Identification of low-frequency magnetosheath waves

Four low-frequency modes may propagate in a, high-beta, nearly bi-Maxwellian plasma, the magnetosonic, Alfven, ion acoustic, and mirror modes. This study uses a procedure based on Linear Vlasov theory for the identification of these modes by use of transport ratios, dimensionless ratios of the fluctuating field and plasma quantities. A single parameter, the mode, deviation, is calculated which characterizes the difference between the theoretical transport ratios of a particular mode and the observed ratios. As well as determining the mode that best describes the observed fluctuations, it gives us a measure of whether or not the resulting identification is unique, Unfortunately, a unique identification is not always possible because of problems discussed herein. One problem is that the parallel phase ratio (related to the phase angle between density and parallel magnetic fluctuations) is often not well defined. Using the plasma and magnetic field data gathered by the Active Magnetospheric Particle Tracer Explorers/Ion Release Module spacecraft, we calculate the mode deviations and identify the modes observed in the magnetosheath. The quasi-perpendicular (with wave vector at a large angle to the background magnetic field) mirror mode is clearly identified in the inner (close to the magnetopause) and middle magnetosheath. The quasi-parallel mirror mode may be observed in the inner magnetosheath, but that identification is not certain. Alfven-like modes are observed for one event in the outer magnetosheath (near the bow shock) and are probably observed in two others.