A search for dayside geomagnetic Pi2 pulsations in the CHAMP low‐Earth‐orbit data

Abstract

Pi2 pulsations are typically observed at night and often during local daytime at low‐latitude ground stations. Their daytime occurrence is most common and clearest near the dip equator; however, daytime Pi2s are regularly observed at low‐latitude ground stations away from the dip equator. In contrast, a study utilizing satellite data at low latitudes within the plasmasphere could find no evidence for dayside Pi2s. Consequently, in this investigation we used high‐resolution vector magnetometer data from the CHAMP satellite to carry out a search for daytime Pi2 pulsations in the F region ionosphere. We confirm that nighttime Pi2s are observed simultaneously on CHAMP and at low‐latitude ground stations and that their characteristics are consistent with a cavity mode resonance. However, our comparisons of daytime Pi2 pulsation events identified in ground station data with data from CHAMP clearly demonstrate that Pi2s in the dayside hemisphere differ from their nighttime counterparts. Specifically, we find no convincing evidence of their occurrence in CHAMP data. Our results suggest that the generation mechanism for daytime Pi2s may be similar to the mechanisms that other authors have proposed to explain sudden impulses and DP2 current systems. We propose a model in which the electric field imprinted by the field‐aligned currents connected to the substorm current wedge is distributed globally in the atmospheric waveguide. Toroidal currents flowing on the dayside between ionosphere and ground generate magnetic fields that are confined to the waveguide. As a consequence, no Pi2‐related magnetic signals can be detected in the topside ionosphere.