SUBSTORM PRECIPITATION IN THE POLAR-CAP AND ASSOCIATED PC-5 MODULATION

This paper reports substorm-related cosmic radio noise absorption even ts observed deep into the polar cap, well poleward of the typical nigh tside auroral oval. We find that such events usually occur in the late expansion/recovery phase of a substorm during moderate-to-strong plan etary disturbances (K-p = 3-8) and at times of high solar wind speed c onditions. We present riometer data from U.S. manned and automatic geo physical observatories in Antarctica to track the morphology and progr ession of auroral absorption features over 5 hours of local time at gr eater than or equal to 80 degrees magnetic latitude. The azimuthal mot ions observed, westward pre-midnight, eastward post-midnight, are simi lar to the dynamics expected for lower latitude substorm-related auror al absorption. ULF modulation of the absorption (and hence of the caus ative fluxes of precipitating energetic electrons) are often observed. These modulations are usually dominated by spectral peaks between 1-4 mHz and, in the one case examined in detail, were confined to a narro w, similar to 1 degrees latitude band embedded within more uniform pre cipitation. Pc 5 (150 to 600 s periods) magnetic variations with simil ar spectral peaks, and auroral arcs are often observed concurrently. S everal mechanisms for producing the particle modulations are examined within the context of these observations.