Intensification and fading of auroral arcs in the dusk‐midnight sector of the polar cap

Abstract

Observations of the aurora from South Pole station (magnetic latitude = −74.2°) have been used to study the intensification and fading of polar arcs observed near the dusk meridian. Most of the cases examined have the following features in common: (1) a preexisting auroral form intensifies for about 10 min; (2) this activation is followed by a pronounced decrease of luminosity; (3) the auroral fade terminates after 30–60 min with the onset of intense aurora which sweeps rapidly overhead. The brief intensifications involve O (630.0 nm) enhancements of 0.1–0.3 kR and N2+ (427.8 nm) enhancements of 0.3–1.5 kR giving an emission ratio which in each case is consistent with the precipitation of electrons with an average energy of 3–12 keV, assuming a Maxwellian distribution. The auroral brightening is not accompanied by riometer absorption. Ground magnetic perturbations of the order 20–40 nT occur, and in about half the instances, weak broadband impulsive hiss is present. The availability of all‐sky camera, auroral electrojet (AE) index and interplanetary magnetic field (IMF) data for some of the cases enables the following additional characterizations of these events. The preexisting form is a Sun‐ or oval‐aligned arc (or part of a multiple arc system) which disappears following the activation; equatorward drift of the arc (or system) accompanies the luminosity change. There is some evidence to suggest that the arc is poleward of the auroral oval. The brief intensification and/or the onset of fading occurs during the growth phase or near the start of the expansive phase of a substorm; termination of the fade is near the maximum in AE and is probably indicative of the beginning of the recovery phase of the substorm. For all three cases for which IMF data were available the onset of fading occurred 20–30 min after Bz turned southward. Two of these cases were preceded by auroral arc enhancement when Bx was sunward; in the third case there was no prior enhancement when Bx was zero or slightly antisunward. By was negative (dawnward) in each case. Sun‐aligned arcs are a common feature of the polar cap during northward Bz but disappear during the increasingly disturbed conditions that accompany southward Bz. The present results suggest that brief intensifications of southern hemisphere polar cap arcs near dusk may be linked in part to the sunward orientation of the IMF which favors enhanced electron fluxes in the southern lobe of the magnetotail.