Possible control of plasma transport in the near-Earth plasma sheet via current-driven Alfven waves (f ≃ fH+)

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 electric E x B/B-2 drift. These calculations are compared with independent estimates of the ion flow direction deduced from ion flux measurements. During active periods, the two flow directions coincide to a large degree. Evidence is given for two regimes of transport: (1) During the growth phase, and after the active phase, the electric field (radial and azimuthal) and hence the azimuthal and radial flow velocities are small in the near-equatorial region. This is interpreted as the consequence of an electrostatic field that tends to shield the induced electric field associated with time-varying external conditions. (2) During active chases (breakup and pseudobreakup), however, large-amplitude bursts in E x B/B-2 radial and azimuthal components (interpreted 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 reversals in the flow direction are observed. These fast flow bursts coincide with intense low-frequency electromagnetic fluctuations: current-driven Alfven waves (CDA waves) with frequency f similar or equal to f(H+), the proton gyrofrequency. CDA waves produce anomalous collisions on timescales shorter than the electron bounce period, thus violating the second adiabatic invariant for electrons. As a consequence, the electrostatic shielding is destroyed, which leads to enhanced radial transport. Thus the transport in the NEPS seems to be controlled by a microscopic current-driven instability.