Coherency and ellipticity of electromagnetic ion cyclotron waves: Satellite observations and simulations

Coherency (C) and ellipticity (c) of electromagnetic ion cyclotron (EMIC) waves are studied using Cassini data in the Earth's dayside low-latitude magnetosphere from L = 7 to 10. The results are compared with linear kinetic theory, 1-D and 2-D simulations. The EMIC waves are observed to occur in packets with multiple wave cycles. The wave cycles within a wave packet are observed to have the same general propagation angle theta(kB0) and polarization. In observations and 2-D simulations, EMIC waves have a mixture of circular and elliptical polarization for theta(kB0) < 30 degrees. This scattered ellipticity values are due to the superposition of multiple wave modes. For wave propagation angles 30 degrees < theta(kB0) < 60 degrees, the waves are highly ;elliptical (0.2 < |epsilon| < 0.7), where epsilon is the ratio of minor to major axis of the polarization ellipse. For theta(kB0) > 60 degrees, the waves are nearly linearly polarized (|epsilon| <= 0.1). This general trend is in good agreement with linear kinetic theory and 1-D simulations. Observations indicate right-hand (RH) wave packets to be interspersed with the left-hand (LH) wave packets. We show for the first time from linear theory that ion temperature anisotropies can generate RH waves at large propagation angles and for plasma beta.. beta(i) > 0.05. The observed mixture of RH and LH waves in the magnetosphere could be due to this direct generation of RH waves. Observations and simulations show that EMIC waves are coherent with 0.5 < C < 1.0 for theta(kB0) <= 50 degrees. Here C is measured as the maximum value of cross-correlation coefficient between the transverse magnetic field components of the wave.