Modes of magnetohydrodynamic waves in the magnetosphere

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.