Possible control of plasma transport in the near‐Earth plasma sheet via current‐driven Alfvén waves (ƒ ≃ ƒ_H+)

Abstract

Abstract. 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 BIB • 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 phases (breakup and pseudobreakup), however, large-amplitude bursts in E x B/B e radial and azimuthal components (interpreted as flow bursts), with typical velocities of the order of 100 km s -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 Alfv•n waves (CDA waves) with frequency f _• f•r+, 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.

The Search-Coil Magnetometer for MMS

“…Such effects do not occur when the frequency of the perturbations is larger than the particle bounce frequency; then a fluid approach can safely be used to describe plasma dynamics (Weiland 2000). They can be also ignored if electrons are prevented from bouncing, for instance due to interaction with some electromagnetic waves (Le Contel et al 2001). The Earth's magnetotail is a classical example of collisionless and magnetically confined plasma.…”

Two satellite study of substorm expansion near geosynchronous orbit

“…8, second panel), shows a distinct in phase correlation for the period after 18:40 UT. Thus, the azimuthal electric field, with the associated E×B-drift, dominates the radial particle transport and hence the development of the substorm, as discussed in detail by Le Contel (2001) for two other events recorded on GEOS-2. The observed correlation of the E×B-drift and the radial ion flux intensity represents supporting evidence for the presence of ballooning modes during the azimuhal expansion of the substorm.…”

Alternating bursts of low energy ions and electrons near the substorm onset

“…However, the transient bursts of fast flows were also observed in the plasma sheet (Angelopoulos et al, 1992;Nakamura et al, 1994;Le Contel et al, 2001). The standard convection models, based on constant electrostatic fields, do not describe this regime of flows.…”

“…Impulsive increases (as short lasting bursts) of these waves were regularly observed at the GEOS 2 during the substorm magnetic field dipolarization (Perraut et al, 1993(Perraut et al, , 2000Le Contel et al, 2001). These spatially localized waves are related to field-aligned currents, Perraut et al (2000) called them the current-driven Alfvén (CDA) waves.…”

“…The standard convection models, based on constant electrostatic fields, do not describe this regime of flows. Le Contel et al (2001) showed that the level of the ULF fluctuations at the proton gyrofrequency seems to control the transition between the two regimes of the plasma transport in the near-Earth plasma sheet: a slow transport during the growth phase and a fast transport during the active phase of the substorm.…”