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 identificat ions through the probability of observation P at downtail distances of similar to 30 < x(agsm) < 210 R-E, in an aberrated GSM coordinate sys tem. The data are from Geotail spacecraft measurements obtained from O ctober 1992 through October 1994. Earth's local plasma environment has been classified into five principal plasma regimes: PS, the plasma sh eet, LB, the lobe (vacuum lobe, with only trace plasma content), and B L, the magnetospheric boundary layer, which together comprise the magn etosphere 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 :ma gnetotail (30 < X-agsm < 100 R-E): At geomagnetic activity index value s Kp < 3o, (a) The average magnetospheric morphology appears nominal, with no large-scale dawn-dusk asymmetries; (b) PS is the regime most l ikely 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 magne tosheath 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 asym metry; (e) P(BL) increases sharply at x(agsm) > 60 R-E and remains hig h 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) Co nditions appear more chaotic. PS, BL, or MS can be observed anywhere i nside the nominal magnetotail domain; (b) Compared to the middle magne totail, 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 equa l to 3o); (c) P(PS) is centered dawnward of the X-AGSM axis for both l ow 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 strong ly on x(agsm) than on Kp, however the Y-AGSM distribution of P(PS) dep ends strongly with both x(agsm) and Kp. Magnetotail width varies weakl y with x(agsm) and Kp.