Microscale dynamics within Kelvin-Helmholtz waves: A probe of localized reconnection occurrence

Abstract:

During periods of northward IMF, an absence of low latitude reconnection at the magnetopause is expected. However, this gives rise to questions as to how the low latitude boundary layer (LLBL) can be populated with magnetosheath-like plasma under northward IMF conditions. Multi-spacecraft observations have shown the existence of Ultra-Low Frequency waves at the magnetopause that are thought to be results of the Kelvin-Helmholtz (K-H) instability (Miura, 1982). These K-H waves can grow into non-linear waves and form rolled-up vortices. It has been postulated that magnetic reconnection within these vortices may be responsible for the transfer of solar wind plasma into the magnetosphere (e.g. La Belle-Hamer et al., 1988; Nykyri and Otto, 2001; Hasegawa et al., 2004; Bavassano Cattaneo et al., 2010). Thus, high temporal resolution plasma observations sufficient to resolve reconnection signatures inside K-H vortices (e.g.) are key to understanding how reconnection bursts contribute to coupling of the magnetosheath plasma with magnetosphere.

We study an event in Dec 2006, in which Cluster encountered on-going K-H waves as the spacecraft were inbound and crossed the Earth’s dusk flank magnetopause through the LLBL. During this event, the spacecraft were operating in burst mode, and the magnetic field remained closely aligned with the spacecraft spin axis. Thus we have used the 3D particle data to reconstruct near-full pitch angle distribution of electrons and ions at sub-spin resolution (0.125s c.f., 4s spin resolution). These observations, up to 32 times faster than normal mode data, provided new insight into particle dynamics during the outbound movements of Cluster across the magnetopause. We present observations of regions where boundary plasma was traveling faster than bulk magnetosheath velocity, which is a plasma signature indicative of rolled up K-H vortices. Within these vortices, we show evidence of reconnection, which consist of D-shape accelerated ion population, field-aligned magnetosheath electrons along with anti-field-aligned magnetospheric electrons. We discuss the ramifications of this result in context for the soon to be launched MMS mission.