Multisatellite observations of a giant pulsation event

摘要

Giant pulsations (Pgs; frequency ~10 mHz) were detected with ground magnetometers on the North American continent on 19 October 2008, when the GOES‐10, ‐11, ‐12, and ‐13 geostationary satellites and the THEMIS‐A probe were magnetically connected to the region of the ground pulsation activity. This unique configuration allowed us to determine the properties of magnetospheric ultra‐low‐frequency (ULF) waves that caused the Pgs on the ground. All spacecraft detected monochromatic ULF waves at ~10 mHz, and the coherence between the Pg at the Gillam ground station and the ULF wave at THEMIS‐A was high when the magnetic field foot point of the spacecraft came close to the ground station. The ULF waves observed by the five spacecraft had perturbations in the radial and compressional components of the magnetic field and in the azimuthal component of the electric field, which are attributed to poloidal mode standing Alfven waves. The poloidal waves were accompanied by multiharmonic toroidal waves, and from the frequency relationship among these, it is concluded that the ~10 mHz oscillations correspond to the fundamental (odd, or symmetric) mode. The standing wave mode also explains the amplitude variation with latitude and the phase delay between the magnetic and electric fields. Numerical models of poloidal waves incorporating finite height integrated ionospheric conductivity indicate that the fundamental mode interpretation is valid even when the damping of the standing waves is strong. Our observations are the most comprehensive to date in terms of spacecraft data, and we believe that theoretical work on the Pg generation mechanism should focus on mechanisms specific to odd mode standing waves, such as drift resonance of ring current ions.