Kelvin‐Helmholtz waves at the Earth's magnetopause: Multiscale development and associated reconnection

Abstract

[1] We examine traversals on 20 November 2001 of the equatorial magnetopause boundary layer simultaneously at $1500 magnetic local time (MLT) by the Geotail spacecraft and at $1900 MLT by the Cluster spacecraft, which detected rolled-up MHDscale vortices generated by the Kelvin-Helmholtz instability (KHI) under prolonged northward interplanetary magnetic field conditions. Our purpose is to address the excitation process of the KHI, MHD-scale and ion-scale structures of the vortices, and the formation mechanism of the low-latitude boundary layer (LLBL). The observed KH wavelength (>4 Â 10 4 km) is considerably longer than predicted by the linear theory from the thickness ($1000 km) of the dayside velocity shear layer. Our analyses suggest that the KHI excitation is facilitated by combined effects of the formation of the LLBL presumably through high-latitude magnetopause reconnection and compressional magnetosheath fluctuations on the dayside, and that breakup and/or coalescence of the vortices are beginning around 1900 MLT. Current layers of thickness a few times ion inertia length $100 km and of magnetic shear $60°existed at the trailing edges of the vortices. Identified in one such current sheet were signatures of local reconnection: Alfvénic outflow jet within a bifurcated current sheet, nonzero magnetic field component normal to the sheet, and field-aligned beam of accelerated electrons. Because of its incipient nature, however, this reconnection process is unlikely to lead to the observed dusk-flank LLBL. It is thus inferred that the flank LLBL resulted from other mechanisms, namely, diffusion and/or remote reconnection unidentified by Cluster.

Global reconnection topology as inferred from plasma observations inside Kelvin-Helmholtz vortices

“…The issue is therefore to understand where does reconnection occur: does it occur locally, within the vortex, as observed in other events reported in the literature (e.g. Nykyri et al, 2006;Hasegawa et al, 2009), or does it occur on a global scale, the reconnection site being far from the vortex? In Sect.…”

“…Nishino et al (2007) analysed the ions and electrons distribution functions of a KH event and also interpreted their observations in terms of reconnection. Hasegawa et al (2009) identified reconnection signatures in a current sheet in the vortices.…”

Multipoint observations of plasma phenomena made in space by Cluster

“…Vorticity is expected in the magnetosheath due to, for example, reconnection along the dayside magnetopause (Lockwood and Smith 1992;Retinò et al 2007;Dunlop et al 2011) and along the flanks (Lin and Lee 1994), as well as due to velocity shear along the magnetopause/magnetosheath boundary (Hasegawa et al 2004;Hasegawa et al 2009). Figure 22 shows an interval of Cluster data when the spacecraft were in a high inclination orbit near apogee, just crossing the northern magnetopause and entering the magnetosheath.…”

“…These processes can trigger plasma transport and mixing within or at the edge of rolled-up KHVs. Decaying KHVs ultimately form a broad mixing layer at the dawn and dusk flanks (Nakamura et al 2006;Hasegawa et al 2009;Cowee 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).…”