Hydromagnetic (HM) chorus events are ULF waves with typical frequenices from 0.2 to 0.6 Hz; their frequency-time spectrum consists of a combination of band-limited unstructured emissions and discrete elements. These waves are one of a class of short-period ULF emissions (in the Pc 1 and 2 frequency range) that may be of value in ground-based identification 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 data from South Pole Station (-74.2¿ geomagnetic latitude, local noon ~1530 UT) and McMurdo, Antarctica (-80.2¿ geomagnetic latitude, local noon ~2030 UT) during 1990, and at McMurdo and Sondrestromfjord, Greenland (74.2¿ geomagnetic latitude, local noon ~1330 UT) during 1988. In agreement with previous studies, these events tend to occur within a few hours of local noon.

Earlier studies reported that HM chorus events are typically seen in ground records at auroral zone latitudes and suggested that they originated in the outer dayside magnetosphere. Study of DMSP particle boundary data indicated that the events reported here occurred during conditions of extremely contracted auroral ovals and are thus also consistent with an outer dayside magnetospheric source region. However, the occurence of HM chorus events in this high-latitude data set was limited to the months of October through May, with an occurrence peak in February, and no events were found in the months of July through September in either of the years studied. Since both hemispheres are covered, this suggests not a seasonal variation (with late winter minimum) but rather an apparent annual variation. We speculate that such an annual variation in very high latitude occurrence of HM chorus may be related to the MLT-UT offset of the Earth's geomagnetic and geographic pole, in conjunction with wave propagation cutoffs at the high-latitude magnetic field minima occurring on dayside magnetic field lines very near the magnetopause. ¿American Geophysical Union 1995