MAGNETIC FIELD-ALIGNED ELECTRODYNAMICS OF ALFVEN ION-CYCLOTRON WAVES

We report on the first experimental determination of the magnetic field-aligned electric field (E(parallel-to)) of an Alfven/ion cyclotron (A/I) wave. The wave was observed propagating in the geostationary regions of the Earth's magnetosphere with a frequency of 1/3 the proton cyclotron frequency. Electron oscillations parallel to (B) are used in the determination of E(parallel-to). E(parallel-to) is found to be a factor of approximately 18 greater than anticipated from cold plasma theory(i.e., electron inertial effects). The inclusion of electron pressure and pressure anisotropy brings reasonable agreement between the observations and linear wave theory.