High-m ULF waves in the terrestrial magnetosphere: spatial structure and generation mechanisms

Abstract:

The ultra-low-frequency (ULF) waves with high azimuthal wave numbers (m~20-150) are supposed to be generated by sources inside the magnetosphere. They have large poloidal component and often compressional of the perturbed magnetic field. The report is concentrated on three issues. 1) The nature of the high-m ULF waves. It is usually supposed that they are poloidal Alfven waves, but other interpretations (like drift compressional modes and slow modes) are also suggested. 2) The spatial structure of these waves. There are two possibilities: the waves standing across the L-shells and the waves running across L-shells with changing polarization from the poloidal to toroidal. Moreover, they can have non-trivial stricture along the field line, being captured in the resonators near the ionosphere. 3) The generation mechanisms. The proposed mechanisms include plasma instabilities due to the non-maxwellian particle distribution function, and alternating currents of substorm-injected energetic particles drifting in the azimuthal direction. These issues are considered both from the observational and theoretical perspective with the focus on some recent advances.