The 2-D Structure of Foreshock-Driven Field Line Resonances Observed by THEMIS Satellite and Ground-Based Imager Conjunctions

Abstract

Recent studies of Pc5-band (150–600 s) ultralow frequency waves found that foreshock disturbances can be a driver of dayside compressional waves and field line resonance, which are two typical Pc5 wave modes in the dayside magnetosphere. However, it is difficult to find spatial structure of dayside Pc5 waves using a small number of satellites or ground magnetometers. This study determines 2-D structure of dayside Pc5 waves and their driver by utilizing coordinated observations by the THEMIS satellites and the all-sky imager at South Pole during two series of Pc5 waves on 29 June 2008. These Pc5 waves were found to be field line resonances (FLRs) and driven by foreshock disturbances. The ground-based all-sky imager at South Pole shows that periodic poleward moving arcs occurred simultaneously with the FLRs near the satellite footprints over ~3° latitude and had the same frequencies as FLRs. This indicates that they are the auroral signature of the FLRs. The azimuthal distribution of the FLRs in the magnetosphere and their north-south width in the ionosphere were further determined in the 2-D images. In the first case, the FLRs distribute symmetrically in the prenoon and postnoon regions with out-of-phase oscillation as the odd toroidal mode in the equatorial plane. In the second case, the azimuthal wavelengths of the 350–500 s and 300–450 s period waves were ~8.0 and ~5.2 Re in the equatorial plane. It also shows a fine azimuthal structure embedded in the large-scale arcs, indicating that a high azimuthal wave number (m ~ 140) mode wave coupled with the low-wave number FLRs.