The cusp in rapid transition


We report on a transitional state of the Northern Hemisphere winter cusp following a strong bipolar rotation of the interplanetary magnetic field (IMF) from south to north (−20 to 20 nT in 8 min). We combine ground optical data (meridian scanning photometer and an all-sky camera at Ny Ålesund, Svalbard (76°MLAT)) with magnetometer and particle precipitation data (ions and electrons) and ion drift observations obtained during an overpass of spacecraft DMSP F13. The isolated IMF signature (southward turning followed by northward turning) excited a specific two-phase response in the cusp ionosphere, consisting of an initial 5 min long activation of the type 1 (southward IMF) cusp aurora which was followed by a type 2 (northward IMF) aurora of similar duration, located at slighty higher latitude. The type 1 aurora persisted during this phase but at diminished intensity. The type 2 aurora was accompanied by the excitation of a double arc with very strong green line emission at the poleward boundary of the cusp. These signatures are ascribed to a rapid transition in the IMF-magnetosphere interconnection geometry, with an initial pulse of enhanced reconnection rate at subcusp latitudes being followed by an event at high latitudes. The most important findings are (1) the rapid (2 min long) transition between the two distinct cusp states, (2) the attendant excitation of discrete auroral arcs in the region of strong flow shear at the poleward boundary of the northward IMF (Bz ≫ 0; By ≫ 0) cusp, and (3) the existence of a bifurcated cusp geometry during the latter IMF condition.

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JGR: Space Physics



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