Eastward sub-auroral ion drifts or ASAID

Abstract

Abstract. From satellite data sampling the top ionosphere in the Northern Hemisphere we have identified strong eastward ion drifts, with speeds larger than 1 km/s, widths of 1 • -2 • , occurring at similar temporal and spatial locations as rapid westward ion drifts known as sub-auroral ion drifts (SAID). We have called these events "abnormal sub-auroral ion drifts" (ASAID). Two events observed in the 20:00-22:00 MLT interval are discussed: the first occurring on 21 September 2003 and the other on 12 October 2003. Tomographic reconstructions of the electron density in the Fregion, based on satellite data, provided by the Scandinavian tomography chain, were also available. We have observed that ASAID are accompanied by upward flows with a speed of the same order as that of the zonal ion drift. They coincide with deep, narrow troughs in the total ion density, both at the altitude of the F15 DMSP satellite (850 km) and in the F-region of the ionosphere, but do not seem to be a feature of the convective transport. During the entire duration of ASAID the electron temperature is very high while, contrary to SAID, the ion temperature has no clear variation. Both events described in this paper end up turning into classical SAID. Satellite data indicate that the generator of ASAID could be located inside the plasmasphere close to the plasmapause and we suggest a possible mechanism for their formation.

Statistical study on the occurrence of ASAID electric fields

“…Two eastward flow events observed in the predusk region (between 14.5 and 16:00 MLT) associated with an intense magnetic storm were reported. These events were however associated with relatively slower drift velocities (∼50-70 m/s) and with broader widths (∼3 • ), as compared to the observations by Voiculescu and Roth (2008). The first event occurred immediately after a northward turning of the IMF that pointed southward for about 6 h during the main phase of a storm.…”

“…Hence, it corresponds to a rather thin region of rapid eastward ion drift (∼2700 m/s) located at the equatorward edge of the auroral oval. It can therefore be considered to be the opposite signature of that of a SAID, or in other words, it is the signature of an abnormal subauroral ion drift (ASAID), as termed by Voiculescu and Roth (2008).…”

“…Observations on a new type of subauroral phenomena has been reported recently by Voiculescu and Roth (2008). It consists of regions of enhanced eastward ion drifts at subauroral latitudes named by the authors as Abnormal SubAuroral Ion Drifts, ASAID.…”

“…Voiculescu and Roth (2008) proposed that intense radial electric fields directed inward (toward magnetic shells with lower L-values) may occur when the outer edge of the ring current coincides with the plasmapause due to finite gyroradius effects at this interface: the larger Larmor radius of the hot ring current ions creates an excess positive charge in the cold plasmaspheric trough and leaves a deficit of negative charges inside the ring current region. The resultant electric field points radially inward and maps down to the ionosphere as a subauroral equatorward-directed electric field, associated with eastward ion drift flow.…”

Auroral and sub-auroral phenomena: an electrostatic picture

“…A range of diverging field strengths exists (Karlsson et al, 1998); very strong fields and the associated drifts might produce stable auroral red arcs as a consequence of the ion and electron heating due to ion-neutral collisions (Hoch and Lemaire, 1974;Foster et al, 1994;Moffett et al, 1998). SAPS and SAID have substantial effects on the plasmasphere and the ring current, and on the overall configuration of the inner magnetosphere (Foster et al, 1994;Ober et al, 1997;De Keyser, 1999;Goldstein et al, 2003Goldstein et al, , 2005Gurgiolo et al, 2005;Foster et al, 2007;Voiculescu and Roth, 2008).…”