Phase Mixing, Density Cavities And Ion Outflow On Auroral Field Lines (Invited)

Details Phase Mixing, Density Cavities And Ion Outflow On Auroral Field Lines (Invited) Phase Mixing, Density Cavities And Ion Outflow On Auroral Field Lines (Invited)

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsm24b..02l&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #SM24B-02

Physics

Plasma Physics

[2704] Magnetospheric Physics / Auroral Phenomena, [2736] Magnetospheric Physics / Magnetosphere/Ionosphere Interactions, [2752] Magnetospheric Physics / Mhd Waves And Instabilities, [7845] Space Plasma Physics / Particle Acceleration

Scientific paper

While nonlinear interactions are often invoked to explain cross-scale coupling in the propagation of Alfven waves on auroral field lines, phase mixing at perpendicular gradients in the Alfven speed can be an effective means of transferring energy from large to small scales. Such phase mixing can be effective even for relatively weak gradients for waves trapped in the ionospheric Alfven resonator. At strong gradients such as the plasma sheet boundary or the edges of auroral density cavities, phase mixing can occur on time scales of seconds. These small-scale Alfven waves have a number of important consequences for auroral particle acceleration. At scales of the electron inertial length or the ion acoustic gyroradius, parallel electric fields develop that can accelerate auroral particles. In addition, if the scale becomes comparable to an ion gyroradius, then the ions can become demagnetized and are accelerated perpendicular to the magnetic field. Perpendicular ion acceleration is also possible if the scale becomes comparable to the effective gyroradius defined by the ExB motion of the ion in the wave field. Since these critical scale lengths depend on the ion mass, these mechanisms give a preferential acceleration to heavier ions. In turn, this perpendicular ion acceleration in conjunction with the mirror force and parallel electric fields can deepen the density cavity, providing a positive feedback mechanism. Thus, even in the absence of higher frequency turbulence providing wave-particle interactions (which may also be operative), phase mixing of Alfven waves and the subsequent energization of particles can lead to ion outflow and the formation of density cavities.

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