Plasma transport during substorm growth phase and relation to breakup

Abstract

For ω<ωbi,ωbe, the electron and ion bounce frequencies, the response of a plasma to an externally applied electromagnetic perturbation is nonlocal. This implies, via the quasi-neutrality equation, the development of an electrostatic potential which is constant for a given magnetic field line. In the near equatorial region the corresponding potential electric field is shown to oppose the effect of the induced electric field associated with the externally applied perturbation. Thus the effect of the induced electric field is partially shielded; the total azimuthal electric field (i.e. induced plus potential) tends to be small, which explains why the radial flow velocity is slow during quasi-steady conditions prevailing during the growth phase and after the active phase. The nonlocal response of the plasma also leads to the development of a parallel current that may generate current driven Alfven (CDA) waves, which mode convert into shear Alfven (SA) waves. CDA/SA waves are systematically observed at early breakup; they grow very fast and produce a parallel diffusion of electrons. As soon as the diffusion time is shorter than the bounce time (τd<τb), the nonlocal response vanishes. Thus the shielding disappears, and an enhanced transport is restored at the rate fixed by the induced electric field alone. We show that fast flows effectively occur when CDA waves have enough power to diffuse electrons (over τd<τb). Electron parallel diffusion also leads to an interruption of the parallel current and therefore to a disruption of the perpendicular current.