Equatorial bubbles updrafting at supersonic speeds

Abstract

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 ≈ − g . Vector magnetic field measurements, available during one of the bubble encounters show a perturbation of ∼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 × B current along the bubble's edge. Rather, it is Alfvénic 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.