EQUATORIAL BUBBLES UPDRAFTING AT SUPERSONIC SPEEDS

We present plasma and electric field observations from two satellite encounters with equatorial plasma bubbles updrafting at velocities of -2 km/s. These large, upward velocities are consistent with an adaptation of Chandrasekhar's model for the motion of plasma blobs supported against gravity by a magnetic field; that is, V(z) almost-equal-to -g. Vector magnetic field measurements, available during one of the bubble encounters show a perturbation of approximately 150 nT, directed radially outward from the Earth, near the western wall of deepest plasma depletion. This magnetic variation is too large to be caused by simple shunting of the g x B current along the bubble's edge. Rather, it is Alfvenic in nature, radiating from a generator located near the magnetic equator, in the plasma outside the bubble's leading edge. A heuristic model of a depleted flux tube with constant circular cross section moving upward through a background plasma predicts most of the measurements' qualitative features.