Electromagnetic ion cyclotron waves in the near-Earth magnetotail

This study considers the characteristics and generation mechanisms of bursts 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 components found in this region of space, we loosely term these bursts "electromagnetic ion cyclotron waves" (EMICs). Nearly all wave events fall in the frequency and wavelength ranges of 50 to 150 mHz and 1000 to 6700 km, respectively, with amplitudes of the order of 1 nT and may exhibit both left- and right-hand polarizations. These waves propagate at wavenormal angles, theta(k)<45 degrees, and exhibit wave amplitudes where the ratio of the amplitude parallel (b(parallel to)) to the background magnetic field (B-0) to that perpendicular (b(perpendicular to)) to B-0 varies proportionally with theta(k), and this ratio is typically less than 0.5. ISEE 1 LEPEDEA distribution function data indicate that the polarized bursts are often accompanied by anisotropic ion distributions and or significant field-aligned currents. It is demonstrated that a positive correlation between the wave properties predicted from linear Vlasov theory and the observed field fluctuations exists for most wave events considered when the plasma is modeled using both bi-Maxwellian and bi-Lorentzian functional forms. The occurrence of these wave bursts is commonly synchronous with the decreasing phase of large-scale surface waves permeating the magnetotail, thereby suggesting that these waves provide a means of energy dissipation for global processes.