H instability is favorable under conditions of northward B z (Hasegawa et al, , ) or high solar wind speed (Rae et al, ), or both (Kavosi & Raeder, ). The instability nevertheless operates under every solar wind speed in the typically observed range, 200–600 km/s, and under virtually every IMF configuration (Farrugia et al, ; Hwang et al, ; Kavosi & Raeder, ; Yan et al, ), which could explain the persistence of the prenoon and postnoon enhanced broadband number flux for every ϕ IMF orientation in Figure . However, this explanation offers no obvious clues as to the apparent preferential generation of prenoon IAW power.…”

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.…”