Multisatellite low-altitude observations of a magnetopause merging burst

Abstract

A fortuitous multisatellite observation of a dayside merging event driven by a sharp southward interplanetary magnetic field (IMF) turning illuminates several ionospheric particle and auroral reconnection signatures. Shortly after the southward IMF turning, as propagated from ACE, DMSP F13 observed a region equatorward of the cusp emptied of magnetospheric ions and bounded by two sharp intensifications of electron energy flux. The equatorward electron burst occurred at 74.4° magnetic latitude. That is also both the boundary to which magnetospheric ions were depleted and to which magnetosheath ions appeared in a DMSP F12 satellite pass 4 min later. These latter ions evinced a low-energy cutoff, implying that about 4-5 min elapsed since merging. The equatorward electron burst observed by F13 reached an intensity of 8.6 ergs/cm² s, and thus must have produced a strong auroral signature. Indeed, Polar UVI observed a dayside auroral transient at the appropriate latitude and local time. This burst was likely Alfvénic (it was not monoenergetic (not associated with a quasi-static electric field)). The poleward electron burst observed by F13 (up to 4.6 ergs/cm² s), was at the equatorward edge of the old cusp and also had a broadband (Alfvénic) signature. Drift meter data on F13 indicates that the merging burst produced flow speeds up to 2 km/s and a 47 kV cusp contribution to the polar cap potential, of which 25 kV was in a narrow high-speed spike. Thus, this event probably constituted a significant fraction of the cross-polar cap potential.