Ionization and electric field properties of auroral arcs during magnetic quiescence

Abstract

Studies of the morphology of auroral precipitation during times of magnetic quiescence indicate that the polar cap shrinks and becomes distorted into a teardrop or pear‐shaped region. The narrowing of the polar cap in the afternoon and morning sectors is a result of discrete aurora that appears at very high latitudes. On November 16, 1987, incoherent scatter radar and all‐sky imaging photometer measurements were made of auroral arcs over Sondre Stromfjord, Greenland. The arcs were generally oriented in a geographic east‐west direction which is approximately Sun aligned at a local time just after dusk. Kp was 1, and the interplanetary magnetic field was northward during the time of observation, so that the arcs occurred under magnetically quiet conditions. The Sondrestrom radar measurements were used to determine the electron density and plasma drifts associated with the arcs; the all‐sky imaging photometer data were used to relate the radar measurements to the arc morphology. Assuming the arcs were produced by precipitating electrons, the height profiles of electron density indicate average energies less than about 2 keV and energy fluxes of 1 erg/(cm² s). F region electron densities were high in the polar cap north of the arcs and low within the region of the arcs. The poleward boundary of the arc system was a convection reversal boundary across which plasma exited the polar cap region moving antisunward and then turned sunward (westward). The observed arc‐associated convection is consistent with that expected under these geomagnetic conditions. Comparison of these results with the electrodynamic properties of other arcs observed in the afternoon and early evening suggests that there is a system of arcs that delineates the afternoon convection cell. As the location and extent of this cell change in response to interplanetary magnetic field conditions, the arcs change accordingly. The observed gradient in F region electron density across the arc can be explained in terms of the recombination of ionization drifting in response to the arc‐associated convection pattern.