A daytime Pi3 wave event observed by Cluster and ground stations

Pi3 waves (1.67-6.7 mHz) are in the same frequency band as Pc5 waves but have an irregular wave form, are usually thought to be induced by ionospheric currents, and are associated with geomagnetic substorms. Here we present observations of a global Pi3 wave event, frequency 3.1 mHz, recorded by the Cluster satellites and at multiple ground stations during local daytime on 1 March 2004. The event occurred in ground data over a wide range of latitudes from the magnetic equator to the polar cap, with power increasing with geomagnetic latitude and maximizing near the magnetospause. This suggests that the Pi3 waves were excited by an exterior source. At low and mid-latitudes, wave amplitude maximized prenoon and postnoon respectively, and decreased rapidly toward nighttime, indicating the Pi3 wave event was probably not caused by geomagnetic substorms. The wave H component was in phase and the D component out of phase at conjugate sites, suggesting the waves were field line guided. The waves were seen by Cluster at the high latitude magnetopause boundary near the southern cusp, with their transverse components in phase with those at the ground. During this event, the magnetosphere was rather quiet. We suggest that the Pi3 wave be generated by a jump of the solar wind speed which induced a surface wave at the magnetopause via the Kelvin-Holmhelts instability and then propagated into the magnetosphere.; Figure 1 The magnetic H component of the Pi3 waves observed by multiple ground stations and Cluster in the high latitude boundary near the southern cusp. The H components were nearly in phase among the stations. ; Figure 2 Same as Figure 1 but for the magnetic D components which were nearly out of phase between the two hemisphere.