The spatial structure of ultralow frequency (ULF) waves is usually, though not exclusively, estimated from ground-based magnetometer measurements. This 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 magnetometer and radar data were quite different for the October 23, 1994, event presented by Ziesolleck et al. [1998>. Azimuthal wave numbers (m) were 3--5 and 12 for the ground and ionosphere, respectively. We reexamine this event and attempt to explain why the spatial structure of the ULF wave in the ionosphere, seen by the Saskatoon Super Dual Auroral Radar Network (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 distribution of ULF amplitude and phase in the ionosphere. Our modeling shows that the differences between ground and ionosphere measurements may be explained by spatial integration. In general, m numbers deduced from ground measurements should be smaller than the ionospheric values, and they are strongly dependent on the ionospheric ULF amplitude and phase distribution in both latitude and longitude. ¿ 2001 American Geophysical Union