Observations of nightside auroral cavities

Abstract

Radar observations of field‐aligned auroral F region density depletions (cavities) have been identified in a portion of the Sondre Stromfjord incoherent scatter radar (ISR) data base covering the period February 1986 to January 1988. These “auroral cavities” are nightside phenomena with localized, field‐aligned F region density depletions of 20 to 70 percent below surrounding values. They occur during moderate to quiet geomagnetic conditions when the poleward edge of the auroral oval is within view of Sondre Stromfjord. Seasonally, they are a wintertime phenomena occurring just poleward of the statistical auroral oval. Unlike the previously reported “polar hole,” the average width of the cavities is less than 100 km. Case studies show that the cavities closely track the poleward edge of the most poleward auroral arc. Sequential radar scans show that cavities appear on time scales as short as several minutes, suggestive of local electrodynamic formation or rapid transport. Data from January 24, 1987, collected during coordinated optical, radar, and satellite observations spanning an hour of local time, were examined for possible cavity formation mechanisms. The cavity formation processes examined herein include locally enhanced chemical loss, vertical diffusion, drifting horizontal gradients in the background plasma, and evacuation as a result of field‐aligned currents. The formation time scales, calculated evacuation fluxes, close proximity to E region aurora, and field‐aligned current signatures seen in magnetometer and radar observations suggest a strong association of the cavities with upward flowing electrons carrying region 1 downward field‐aligned currents.