Characteristics of MHD waves associated with storm sudden commencements observed by SABRE and ground magnetometers

ULF pulsations occurring immediately after storm sudden commencements (SSCs) are caused by an impulse to the magnetosphere and might be expected to have a considerable compressional mode wave component. Examples of such pulsations have been studied with the SABRE VHF coherent radar in conjunction with a ground magnetometer at Lerwick and the SAMNET magnetometer array. The pulsations modulate SABRE backscatter intensities, but modulations of the ionospheric flow velocities are not observed. The backscatter intensity modulations are thought to be a consequence of both a greater radar sensitivity to fluctuations in backscatter intensity than velocity changes and also enhanced electron precipitation associated with the pulsations. These results are indicative of a compressional mode wave, which is expected to have a small ionospheric electric field and represent the first direct, ground based observational evidence of such modes away from their point of coupling to field line resonances. No appreciable period or phase variations with latitude occur in the radar data and the pulsation azimuthal wave number, m, is typically in the range 0-10. A maximum entropy spectral analysis indicates that the magnetometer pulsation may be dominated by the same frequency as the radar or by a higher harmonic. The dominant frequency in the magnetometer pulsation is dependent on the pulsation azimuthal wave number, with higher harmonic observations correlating with higher m-values. This behaviour is consistent with recent models of global modes.