Poloidal Pc 4
Typical Pc 4 pulsations (period close to 100 s, but reaching sometimes to Pc 5 range) are radially polarized transverse ULF waves localized close to the equator to a region which is radially very narrow, but longitudinally extending over several hours in the dusk sector (L being close to 6 at noon, and 8 at midnight). They are locally generated by freshly injected ions, but since the occurrence reaches its peak during more quiet period after the substorms , some other mechanism is also affecting. For example, plasmaspheric refilling may be an important factor. Ion fluxes are shown to oscillate at the same frequency with the waves, with energy dependent change in phase (the phase being same for about 150 keV protons). The fluxes from the north and south look directions have opposite phase indicating a bounce resonant motion. In addition, there is a phase lag between fluxes in the eastside and westside particles, indicating westward propagation, as expected for drifting ions. See, e.g., Anderson (1994).
Because the waves also occur at the second harmonic field line resonance frequency, they are likely to be guided poloidal mode waves (expected to occur with high azimuthal wave numbers, as observed) and are thought to be driven by bounce resonance with energetic (150 keV) ions.
Note that the dayside multi-harmonic pulsations sometimes called toroidal Pc 3 waves reach occasionally frequencies that are in the Pc 4 and Pc 5 range.
On rare occasions (few times a year or so), ground based observations in the morning sector show highly monochromatic amplitude modulated oscillations in the Pc 4 range (see the figure below). These pulsations have been termed 'giant pulsations' or pg's (see, e.g., Rostoker et al., 1979; Green, 1985), although maximum amplitude is typically some tens of nT (small compared to Pc 5; however, other Pc 4 pulsations have smaller amplitude). It is thought that protons with energies of about 5-30 keV drifting from the nightside are causing the pulsations. As these particles circle the Earth and reach the morning hours only when the ExB drift is small, giant pulsations are observed only during geomagnetically quiet times (Chisham, 1996). During stable quiet conditions they can continue for two or more days, being quashed during substorm times.
The giant pulsations are highly localized in latitude and have large azimuthal wave number. They are most typical around equinoxes and solar cycle minimum. The region of the pulsations moves poleward during the course of the pulsation activity. The period increases with latitude, as expected for locally resonant field lines. The polarization of the waves is azimuthal on the ground, i.e., radial in space. (For more details about the polarization, see Chisham et al., 1997.) Engebretson et al. (1988) has discovered a subset of Pc 3-4 pulsations localized at or within 1-2 degrees of the geomagnetic equator, that may be related to the giant pulsations. In these events, the pulsation activity is characterized by intense fluxes of warm, highly trapped light ions and/or electrons outside the plasmapause and the ring current, at times in association with streaming low energy plasma.
- Anderson, B. J., An overview of spacecraft observations of 10 s to 600 s period magnetic pulsations in the Earth's magnetosphere, in Solar Wind Sources of Magnetospheric Ultra-Low-Frequency Waves, eds. M. J. Engebretson, K. Takahashi, and M. Scholer, AGU Geophysical Monograph 81, 25-43, 1994.
- Chisham, G., Giant pulsations: An explanation for their rarity and occurrence during geomagnetically quiet times, J. Geophys. Res., 101, 24755-24763, 1996.
- Chisham, G., I. R. Mann, and D. Orr, A statistical study of giant pulsation latitudinal polarization and amplitude variation, J. Geophys. Res., 102, 9619-9629, 1997.
- Engebretson, M. J., L. J. Zanetti, T. A. Potemra, D. M. Klumpar, R. J. Strangeway, and M. H. Acuna, Observations of intense ULF pulsation activity near the geomagnetic equator during quiet times, J. Geophys. Res., 93, 12795-12816, 1988.
- Green, C. A., Giant pulsations in the plasmasphere, Planet. Space Sci., 33, 1155-, 1985.
- Rostoker, G., H.-L. Lam, and J. V. Olson, Pc 4 giant pulsations in the morning sector, J. Geophys. Res., 84, 5153-5166, 1979.