Simultaneous observations of polar cap patches and Sun-aligned arcs duringtransitions of the IMF

This paper presents the first observations of simultaneous polar cap patche s and polar cap arcs in a single common 1000-km field of view, and identifi es a model that explains the interplanetary magnetic field (IMF) dependenci es of the observed phenomenology. To study the characteristics of the polar cap optical emissions in the 630.0 nm line during transitions of the IMF B -z, we have scanned images taken at Qaanaaq, Greeland, between 1989 and 199 4. We found that on a few occasions, when B-z changed from a south to a nor th orientation, a particular pattern of polar cap patches and Sun-aligned a rcs coexisted. No similar pattern of coexisting arcs and patches was found during north-to-south IMF transitions. The detailed analyses of three of th ese events are presented here in which patches and polar cap arcs are clear ly identified to reside simultaneously within the Qaanaaq imager field of v iew. The digisonde located also at Qaanaaq is used to confirm that the opti cal patches correspond to enhancements in the number density and a simultan eous decrease of the h(m)F(2) value. These two factors increase the capabil ity of the imager to differentiate between patches and the background airgl ow. Data collected by the DMSP F8 satellite during one of the events reaffi rm the appearance of polar cap precipitation during the B-z positive period . The J4 sensor on board DMSP F8 detected typical electron fluxes commonly associated with polar cap arcs. The coexistence of patches and arcs is due to a slower response of the patches in exiting the polar cap, and then the relatively sudden appearance of polar cap arcs presumably driven by dayside reconnection between the IMF and open flux drawn initially equatorward tow ard the cusp. This model, of dayside reconnection switching from equatorwar d of the cusp for B-z south to poleward of the cusp for B-z north, likewise explains why arcs and patches are seen by the imager to coexist for rapid B-z reversals only from south to north and not from north to south.