Possible control of plasma transport in the near-Earth plasma sheet via current-driven Alfven waves (f similar or equal to f(H+))

Two time periods, each covering both quiet and disturbed conditions (growth phase, breakup, and postbreakup phase), are studied. Electric and magnetic field measurements, carried out in the near-Earth plasma sheet (NEPS), are used to calculate the two components (radial and azimuthal) of the electri c E x B/B-2 drift. These calculations are compared with independent estimat es of the ion flow direction deduced from ion flux measurements. During act ive periods, the two flow directions coincide to a large degree. Evidence i s given for two regimes of transport: (1) During the growth phase, and afte r the active phase, the electric field (radial and azimuthal) and hence the azimuthal and radial flow velocities are small in the near-equatorial regi on. This is interpreted as the consequence of an electrostatic field that t ends to shield the induced electric field associated with time-varying exte rnal conditions. (2) During active chases (breakup and pseudobreakup), howe ver, large-amplitude bursts in E x B/B-2 radial and azimuthal components (i nterpreted as how bursts), with typical velocities of the order of 100 kms( -1), are observed. The direction of these flow bursts is somewhat arbitrary , and in particular, for the two substorm events described here, sudden rev ersals in the flow direction are observed. These fast flow bursts coincide with intense low-frequency electromagnetic fluctuations: current-driven Alf ven waves (CDA waves) with frequency f similar or equal to f(H+), the proto n gyrofrequency. CDA waves produce "anomalous" collisions on timescales sho rter than the electron bounce period, thus violating the second adiabatic i nvariant for electrons. As a consequence, the electrostatic shielding is de stroyed, which leads to enhanced radial transport. Thus the transport in th e NEPS seems to be controlled by a microscopic current-driven instability.