Plasmasphere dynamics in the duskside bulge region: A new look at an old topic

Abstract

Data acquired during several multiday periods in 1982 at ground stations Siple, Halley, and Kerguelen and on satellites DE 1, ISEE 1, and GEOS 2 have been used to investigate thermal plasma structure and dynamics in the duskside plasmasphere bulge region of the Earth. The distribution of thermal plasma in the dusk bulge sector is difficult to describe realistically, in part because of the time integral manner in which the thermal plasma distribution depends upon the effects of bulk cross‐B flow and interchange plasma flows along B. While relatively simple MHD models can be useful for qualitatively predicting certain effects of enhanced convection on a quiet plasmasphere, such as an initial sunward entrainment of the outer regions, they are of limited value in predicting the duskside thermal plasma structures that are observed. Furthermore, use of such models can be misleading if one fails to realize that they do not address the question of the formation of the steep plasmapause profile or provide for a possible role of instabilities or 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 plasmasphere appears to become divided into two entities, a main plasmasphere and a duskside bulge region. The latter consists of outlying or outward extending plasmas that are the products of erosion of the main plasmasphere; (2) in the aftermath of an increase in convection activity, the main plasmasphere tends (from a statistical point of view) to become roughly circular in equatorial cross section, with only a slight bulge at dusk; (3) the abrupt westward edge of the duskside bulge observed from whistlers represents a state in the evolution of sunward extending streamers; (4) in the aftermath of a weak magnetic storm, 10 to 30% of the plasma “removed” from the outer plasmasphere appears to remain in the afternoon‐dusk sector beyond the main plasmasphere. This suggests that plasma flow from the afternoon‐dusk magnetosphere into the boundary layers is to some extent impeded, possibly through a mechanism that partially decouples the high altitude and ionospheric‐level 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‐night plasmatrough boundary may be identified in equatorial satellite data; (7) factor‐of‐2‐to‐10 density irregularities appear near the plasmapause 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 trapped light ion pitch angle distributions give way to a predominantly isotropic 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 det...

Cold dense plasma in the outer magnetosphere

“…Such a case happens when the plasmasphere has been recently eroded owing to a prior period of enhanced activity and the outer plasmasphere has not had enough time of plasma supply. Another problem with the above interpretation is that it is hard to explain the following configurations that were clarified previously: the eddy flows [Carpenter et al, 1993] and detachment [Chappell, 1974] of the plasmaspheric material. Although it is not clear whether we have observed such configurations, we should have a consistent model with previous and current observations in the future.…”

New plasmapause model derived from CHAMP field-aligned current signatures

“…to the presence of sub-auroral polarisation streams (SAPS), plumes, etc., and the abrupt westward edge of the bulge as described in the introduction (e.g. Carpenter et al, 1993). SAPS partly overlap with the plasmasphere forming badly disturbed plasmapause shapes (e.g.…”

“…The plasma in the bulge is already detached from the main plasmasphere, but not yet escaped from the magnetosphere. From the observational point of view, the main plasmasphere and the bulge are "essentially two separate entities" (Carpenter et al, 1993). Disturbance associated sunward plasma flows, such as plumes, tails and other structures driven by, for example SAPS electric field, are found outside the R2 FACs.…”

Bursts of ULF noise excited by sudden changes of solar wind dynamic pressure

“…4.2), we think that the interaction with cold plasma of the plasmaspheric origin rather than the magnetospheric compression is the reason for their precipitation. Such plasma is often observed in the afternoon-dusk sector after magnetic storms (Carpenter et al, 1993).…”