Detached auroral arcs in the trough region

Abstract

The region of auroral emissions is normally terminated on its equatorward side by a regular and well‐defined boundary of diffuse aurora. We report here the observation of arclike auroral emissions in the northern hemisphere equatorward of this boundary, apparently in the ionospheric trough region. These ‘detached arcs’ always have a pronounced drop in intensity at their eastern extremity, equatorward of the diffuse aurora. The arcs show a pronounced local time and longitude dependence which yields an observing probability (for the Isis 2 satellite scanning photometer) in excess of 25% in the vicinity of 270° corrected geomagnetic longitude at 1800 hours corrected geomagnetic time. The arcs are not correlated in any obvious way with Kp or Dst, and they do not show any enhancement in 6300‐Å emission or electron temperature relative to normal aurora findings, leading to the conclusion that they are probably not related to stable auroral red arcs.