Coherent EMIC Waves and Pitch Angle Scattering of Energetic Electrons in the Magnetosphere

Coherency (C) and ellipticity (ɛ) of electromagnetic ion cyclotron (EMIC) waves are studied in detail using spacecraft magnetometer data in the Earth's dayside subsolar magnetosphere between L = 7 and 10 during the Earth flyby of Cassini on 18 August 1999. The study covers the equatorial magnetic latitudes from 3.1° to 1.5° MLAT where EMIC waves are believed to be generated. The observations are compared to linear dispersion theory and 1D and 2D simulations. The EMIC waves are found to occur in packets with multiple wave cycles. EMIC waves have a mixture of circular and elliptical polarization for waves propagating within 30° of the magnetic field, θkB0 < 30°. The wave cycles within a wave packet are observed to have the same general θkB0 values and polarization, so the ellipticity variation is typically from packet to packet. The 2D simulation results show a similar feature. The variation of the packet to packet ellipticity in satellite observations and in the 2D simulations for θkB0 < 30° is speculated to be due to the interference from various other wave modes. For wave propagation angles θkB0 between 30° and 60°, the waves are highly elliptical and for θkB0 > 60°, the waves are nearly linearly polarized. This general trend is in good agreement with linear kinetic theory and the 1D simulations. From linear theory and 1D simulations, we find that RH EMIC waves are generated when waves have quasi-perpendicular propagation directions (θkB0 > 60°). EMIC waves are observed to be coherent to quasi-coherent with 0.5 < C < 0.8 for θkB0≤ 45°. EMIC waves are generally noted to be coherent (C=1.0) to quasi-coherent (C > 0.5) in nature (C is the maximum cross correlation coefficient value). The coherency is independent of wave ellipticity. For specific cases, coherent EMIC waves are shown to be highly effective in the pitch angle scattering of 1 MeV relativistic electrons to their loss cones. If we assume that relativistic electrons stay in cyclotron resonance for 4 EMIC wave cycles, then they would be pitch angle transported by 10° in a single, sustained 8 ms coherent cyclotron resonant interaction. A statistical survey of the resonant wave-particle interactions could possibly give an answer for the dominant loss mechanism for protons and energetic electrons in the Earth's magnetosphere.