Transverse ion acceleration mechanisms in the aurora at solar minimum: occurrence distributions

We present a statistical study of 714 ion conic events detected by the Fast Auroral Snapshot Explorer (FAST) from September 1996 to February 1997. Nin ety-nine percent of the events found are associated with either broadband e xtremely low frequency (BBELF) emissions or electromagnetic ion cyclotron ( EMIC) waves. Lower hybrid waves are much less important in transverse ion a cceleration above 2000 km. The BBELF events are more numerous, comprising s ome 84% of ion conic events identified, and occur at all local times, with a peak near noon and a minimum near dusk. The EMIC events are concentrated in the dusk to midnight sector and are more likely to occur at lower latitu des compared to the BBELF events. The occurrence rate of EMIC conics has an apparent local minimum at 2000-2500 km, while the BBELF conic occurrence r ate varies only slowly between 2000 km and the FAST apogee of 4200 km. The occurrence rate of EMIC conics is more strongly correlated with K-p than th e BBELF conic occurrence rare. These results are consistent with previous s tudies of ion conics at lower altitudes. The correlation of both BBELF and EMIC ion conics with phenomena that are associated with parallel electric f ields suggests that parallel electric fields play a significant role in tra nsverse ion heating in the aurora. (C) 2000 Elsevier Science Ltd. All right s reserved.