Statistical study of the late substorm recovery phase and quiet time plasma sheet based on ISEE 1~30-keV ion observations

Details Statistical study of the late substorm recovery phase and quiet time plasma sheet based on ISEE 1~30-keV ion observations Statistical study of the late substorm recovery phase and quiet time plasma sheet based on ISEE 1~30-keV ion observations

Jun 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994jgr....9910981t&link_type=abstract

Journal of Geophysical Research, Volume 99, Issue A6, p. 10981-10994

Physics

2

Magnetospheric Physics: Plasma Sheet, Magnetospheric Physics: Magnetospheric Configuration And Dynamics, Magnetospheric Physics: Magnetotail, Magnetospheric Physics: Storms And Substorms

Scientific paper

We present a statistical survey of the near-Earth plasma sheet (r=7-23RE, distance from the neutral sheet center Zns=0-10RE,LT=2100-0200) during (late) substorm recovery phase and quiet time based on ISEE 1 medium energy particle instrument observations of first-order ~30-keV ion anisotropies, comprising ~112 hours of the 1978 tail season. The data reveal predominantly isotropic conditions, with intervals of generally weakly anisotropic (<10 highest-to-lowest flux ratio) field-aligned ion flows (FAIFs) and ion flux gradients. Both earthward and tailward streaming FAIFs are observed for Zns between 0 and 10RE and are probably unassociated with the plasma sheet boundary layer. The occurrence rate of tailward FAIF (~4.5%) is relatively insensitive to AE conditions. The occurrence rate of earthward FAIF increases during ``active'' conditions to as much as 12.4% and decreases to as low as 1.3% during ``quieter'' conditions. An increase in the occurrence rate of tailward FAIFs is observed with increasing Z (mapping along B to the more distant current sheet). We suggest that the tailward streaming flows may arise during stretching of especially distant closed field lines during extended recovery and quiet time. Ions will lose energy as they cross the tailward moving current sheet, just as they gain energy in crossing an earthward moving field reversal region.
No evidence for a quiet time distant tail source is seen in the energetic (24 keV to 2 MeV) ion data during these late recovery phase and quiet time periods. A weak local time asymmetry in the ratio of occurrence rates for tailward and earthward FAIFs (T:E) is observed, with earthward streaming FAIFs outnumbering tailward streaming FAIFs in the dusk sector (T:E=0.9-1.0) and vice versa in the dawn sector (T:E=1.4-2.1). The occurrence rate of dawn-to-dusk flux anisotropies, interpreted here as gradients indicating higher ion flux toward the plasma sheet equatorial plane, increases during active conditions and decreases during quieter conditions. North-to-south and south-to-north flux anisotropies, interpreted here as dawnward and duskward flux gradients (3-5%), are independent of AE conditions.

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