Four low-frequency modes may propagate in a high-beta nearly bi-Maxwellian plasma, the magnetosonic, Alfvén, ion acoustic, and mirror modes. This manuscript 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 which 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 that we discuss. 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 (AMPTE/IRM) 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. Alfvén-like modes are observed for one event in the outer magnetosheath (near the bow shock), and are probably observed in two others.
AGU Index Terms: 2752 MHD waves and instabilities; 2728 Magnetosheath; 7871 Waves and instabilities
Keywords/Free Terms: ULF waves, mode identification, magnetosheath, transport ratios, quasi-parallel mirror mode.
JGR-Space 98JA02196
Vol. 103
, No. A10
, p. 23,661