Production of narrow-scale Alfvén waves by E-region depletion, phase-mixing and feedback instability

Details Production of narrow-scale Alfvén waves by E-region depletion, phase-mixing and feedback instability Production of narrow-scale Alfvén waves by E-region depletion, phase-mixing and feedback instability

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufmsm43a2041r&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #SM43A-2041

Physics

[2407] Ionosphere / Auroral Ionosphere, [2431] Ionosphere / Ionosphere/Magnetosphere Interactions, [2736] Magnetospheric Physics / Magnetosphere/Ionosphere Interactions, [2752] Magnetospheric Physics / Mhd Waves And Instabilities

Scientific paper

Narrow-scale Alfvén waves have been observed in strong downward current channels, and numerical simulations have associated their formation with self-consistent magnetosphere-ionosphere (M-I) coupling. Here, we present three simulations, of increasing physical complexity, that together reveal the processes producing these waves. An important part of magnetosphere-ionosphere coupling is removal of electrons from Earth's ionosphere by downward-directed electric currents, capable of significantly eroding E-region plasma density. If initial downward current density exceeds a threshold, so that electrons are removed from the E-region faster than they can be produced by local ionization, then the magnetospheric current channel and ionospheric depletion region broaden. If the magnetosphere is modelled using ideal MHD, then broadening forms a discontinuity in E-region plasma density, and a corresponding current sheet in the magnetosphere. This structure steps between two steady-states and moves in the direction of the horizontal electric field. In this scenario, the smallest length-scale collapses to zero, but at a single point. At the next level of complexity, adding electron inertia to the magnetosphere introduces dispersion. The system evolves much as before, but the `discontinuity' is smoothed slightly and a series of ripples are left behind it at the M-I boundary, sending upgoing inertial Alfvén waves into the magnetosphere. Narrow scales are formed in two distinct stages: depletion and broadening rapidly access electron inertial scales, and then shorter scales are accessed gradually as inertial waves phase-mix on the M-I boundary. Our third simulation considers the possibility that upgoing inertial Alfvén waves, formed by the processes described, may reflect from the steep Alfvén speed gradient above the E-region, becoming partially trapped in the ionospheric Alfvén resonator. With this addition, waves spread to fill the entire downward current channel. Just as importantly, trapped inertial Alfvén waves are typically subject to ionospheric feedback instability, which can amplify the narrow-scale waves considerably.

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