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Two extensive series of measurements of magnetospheric ULF waves in the Pc 3 frequency range (22–100 mHz) have been carried out in pairs of conjugate areas (Kerguelen-Sogra and Siple-Lac Rebours) which have approximately the same geomagnetic latitudes (L ∼ 3.5–4.0) but are separated by more than 8 hours in local time. The results of these measurements are compared in this review, and some differences in the results are discussed in terms of possibly differing ionospheric and ground conditions in the conjugate areas. Additional, possibly cooperative, studies to investigate further the magnetospheric wave phenomena in this frequency range are outlined.

Magnetohydrodynamic plasma waves (waves with frequencies less than the local ion gyrofrequency) constitute an important part of the wave frequency spectrum in the earth's magnetosphere and in the solar wind. Such plasma waves also probably play an important role in astrophysical plasmas. The nature and modes of the plasma waves excited in the earth’s magnetosphere are of interest because they are generated and propagate in a plasma environment imbedded in a dipole magnetic field configuration. As such, analyses of the nature of these waves may prove useful in studies of some planetary and stellar plasma environments. Measurements made at conjugate points on the earth's surface and in the magnetosphere between spacecraft altitudes and the ground provide information on the nature of the modes of the magnetohydrodynamic waves excited in the magnetosphere. This paper reviews the available data on wave modes and concludes that near L=4 (corresponding to geomagnetic latitudes ∼60°) the mode for waves in the period range ∼20 to ∼200 s is predominantly odd mode. This may be a fundamental aspect of the physical process that couples the driving source to the resonant field line. For other geomagnetic and frequency ranges there is presently a disappointing dearth of experimental observations.

Since the original study of PCA riometer measurements, solar energetic particles have been increasingly employed as test particles in the magnetosphere. The characteristics of the access of these particles to the magnetosphere are studied to determine if substantial connection exists between the earth’s dipole field and the solar-generated interplanetary field. Satellite measurements over the polar caps indicate strong north pole-south pole proton flux asymmetries that may be related to existing interplanetary particle anisotropies. High-latitude measurements indicate significant pseudo-trapping of the solar fluxes on closed magnetospheric field lines. Particle flux comparisons between satellites in interplanetary space and within the magnetosphere suggest that the particle access point is probably not greater than 1000 RE (earth radii) behind the earth in the magnetotail.