The flow of plasma in the earth's magnetotail has been measured with an electrostatic analyzer on Vela 4B at geocentric distances of ∼18 RE. The analyzer on the rotating (64-sec period) satellite measures proton energy spectra from 79 ev to 19 kev, and the plasma, flow is detected and measured by the substantial spin modulation that it often causes in the measured proton fluxes. The satellite's spin axis is kept directed radially outward along a radius vector from the earth, and so the analyzer, whose aperture is in the satellite's equatorial plane, most effectively senses flows in the direction perpendicular to the radius vector. Some results of the measurements are that (1) plasma flow speeds of several hundred km/sec are frequently measured in the plasma sheet, particularly during substorms, and these sometimes approach 1000 km/sec; however, evident flow in a given direction seldom persists for more than a few minutes; (2) these rapid substorm-related flows are usually directed generally sunward; (3) flow in the anti-sunward (tailward) direction is observed early in some substorms as the plasma sheet thins down; this may suggest the formation of a neutral line at geocentric distances <18 RE;, (4) the magnetotail is separated from the surrounding magnetosheath by a boundary layer a few thousand kilometers thick in which magnetosheath-like flow occurs but at reduced particle density and velocity; and (5) averaging of all flow measurements made in the plasma, sheet over many months does not reveal any distinct pattern of flow either sunward or anti-sunward; an average of the flows observed during periods of a few minutes of clearly evident flow, however, does reveal a flow in the general direction of the sun. It appears that the plasma sheet may often be in turbulent motion with turbulence-cell dimensions no greater than a few RE.
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Hones et al. (1972) studied this question.