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

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 similar to 30 < x(agsm) < 210 R-E, 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 < X-agsm < 100 R-E): 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 similar to 20 R-E of Earth; that is, the magnetosphere is inside and the magnetosheath is outside; and (c) P(BL) increases gradually with increasing x(agsm). At Kp greater than or equal to 3o, (d) The centroid of P(PS) lies duskward of the X-AGSM axis, producing a distinct dawn-dusk asymmetry; (e) P(BL) increases sharply at x(agsm) > 60 R-E and remains high tailward; and (f) At x(agsm) < 75 R-E, MS is rarely observed for \Y-AGSM\ < 20 R-E. In the distant magnetotail (x(agsm) > 100 R-E): (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 X-AGSM axis and P(PS) is significantly lower (most distinctly at Kp greater than or equal to 3o); (c) P(PS) is centered dawnward of the X-AGSM axis for both low and high Kp; and (d) At x(agsm) > 170 R-E, P(BL) is greater than P(PS) or P(MS), with P(MS) approximate to P(PS). P(LB) > 0, but low, out to x(agsm) greater than or equal to 200 R-E. In general, the downtail distributions of the magnetospheric plasma regimes depend more strongly on x(agsm) than on Kp, however the Y-AGSM distribution of P(PS) depends strongly with both x(agsm) and Kp. Magnetotail width varies weakly with x(agsm) and Kp.