Ion composition aspects of magnetotail plasma flows

Details Ion composition aspects of magnetotail plasma flows Ion composition aspects of magnetotail plasma flows

Aug 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001jgr...10615621l&link_type=abstract

Journal of Geophysical Research, Volume 106, Issue A8, p. 15621-15634

Physics

3

Magnetospheric Physics: Magnetotail, Magnetospheric Physics: Plasma Convection, Magnetospheric Physics: Plasma Sheet, Magnetospheric Physics: Solar Wind/Magnetosphere Interactions

Scientific paper

Recent archiving of energetic (<18 keV /e) ion composition data from the ISEE 1 satellite has created a 4.5-year World Wide Web based set well suited for statistical investigations of magnetospheric plasmas inside of 23 RE, especially with regard to signatures of solar (He++) or terrestrial (O+) origins. This is an application that investigates tail ion flows in the 10-23 RE radial range, specifically the (spacecraft spin plane) GSE x and y flows of H+, He++, and O+ ions in the 0.1-16 keV/e energy range. The plasma sheet is divided into (1) ``high,'' (2) ``intermediate,'' and (3) ``low'' latitude regions based on magnetic field GSE elevation angle λ and ion beta (sum of H+, He++, He+, and O+ ions), such that regions 1 and 2 both have |λ|<=9°, i.e., |Bz|<<(Bx2+By2)1/2, with region 1 being at β<0.1 and region 2 being at β>0.1, and region 3 has |Bz|>(Bx2+By2)1/2 and β>0.1. A fourth region has remaining plasma sheet samples (30%). In region 1 the most common drift is within +/-30° of the tailward magnetic field direction for all ions, including, somewhat surprisingly, the He++ ions (about half the time). Tailward drift speeds (<100 km s-1) are substantially smaller than thermal speeds (~500 km s-1) for both He++ and H+ ions. Only the O+ ions have a net field-aligned (and tailward) distribution (<2ν∥2><ν⊥2>-1~6). On average, the He++/H+ number density and mean energy ratios are ~4% and ~4, respectively, indicating a mostly solar origin of both species. The second most common drift of He++ and H+ ions (~20%) is aligned near the earthward field direction and has the largest speeds (>100 km s-1). It also has velocity dispersion, however, and seems to be associated with the leading edge of intermittent bursts of ions expanding earthward from the downtail plasma sheet. Superimposed on these drifts is E×B drift of varying speed and direction, which has a net average of the order of 10 km s-1 toward local midnight, away from the tail flanks. Comparing times (2-hour intervals) of northward and southward interplanetary magnetic field, drift speeds are some 50% larger in the southward case, and thermal speeds are 10-20% larger. In region 2, drifts are more random in direction and somewhat slower, whereas thermal speeds are 30-50% larger than in region 1. Only the O+ ions maintain dominantly tailward (field-aligned) drift. The average E×B drift is reversed, away from local midnight, but still of the order of 10 km s-1. In region 3, average E×B drift is sunward and away from midnight. Sunward drift occurs ~2.5 times more often than tailward drift with northward interplanetary magnetic field and almost 8 times more often with southward interplanetary magnetic field. Comparing all four regions, tailward flows of H+ and He++ ions in region 1 can account for some but only part (<=30%) of the sunward flows at lower latitude, implying that solar ions enter the plasma sheet both tailward and sunward of the ISEE 1.

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