Observation of a correspondence between poleward moving auroral forms and stepped cusp ion precipitation

Abstract

In a case study, we document for the first time a one‐to‐one association between three auroral forms which are separated in latitude and moving poleward and three intervals of almost constant, but different, low‐energy cutoffs in the ion precipitation in the cusp region (“stepped cusp”) precipitation. The ground observations are from Ny Alesund at 75° magnetic latitude (MLAT), and the precipitation pattern is observed by the DMSP F13 satellite as it crosses the noon meridian into the morning sector. The energy cutoffs increase with decreasing latitude. The auroral forms brighten up at different times but at practically the same MLAT (∼73°) and start fading 5 min later at ∼76° MLAT. Interplanetary conditions, monitored by WIND, show a stable, slow, and cold solar wind with an interplanetary magnetic field which points south and west ( B y > 0). Staircase ion precipitation signatures such as observed in this example have been successfully interpreted in terms of a sequence of reconnection bursts at the low‐latitude, dayside magnetopause. The time‐history of the auroral activity observed from the ground confirms this interpretation. We exclude the possibility that the auroral forms are triggered by changes in the interplanetary medium because the latter is very steady. In particular, the (low) dynamic pressure varies little about a mean value of 1.6 nPa.