High-latitude magnetic reconnection in sub-Alfvenic flow: Interball Tail observations on May 29, 1996

The Interball Tail spacecraft crossed the high-latitude magnetopause near the cusp region under northward interplanetary magnetic field (IMF) conditions on May 29, 1996, with magnetic local time and magnetic latitude of similar to7.3 hours and similar to65.4degrees, respectively. Under these IMF conditions the Interball Tail spacecraft observed quasi-steady reconnection in progress and evidence for a relatively stable reconnection site at high latitudes. Sunward plasma flow observed by Interball Tail and a determination of the tangential stress balance indicated that reconnection was occurring poleward of the Earth's magnetic cusp, above the spacecraft's location. At these high latitudes the gasdynamic model of the solar wind/magneto-sphere interaction indicates that the magnetosheath flow should be super-Alfvenic, and therefore that the reconnection site should have propagated tailward. However, the spacecraft observed sub-Alfvenic flow in the magnetosheath region adjacent to the magnetopause current layer near the reconnection site indicating that the reconnection site may have moved in the sunward direction. These observations suggest that the region of sub-Alfvenic flow and stable, quasi-steady reconnection extend to very high latitudes under northward IMF conditions. It is shown that the thickness of the magnetopause current layer for this event (estimated as similar to1600 km) is consistent with that found for reconnection at the dayside magnetopause.