email@example.com - last update: 5 January 1998, 1515 UT (RR)
Compressional Pc 5 (ULF) waves, found on the nightside close to the flanks of the magnetosphere, are the dominant pulsations occurring beyond L=8. The compressional (along magnetic field) perturbation of the wave is often accompanied by a comparable radial perturbation, amplitude of the oscillations being large with periods of 5 to 10 minutes. These waves are, most likely, locally generated drift mirror waves driven by positive plasma pressure anisotropy. Accordingly, the events are strongly associated with high beta plasma clouds and are much more likely to be observed near the equator than >10° off the equator (see, e.g., Anderson, 1993). The azimuthal wavelength of the waves is short, m = 50-100, and the particle signatures associated with the waves show anti-phase correlation between the ion and magnetic field pressures.
The dusk and dawn compressional Pc 5 occur under quite different geophysical conditions. While the evening side events occur during enhanced ring current, high AE index, and strongly southward IMF ("storm time Pc 5" related to the substorm particle injection), the dawnside events occur during small negative Dst (weak ring current), low AE, and northward IMF. The dusk side waves propagate westward with the speeds comparable to energetic (10-30 keV) proton drift speed, while the dawn side events propagate eastward because of the general drift direction: in plasma frame they propagate westward (see, e.g., Anderson, 1994).
Azimuthally polarized toroidal Pc 5 pulsations are thought to be fundamental mode toroidal field line resonances (FLR) with magnetic node at the equator. They are considered to be a separate phenomena from the multi-harmonics (discussed in the Pc 3 section) because of the difference in occurrence distribution: toroidal Pc 5 are found close to the flanks of the magnetosphere either both at dawn and dusk (as derived from some ground based studies), or only at dawn side (as derived from satellite measurements). The reason for this discrepancy is still not known, but it can be that the toroidal waves present at dusk are just more difficult to separate (by satellite measurements) from other waves occuring there. (To make things even more complicated, electric field measurements of the waves have shown a broad dayside distribution!) However, because the occurrence rate increases monotonically with L, the energy source of the waves is though to be near the magnetopause boundary. Some measurements support the idea that some form of wave structure commonly propagates anti-sunward at the flanks producing undulating motions of the magnetopause boundary. The present theories for the wave generations are:
Also the nightside injections of electrons seem to have an effect on these Pc 5 waves (e.g., Saka et al., 1992).
The only found relation between toroidal Pc 5 and the solar wind/IMF conditions is that stronger events occur during higher average solar wind velocity. This may, however, be only a magnetospheric compression effect (the source of the waves comes closer to the measurement point). Besides, increasing power with solar wind velocity and density is found for all other pulsations as well, even those driven by local ion instabilities, so this correlation does not tell much about the source mechanism for the waves. See, e.g., Anderson (1994).
When measured from the ground, the toroidal Pc 5 pulsations show a 90 degrees counter clockwise rotation of the magnetic field variation due to modifications in the ionosphere (Hughes, 1974).For a recent study of ground based Pc 5 pulsations measurements, see Chisman and Orr (1997).
Various ground-based measurements have shown that some auroral features correlate with non-FLR wave activity at Pc 5 frequncy range (Elphinstone, ICS-3).