Multi-Satellite Low-Altitude Observations of a Magnetopause Merging Burst

A fortuitous combination of DMSP F12 and F13 observations of a dayside merging event driven by a sudden sharp southward interplanetary magnetic field (IMF) turning illuminates several facets of the ionospheric particle and auroral signatures of reconnection. Nearly coincident with the sharp southward IMF turning (as propagated from ACE), DMSP F13 observed a region equatorward of the cusp apparently emptied of magnetospheric ions (of tens of keV energy), and bounded by two sharp intensifications of electron energy flux. The equatorward electron intensification occurred at 74.3xxx MLAT, which is both the boundary to which magnetospheric ions were depleted, and the boundary to which magnetosheath energy ions reached in the F12 satellite pass 4 minutes later (these latter ions evinced a low-energy cutoff corresponding to xxx minutes elapsed since merging). The equatorward electron burst observed by F13, reaching to 5xxx ergs/cm2 s, had a striking and hitherto unreported signature, which suggests recent disconnection from the x-line merging site. To wit, these very high electron fluxes had a spectacularly clear high energy cutoff corresponding to a transit time from the nose of the magnetospause of from 5-6 seconds. The more poleward electron burst observed by F13, occurring at the equatorward edge of the old cusp, had a more conventional signature suggesting wave aurora. Drift meter data on F13 indicates that the merging burst resulted in ionospheric flow speeds above 1 km/s, and a xxx kV cusp contribution to the cross polar cap potential. Simultaneous observations by Polar UVI suggest a large-scale auroral transient at cusp latitudes, while magnetometer observations suggest sheet currents rather than filamentary currents, also indicative of the broad scale of the event. Taken together, these observations suggest the ability to recognize newly opened dayside regions even prior to the arrival of magnetosheath (cusp) plasma from low-altitude observations.