Magnetospheric plasma regimes identified using Geotail measurements: 2. Statistics, spatial distribution, and geomagnetic dependence

Abstract

We investigate the spatial distribution, solar‐terrestrial correlation, and basic statistics of a long sequence of plasma regime identifications through the probability of observation P at downtail distances of ∼30 < xagsm < 210 RE, in an aberrated GSM coordinate system. The data are from Geotail spacecraft measurements obtained from October 1992 through October 1994. Earth's local plasma environment has been classified into five principal plasma regimes: PS, the plasma sheet, LB, the lobe (vacuum lobe, with only trace plasma content), and BL, the magnetospheric boundary layer, which together comprise the magnetosphere proper, and MS, the magnetosheath, and SW, the nearby solar wind. Eastman et al. [this issue] describe the process, rationale, and instrumentation used in the regime identifications. In the middle magnetotail (30 < xagsm < 100 RE): At geomagnetic activity index values Kp < 3o, (a) The average magnetospheric morphology appears nominal, with no large‐scale dawn‐dusk asymmetries; (b) PS is the regime most likely to be observed, and the spatial interrelationships of the PS, LB, BL, and MS appear similar to those commonly observed within −20 RE of Earth; that is, the magnetosphere is inside and the magnetosheath is outside; and (c) P(BL) increases gradually with increasing xagsm. At Kp ≥ 3o, (d) The centroid of P(PS) lies duskward of the XAGSM axis, producing a distinct dawn‐dusk asymmetry; (e) P(BL) increases sharply at xagsm > 60 RE and remains high tailward; and (f) At xagsm < 75 RE, MS is rarely observed for |YAGSM| < 20 RE. In the distant magnetotail (xagsm > 100 RE): (a) Conditions appear more chaotic. PS, BL, or MS can be observed anywhere inside the nominal magnetotail domain; (b) Compared to the middle magnetotail, PS and BL are observed more dawnward of the XAGSM axis and P(PS) is significantly lower (most distinctly at Kp > 3o); (c) P(PS) is centered dawnward of the XAGSM axis for both low and high Kp; and (d) At xagsm ≥ 170 RE, P(BL) is greater than P(PS) or P(MS), with P(MS) ≈ P(PS). P(LB) > 0, but low, out to xagsm ≥ 200 RE. In general, the downtail distributions of the magnetospheric plasma regimes depend more strongly on xagsm than on Kp, however the YAGSM distribution of P(PS) depends strongly with both xagsm and Kp. Magnetotail width varies weakly with xagsm and Kp.