Satellite observations of spatial and temporal fluctuations in post-midnight auroras

Abstract

ISIS 2 satellite observations of the equatorward boundary of the diffuse aurora in the early morning magnetic local time (MLT) sector reveal irregularities in that boundary which are identified as a pulsating aurora. In the case of "cartwheel" data, repeated scans made through the same meridian reveal clear temporal variations in regions having scale sizes on the order of 300–400 km in north–south extent and intensities up to 60% of the diffuse auroral intensity poleward of the pulsations. When the pulsations are observed there are no corresponding fluctuations in intensity at the poleward boundary.When two-dimensional scans of the morning section are carried out, similar irregularities can be seen which range from quasi-periodic (in MLT) patch-like regions at the equatorward boundary, to north–south elongated irregularities in the diffuse aurora as a whole. Abrupt longitudinal boundaries in the irregularity regions can be seen particularly toward later MLT, suggesting that purely statistical analyses of latitude and local time distributions may not do justice to the phenomena involved.It seems quite likely that these irregularities indicate regions of pulsating auroras, in which case the pulsation regions are associated with remarkably orderly large scale fluctuations in the plasma sheet and thus may need to be viewed as something more than an irregular phenomenon governed entirely by local instabilities.