Simultaneous Determination of Magnetospheric Mass Density at the Time of Large Pc-5 Fluctuations Using an Electrostatic Analyzer and Toroidal Alfven Frequencies

Measuring plasmaspheric mass density with particle detectors is usually a difficult task, as cold ions often dominate the mass density, and these do not have a sufficient flux into the detector to be measurable. When there is a significant bulk flow, however, the cold ions can be detected as they sweep across the spacecraft. Such a circumstance occurs when the toroidal Alfven wave (field line resonance, often at Pc5 frequency) has large amplitude, and therefore a large velocity perturbation. Using the Los Alamos National Laboratory Magnetospheric Plasma Analyzer (MPA) onboard geosynchronous spacecraft, we measure the ion composition when such large amplitude waves are present. Because the fluctuating flow exceeds the thermal speed of the particles, populations of H+, He+, and O+ can be distinguished using the E/q discrimination of the MPA. In this way, the mass density is determined. At the same time, we determine the mass density from the frequency of the toroidal Alfven wave. A Tsyganenko 96 magnetic field model is used, and the field line dependence of the mass density is assumed to be that found from Polar spacecraft electron density measurements. The wave equation is solved, and the mass density is adjusted so that the theoretical and observed frequencies are the same.