Local time‐dependent Pi2 frequencies confirmed by simultaneous observations from THEMIS probes in the inner magnetosphere and at low‐latitude ground stations

Abstract

Using electric and magnetic field data from Time History of Events and Macroscale Interactions during Substorms (THEMIS) fast mode waves away from midnight. probes (TH‐A, TH‐D, and TH‐E) acquired in the inner magnetosphere (L < 4) and low‐latitude ground magnetic field data from Bohyun (BOH, L = 1.35) and Hermanus (HER, L = 1.83) stations, we studied longitudinal variations of Pi2‐associated fast mode waves away from midnight. We selected 48 nightside Pi2 events at BOH identified when THEMIS probes were in the inner magnetosphere for 1 month (February 2008). During that period the probes were located between 4.0 and 20.0 magnetic local time (MLT), i.e., at and on either side of the dayside sector. This choice was motivated by our interest in studying loss of Pi2 wave energy away from its nominal source at the nightside, all the way to the dayside. Between 4.0 and 8.0 MLT the probes often observed poloidal oscillations in space during BOH Pi2 events. The poloidal oscillations had high coherence (>0.6) with BOH Pi2s and radially standing fast mode structures. Thus, these fast mode waves are explained by plasmaspheric resonance. On the duskside, however, few events at THEMIS probes had high coherence with BOH Pi2s. Furthermore, the THEMIS probe data showed no evidence of Pi2 signals at 9.0–18.0 MLT, which is consistent with previous studies. Most of the high‐coherence events were detected when the local time separation between the THEMIS probes and BOH was less than 3 h. These observations suggest that Pi2 wave energy is lost as it propagates azimuthally from a source region localized in longitude. From longitudinally separated simultaneous multipoint observations at THEMIS probes and BOH and HER stations, we found that the Pi2 frequency varies with longitude both in space and on the ground. This implies that although plasmaspheric fast mode waves establish a standing wave structure on a given meridional plane, their frequency changes with longitude if the plasmasphere is not axisymmetric. Finally, we show that a low‐latitude daytime Pi2 is not a fast mode wave propagating to the dayside through the magnetosphere.