Electromagnetic ion cyclotron waves in the near-Earth magnetotail

This study considers the characteristics and generation mechanisms of burst s of polarized transverse electromagnetic waves observed over the frequency range from 50 up to 500 mHz in the near-Earth magnetotail from ISEE 1 and ISEE 2. Since this range contains the cyclotron frequencies of the ion comp onents found in this region of space, we loosely term these bursts "electro magnetic ion cyclotron waves" (EMICs). Nearly all wave events fall in the f requency and wavelength ranges of 50 to 150 mHz and 1000 to 6700 km, respec tively, with amplitudes of the order of 1 nT and may exhibit both left- and right-hand polarizations. These waves propagate at wavenormal angles, thet a(k)<45 degrees, and exhibit wave amplitudes where the ratio of the amplitu de parallel (b(parallel to)) to the background magnetic field (B-0) to that perpendicular (b(perpendicular to)) to B-0 varies proportionally with thet a(k), and this ratio is typically less than 0.5. ISEE 1 LEPEDEA distributio n function data indicate that the polarized bursts are often accompanied by anisotropic ion distributions and or significant field-aligned currents. I t is demonstrated that a positive correlation between the wave properties p redicted from linear Vlasov theory and the observed field fluctuations exis ts for most wave events considered when the plasma is modeled using both bi -Maxwellian and bi-Lorentzian functional forms. The occurrence of these wav e bursts is commonly synchronous with the decreasing phase of large-scale s urface waves permeating the magnetotail, thereby suggesting that these wave s provide a means of energy dissipation for global processes.