Magnetotail plasma flow during plasma sheet expansions: Vela 5 and 6 and Imp 6 observations

Abstract

Plasma flow during plasma sheet expansions is studied with extensive data from Vela 5 and 6 and IMP 6 satellites in the range X ≃ 0 to −30 RE. By combining statistically 44 flow observations by Vela 5 and 6 and 102 flow observations by IMP 6 it is concluded that the three-dimensional plasma flow direction within the expanding plasma sheet at −10 RE > X > −30 RE is, in general, sunward and approximately parallel to the equatorial plane. The flow speed is usually high (∼500–1000 km/s) in the midnight sector and is generally lower (less than ∼500 km/s) in the dawn and dusk sectors. The flow within a geocentric distance of ∼10 RE is less systematic. The average duration of the flow is ∼12 min. Our result suggests either that the high speed plasma flows are mostly confined near the upper and lower boundaries of the expanding plasma sheet (if it is assumed that the estimation of the distance dZ from the neutral sheet is reasonably accurate) or that the flow lasts for a short duration over the entire plasma sheet (if it is assumed that the dZ estimation is not accurate). The possible implication to the production of bright auroral arcs is discussed.