Identification of Magnetospheric Plasma Regimes from the Geotail Spacecraft

T. E. Eastman, S. P. Christon, D. Williams, A. T. Y. Lui, R. W. McEntire, E. Roelof, T. Doke, L. A. Frank, W. R. Paterson, S. Kokubun, H. Matsumoto, H. Kojima, R. R. Anderson, T. Mukai, Y. Saito, T. Yamamoto, K. Tsuruda.
J. Geophys. Res., submitted for publication, 1996.

For many important scientific objectives, the analysis and interpretation of magnetospheric data sets require a sorting by primary plasma regimes. This sorting process is much more difficult in the distant magnetotail than for data sets sampled close to Earth. We have carried out systematic regime identifications for most of the deep-tail portion of Geotail data at time scales down to about one minute. Criteria were developed for selection of five basic plasma regimes: plasma sheet, lobe, magnetospheric boundary layer, magnetosheath, and solar wind. Although low-energy plasma data is central to the identification process, critical supplementary information derives from magnetic field and plasma wave data. Our plasma regime identifications provide a critical baseline for efforts towards automated identification. As a first application, data from the Energetic Particle and Ion Composition (EPIC) instrument, filtered according to our plasma regime identifications, have revealed new and important systematic effects in energetic ion spectra, composition, and anisotropy, including statistical correlations that are substantially masked with less effective filters, such as sorting by radial distance. Our identifications provide a key resource for both Geotail data analysis and interpretation and correlative studies between the Geotail and other ISTP mission data sets.