The first in situ observation of Kelvin‐Helmholtz waves at high‐latitude magnetopause during strongly dawnward interplanetary magnetic field conditions

Abstract

[1] We report the first in situ observation of high-latitude magnetopause (near the northern duskward cusp) Kelvin-Helmholtz waves (KHW) by Cluster on January 12, 2003, under strongly dawnward interplanetary magnetic field (IMF) conditions. The fluctuations unstable to Kelvin-Helmholtz instability (KHI) are found to propagate mostly tailward, i.e., along the direction almost 90 to both the magnetosheath and geomagnetic fields, which lowers the threshold of the KHI. The magnetic configuration across the boundary layer near the northern duskward cusp region during dawnward IMF is similar to that in the low-latitude boundary layer under northward IMF, in that (1) both magnetosheath and magnetospheric fields across the local boundary layer constitute the lowest magnetic shear and (2) the tailward propagation of the KHW is perpendicular to both fields. Approximately 3-hour-long periods of the KHW during dawnward IMF are followed by the rapid expansion of the dayside magnetosphere associated with the passage of an IMF discontinuity that characterizes an abrupt change in IMF cone angle, f = acos B x B j j , from $90 to $10 . Cluster, which was on its outbound trajectory, continued observing the boundary waves at the northern evening-side magnetopause during sunward IMF conditions following the passage of the IMF discontinuity. By comparing the signatures of boundary fluctuations before and after the IMF discontinuity, we report that the frequencies of the most unstable KH modes increased after the discontinuity passed. This result demonstrates that differences in IMF orientations (especially in f) are associated with the properties of KHW at the high-latitude magnetopause due to variations in thickness of the boundary layer, and/or width of the KH-unstable band on the surface of the dayside magnetopause.

Solar wind entry into the high-latitude terrestrial magnetosphere during geomagnetically quiet times

“…3b). This further confirms that the entry plasmas in our observations is unlikely to be sourced by those mechanisms operating at low latitudes (K-H instability at the high latitudes may still operate under some special conditions 41 ), and are more consistent with the near midnight operation of the high-latitude reconnection. The entry region locations at the high latitude that are predicted by the simulations (the yellow or green regions of Fig.…”

Distribution of Field‐Aligned Electron Events in the High‐Altitude Polar Region: Cluster Observations

“…The IMF orientation (i.e., the inward or outward Parker Spiral) should have a key role for the dawn-dusk asymmetries and the spatial distribution of the FAE events. During Parker Spiral IMF KHI can occur both at the LLBL (Moore et al, 2016;Nykyri, Grison, et al, 2006) favoring the dawn flank (Nykyri, 2013) and at the high latitudes (Hwang et al, 2012;Ma et al, 2016). The dependence of FAEs under different solar wind and IMF conditions will be left for future studies with more statistics.…”

Multipoint observations of plasma phenomena made in space by Cluster

“…On January 12, 2003 Cluster observed the generation and evolution of KHW under variable IMF conditions (Hwang et al 2012). This was the first in-situ observation of KHW in the high-latitude magnetopause near the northern duskward cusp under strongly dawnward IMF conditions.…”

“…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).…”

“…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). Cluster multipoint measurements have enabled us to identify KHVs (Hasegawa et al 2004;Hwang et al 2011b) and explore wave propagation, wave periods, boundary normals and the steepness of KHW/KHVs (Owen et al 2004;Nykyri et al 2006;Foullon et al 2008;Hasegawa et al 2009;Foullon et al 2010;Hwang et al 2012).…”