Typical classification scheme for the ULF waves is according to the period of the pulsation (Jacobs et al., 1964):
|Continuous pulsations||Irregular pulsations|
|Pc 1||Pc 2||Pc 3||Pc 4||Pc 5||Pi 1||Pi 2|
|f||0.2-5 Hz||0.1-0.2 Hz||22-100 mHz||7-22 mHz||2-7 mHz||0.025-1 Hz||2-25 mHz|
Pulsations are studied with in situ observations in space of both magnetic and electric fields, and with ground-based magnetometers. In order to study the generation mechanisms of these waves, several points should be studied, including
At lower frequencies continuous pulsations sharing the same frequency range may differ, e.g., in their polarization, harmonic structure, or occurrence region, and have thus different origin. They can be divided according to whether they result from a local wave-particle instability or from coupling of wave energy propagating through the magnetophere and produced either in the solar wind / magnetosheath or at the magnetopause / boundary layer (all referred to as "upstream" below). Latter type relate to resonances of field lines and magnetospheric cavity. One can categorize these waves in the following way (Anderson, 1993, 1994):
|Compressional Pc 3||dayside||upstream||relate to wave-particle interaction in the foreshock and shock|
|Toroidal Pc 3 or multi-harmonics||dayside||upstream||field line resonance harmonics in Pc 3 / Pc 4 range, compressional Pc3 as a driver (coupling with the fundamental toroidal mode)|
|Poloidal Pc 4||afternoon|
|local||related to injections of energetic plasma and subsequent low activity or convection electric field; occurring at the second harmonic field line resonance frequency|
|Compressional Pc 5||nightside dawn and dusk||local||related to of high beta plasma (ion injections)|
|Toroidal Pc 5||dawn and dusk flanks||upstream||fundamental mode field line resonances; source in the flanks|
|Incoherent noise||everywhere||unknown||increases with magnetic activity|
For upstream generated waves, one important task is to establish linkage between waves external and internal to the magnetosphere. The question is, what are the sites of transfer of external to internal wave energy. One model would have compressional oscillations in the foreshock (see bow shock ) propagate directly through the shock, sheath, and subsolar magnetopause into the lower magnetosphere, while the other would have waves entering along cusp /cleft/boundary layer field lines, transferring to the interior dayside magnetosphere via an ionospheric process.
Note that actually all pulsations, ultimately, derive their energy from the solar wind. This is because the particles taking part in the local instabilities are energized via the convection electric field imposed by the solar wind. Similarly the precipitating particles and impinging electric field in the ionosphere are solar wind powered.