STATISTICAL STUDY OF HYDROMAGNETIC CHORUS EVENTS AT VERY HIGH-LATITUDES

Hydromagnetic (HM) chorus events are ULF waves with typical frequencie s from 0.2 to 0.6 Hz; their frequency-time spectrum consists of a comb ination of band-limited unstructured emissions and discrete elements. These waves are one of a class of short-period ULF emissions (in the P c 1 and 2 frequency range) that may be of value in ground-based identi fication of the footpoint of magnetospheric boundary regions. As part of an extensive survey of Pc 1 and 2 waves at very high latitudes, we have identified all occurrences of HM chorus in nearly a full year's d ata from South Pole Station (-74.2 degrees geomagnetic latitude, local noon similar to 1530 UT) and McMurdo, Antarctica (-80.2 degrees geoma gnetic latitude, local noon similar to 2030 UT) during 1990, and at Mc Murdo and Sondrestromfjord, Greenland (74.2 degrees geomagnetic latitu de, local noon similar to 1330 UT) during 1988. In agreement with prev ious studies, these events tend to occur within a few hours of local n oon. Earlier studies reported that HM chorus events are typically seen in ground records at amoral zone latitudes and suggested that they or iginated in the outer dayside magnetosphere. Study of DMSP particle bo undary data indicated that the events reported here occurred during co nditions of extremely contracted auroral ovals and are thus also consi stent with an outer dayside magnetospheric source region. However, the occurrence of HM chorus events in this high-latitude data set was lim ited to the months of October through May, with an occurrence peak in February, and no events were found in the months of July through Septe mber in either of the years studied. Since both hemispheres are covere d, this suggests not a seasonal. variation (with late winter minimum) but rather an apparent annual variation. We speculate that such an ann ual variation in very high latitude occurrence of HM chorus may be rel ated to the MLT-UT offset of the Earth's geomagnetic and geographic po les, in conjunction with wave propagation cutoffs at the high-latitude magnetic field minima occurring on dayside magnetic field lines very near the magnetopause.