On the formation of postnoon auroral bright spots during weak substorm activity

Abstract

Postnoon auroral bright spots occurring around 15:00 magnetic local time (MLT) are typically caused by monoenergetic electrons accelerated by quasi-static electric field. In this study, we conduct an analysis of two special auroral spots observed by the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) onboard the Defense Meteorological Satellite Program (DMSP) spacecraft. The first bright spot, observed on 05 June 2010, appears as a south-north aligned arc, while the second case, observed on 19 August 2005, takes the form of a circular patch. They both occurred at the equatorward edge of and are connected to the discrete aurora in the auroral oval. Particle measurements indicate that these two postnoon spots are primarily produced by precipitating electrons experiencing an acceleration process by quasi-static electric field. The average energy and integrated energy flux for the two cases are approximately 1.4 keV (1.5 keV) and 2.3 × 1012 (1.1 × 1012) eV/(cm2 s sr). It is suggested that the precipitating particles associated with the two cases originate from the boundary plasma sheet located poleward of the central plasma sheet. During the occurrences, the interplanetary magnetic field (IMF) By was nearly zero, while the IMF Bz was slightly negative, with values above −3 nT. Additionally, the geomagnetic AL index decreased to −160 and −200 nT, respectively. This indicates a potential relationship between the postnoon auroral spots and week substorm activities. Surprisingly, significant ion flow velocity was found to be nearly orientated with the postnoon spots. This suggests that the convection transports plenty of plasma along the south-north direction, offering sufficient particles for the formation of the postnoon auroral bright spots.