Spatial structure of ULF waves: comparison of magnetometer and Super Dual Auroral Radar Network data

The spatial structure of ultralow frequency (ULF) waves is usually, though not exclusively, estimated from ground-based magnetometer measurements. Thi s paper compares ULF wave spatial structure obtained from coincident ground magnetometer and HF radar measurements and addresses the interpretation of Pc5 azimuthal wave numbers. ULF spatial structures estimated from magnetom eter and radar data were quite different for the October 23, 1994, event pr esented by Ziesolleck et al. [1998]. Azimuthal wave numbers (m) were 3-5 an d 12 for the ground and ionosphere, respectively. We reexamine this event a nd attempt to explain why the spatial structure of the ULF wave in the iono sphere, seen by the Saskatoon Super Dual Auroral Radar Netwrork (SuperDARN) radar, may differ from that deduced from the magnetometer data. The radar data are used to develop a two-dimensional (2-D) model of the spatial distr ibution of ULF amplitude and phase in the ionosphere. Our modeling shows th at the differences between ground and ionosphere measurements may be explai ned by spatial integration. In general, m numbers deduced from ground measu rements should be smaller than the ionospheric values, and they are strongl y dependent on the ionospheric ULF amplitude and phase distribution in both latitude and longitude.