Magnetospheric plasma regimes identified using Geotail measurements: 1. Regime identification and distant tail variability

Abstract

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 the entire deep‐tail portion of Geotail data at timescales down to ∼l min. Criteria were developed for selection of five basic plasma regimes: plasma sheet, lobe, magnetospheric boundary layer, magnetosheath, and solar wind. In addition to low‐energy plasma data central to the identification process, critical supplementary information derives from magnetic field and plasma wave data. We find that all basic plasma regimes remain physically distinct populations at all downtail distances sampled by the Geotail spacecraft. Preliminary automated separation of subregimes using a B versus δB method shows that plasma mantle intervals increase in probability of occurrence from 18% near Earth to about 50% near 205 RE. In contrast, plasma sheet intervals decrease overall with increasing tailward distance. Beyond about X = −100 RE, the frequency of short regime intervals steadily increases, overall variability increases, and the lobe and plasma sheet are progressively displaced by the boundary layer, here mostly plasma mantle. Our plasma regime identifications provide a critical baseline for the development of automated identification and a key resource for both Geotail data analysis and correlative studies between Geotail and other International Solar Terrestrial Physics mission data sets.