PLASMASPHERE DYNAMICS IN THE DUSKSIDE BULGE REGION - A NEW LOOK AT ANOLD TOPIC

Data acquired during several multiday periods in 1982 at ground statio ns Siple, Halley, and Kerguelen and on satellites DE 1, ISEE 1, and GE OS 2 have been used to investigate thermal plasma structure and dynami cs in the duskside plasmasphere bulge region of the Earth. The distrib ution of thermal plasma in the dusk bulge sector is difficult to descr ibe realistically, in part because of the time integral manner in whic h the thermal plasma distribution depends upon the effects of bulk cro ss-B flow and interchange plasma flows along B. While relatively simpl e MHD models can be useful for qualitatively predicting certain effect s of enhanced convection on a quiet plasmasphere, such as an initial s unward entrainment of the outer regions, they are of limited value in predicting the duskside thermal plasma structures that are observed. F urthermore, use of such models can be misleading if one fails to reali ze that they do not address the question of the formation of the steep plasmapause profile or provide for a possible role of instabilities o r other irreversible processes in plasmapause formation. Our specific findings, which are based both upon the present case studies and upon earlier work, include the following: (1) during active periods the pla smasphere appears to become divided into two entities, a main plasmasp here and a duskside bulge region. The latter consists of outlying or o utward extending plasmas that are the products of erosion of the main plasmasphere; (2) in the aftermath of an increase in convection activi ty, the main plasmasphere tends (from a statistical point of view) to become roughly circular in equatorial cross section, with only a sligh t bulge at dusk; (3) the abrupt westward edge of the duskside bulge ob served from whistlers represents a state in the evolution of sunward e xtending streamers; (4) in the aftermath of a weak magnetic storm, 10 to 30% of the plasma ''removed'' from the outer plasmasphere appears t o remain in the afternoon-dusk sector beyond the main plasmasphere. Th is suggests that plasma flow from the afternoon-dusk magnetosphere int o the boundary layers is to some extent impeded, possibly through a me chanism that partially decouples the high altitude and ionospheric-lev el flow regimes; (5) outlying dense plasma structures may circulate in the outer duskside magnetosphere for many days following an increase in convection, unless there is extremely deep quieting; (6) a day-nigh t plasmatrough boundary may be identified in equatorial satellite data ; (7) factor-of-2-to-10 density irregularities appear near the plasmap ause in the postdusk sector in the aftermath of weak magnetic storms; (8) during the refilling of the plasmatrough from the ionosphere at L = 4.6, predominantly bidirectional field aligned and equatorially trap ped light ion pitch angle distributions give way to a predominantly is otropic distribution (as seen by DE 1) when the plasma density reaches a level a factor of about 3 below the saturated plasmasphere level; ( 9) some outlying dense plasma structures are effectively detached from the main plasmasphere, while others appear to be connected to that bo dy.