This paper reports substorm-related cosmic radio noise absorption events observed deep into the polar cap, well poleward of the typical nightside auroral oval. We find that such events usually occur in the late expansion/recovery phase of a substorm during moderate-to-strong planetary disturbances (Kp=3-8) and at times of high solar wind speed conditions. We present riometer data from U.S. manned and automatic geophysical observatories in Antarctica to track the morphology and progression of auroral absorption features over 5 hours of local time at ≥80¿ magnetic latitude. The azimuthal motions observed, westward pre-midnight, eastward post-midnight, are similar to the dynamics expected for lower latitude substorm-related auroral absorption. ULF modulation of the absorption (and hence of the causative 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 narrow, ~1¿ latitude band embedded within more uniform precipitation. Pc 5 (150 to 600 s periods) magnetic variations with similar spectral peaks, and auroral arcs are often observed concurrently. Several mechanisms for producing the particle modulations are examined within the context of these observations.¿ 1997 American Geophysical Union