Outlying plasmasphere structure detected by whistlers

Abstract

Whistlers recorded at Eights ( L ⋍ 4) and Byrd ( f ⋍ 7), Antarctica have been used to study large-scale structure in equatorial plasma density at geocentric distances ⋍3–6 R E . The observations were made during conditions of magnetic quieting following moderate disturbance. The structures were detected by a “scanning” process involving relative motion, at about one tenth of the Earth's angular velocity or greater, between the observed density features and the observing whistler station or stations. Three case studies are described, from 26 March 1965, 11 May 1965 and 29 August 1966. The cases support satellite results by showing outlying high density regions at ⋍4–6 R E that are separated from the main plasmasphere by trough-like depressions ranging in width from ⋍0.2 to 1 R E. The structures evidently endured for periods of 12 hr or more. In the cases of deepest quieting their slow east-west motions with respect to the Earth are probably of dynamo origin. The cases observed during deep quieting (11 May 1965 and 29 August 1966) suggest the approximate rotation with the Earth of structure formed during previous moderate disturbance activity in the dusk sector. The third case, from 26 March 1965, may represent a structure formed near local midnight. The reported structures appear to be closely related to the bulge phenomenon. The present work supports other experimental and theoretical evidence that the dusk sector is one of major importance in the generation of outlying density structure. It is inferred that irregularities of the type reported here regularly develop near 4–5 R E during moderate substorm activity. This research suggests that at least a major class of the density structures that develop near 4 R E are tail-like in nature, joined to the main body of the plasmasphere. The apparent disagreement with Chappell's results from OGO 5, which are interpreted as showing regions of “detached” plasma beyond 5 R E , may be related to the pronounced spatial structure of electric fields observed in high-latitude ionospheric regions that are conjugate to the magnetospheric regions in which the OGO-5 observations were made.