Mauk et al. (1981) have presented data on modulated precipitation observed by a new X ray camera, capable of imaging an 80-km field of view. There is some uncertainty, however, over the nature of the modulation and whether it represents a standing or traveling wave structure. A similar event showing a periodic modulation of the precipitating electron flux observed with a multi-narrow-beam riometer and concurrent Scandinavian twin auroral radar experiment radar data is considered. These data suggest that the observations of Mauk et al. are consistent with the apparent poleward motion of tracers associated with resonant Pc 5 events.