Northward interplanetary magnetic field cusp aurora and high‐latitude magnetopause reconnection

Abstract

We present observations of two types of auroral forms located at different latitudes in the cusp region. Type 1 (south) auroras are located at ∼71° – 75° MLAT and occur during intervals of southward directed interplanetary magnetic field (IMF). Higher‐latitude (77°–78°) type 2 (north) auroras are associated with northward IMF (Bz > 0). Intervals are studied which are characterized by transitions from one auroral form to the other in response to IMF switches from north (clock angle <50°) to south (clock angle >90°) and vice versa. These observations are found to be consistent with the interpretation that type south auroras are a signature of low‐latitude magnetopause reconnection and that the type north auroras are associated with high‐latitude reconnection, tailward of the cusp. The latter is supported by satellite (DMSP F11) observations of particle precipitation and ionospheric convection. The simultaneous existence of type north and type south auroras is observed during intermediate IMF states (clock angle ∼60°). Observations of type north auroras may be used to study the relationship between the IMF Bx component and high‐latitude reconnection. We find that type north auroras occur in the northern hemisphere for both Bx polarities. Enhanced emission intensity of the auroral green line seems to be a feature occurring predominantly during negative Bx conditions. The enhanced green line intensity is considered to be an indication of low‐altitude particle acceleration in regions of strong field‐aligned current intensities. Such particle accelerations (green line emission) may occur predominantly in regions of upward directed IMF By‐related currents in association with the type north cusp aurora.