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

Abstract

[1] The Geotail spacecraft made an inbound passage perpendicular to the dusk equatorial magnetopause on 1 August 1998 when the interplanetary magnetic field had been very northward for more than 10 hours. As the spacecraft moved through the low-latitude boundary layer, it detected waves with $3 min period that caused transitions between cool dense magnetosheath plasma and mixed magnetosheath and magnetosphere plasmas. A region of highly fluctuating magnetic fields with variations up to 50 nT/s was observed on leaving the magnetosheath plasma but not on its return to this region. This observation suggests that the boundary region is not laminar but more likely consists of KelvinHelmholtz vortices. Such fluctuations are probably caused by twisting of the field by the vortices, and they could be important in transporting plasma to the magnetosphere where densities of 5/cc were observed. A global MHD simulation of the event reproduced the observed low-frequency waves, their counterclockwise polarization, and a region of high-velocity magnetosheath flow outside the magnetopause. This flow was associated with a decreased density and increased field strength which are characteristics of a plasma depletion layer. When Geotail moved further inward into the magnetosphere, it detected waves that are probably the magnetotail vortices which had been detected earlier by the ISEE spacecraft.

Impact of solar wind depression on the dayside magnetosphere under northward interplanetary magnetic field

“…In our endeavor to carry out these tasks, the dayside magnetosphere (magnetopause) was targeted. In the past, the magnetopause motion was reported often oscillating more or less uniformly in and out a reference position (Formisano et al, 1979) over distances of the order of 0.1 earth radii and with periods of a few minutes (Kaufmann and Konardi, 1969;Sibeck et al, 1991, Safrankova et al, 1997Haaland et al, 2004;Fairfield et al, 2007;Farrugia et al, 2008;Plaschke et al, 2009;Zhang et al, 2009). It is clear that when the parameters defining the pressure balance change, the position of the magnetopause will also vary (Semenov et al, 2002).…”

“…Also, it is interesting to note recent reports on formation and retreat of many X-lines at high latitudes during a steady northward IMF (Hasegawa et al, 2008;Retinò et al, 2008). From existing observations and simulations, both the KH instability and reconnection appear as acting processes in the high latitude cusp regions during northward IMF condition, yet their respective efficiency remains uncertain (Frey et al, 2003;Lavraud et al, 2006;Fairfield et al, 2007;Li et al, 2008). More generally, in past studies, the dynamic motion of the MP was mostly considered under the impact of interplanetary shocks that compress the magnetosphere (Zhang et al, 2009).…”

“…Usually, the Kelvin-Helmholtz (KH) instability is advocated to explain observation of the MP flanks distortion, a process that may also occur without any solar wind forcing with amplitudes that may exceed a factor of 2, and may lead to plasma entry in the magnetospheric cavity under northward IMF condition (Fairfield et al, 2007). More recently, the observation and 2-D simulation of flux transfer events (FTE) revealed the formation of plasma vortex islands that could be related to time evolving reconnection events near the low-latitude boundary layer (LLBL) (Eriksson et al, 2009).…”

“…Most of studies during northward IMF conditions focused on wave generation by solar wind disturbances and resulting propagation in the magnetospheric cavity with the possibility of plasma entry from the magnetosheath through the MP flanks (Terasawa et al, 1997;Sibeck, 2000;Fairfield et al, 2003Fairfield et al, , 2007Farrugia et al, 2008;Plaschke et al, 2009). Usually, the Kelvin-Helmholtz (KH) instability is advocated to explain observation of the MP flanks distortion, a process that may also occur without any solar wind forcing with amplitudes that may exceed a factor of 2, and may lead to plasma entry in the magnetospheric cavity under northward IMF condition (Fairfield et al, 2007).…”

Multipoint observations of plasma phenomena made in space by Cluster

“…Recently, in-situ observations from Cluster, THEMIS, Geotail, Double Star, MESSENGER, and Cassini have indicated that the KHI appears capable of generating surface waves (e.g., Fairfield et al 1990;Hasegawa et al 2004;Nykyri et al 2006;Fairfield et al 2007;Volwerk et al 2007;Taylor and Lavraud 2008;Boardsen et al 2010;Masters et al 2010;Cutler et al 2011;Hwang et al 2011bHwang et al , 2012Sundberg et al 2012). KHW have also been identified in the magnetosheath (Walker et al 2011b), the solar corona (Ofman and Thompson 2011), and the ionospheres of Mars and Venus (Pope et al 2009).…”