Benchmarking Fast-to-Alfven Mode Conversion in a Cold MHD Plasma

Paul S. Cally; Shelley C. Hansen

doi:10.1088/0004-637X/738/2/119

paper

Benchmarking Fast-to-Alfven Mode Conversion in a Cold MHD Plasma

arXiv:1105.5754 · doi:10.1088/0004-637X/738/2/119

Abstract

Alfvén waves may be generated via mode conversion from fast magneto-acoustic waves near their reflection level in the solar atmosphere, with implications both for coronal oscillations and for active region helio-seismology. In active regions this reflection typically occurs high enough that the Alfvén speed $a$ greatly exceeds the sound speed $c$ , well above the $a = c$ level where the fast and slow modes interact. In order to focus on the fundamental characteristics of fast/Alfvén conversion, stripped of unnecessary detail, it is therefore useful to freeze out the slow mode by adopting the gravitationally stratified cold MHD model $c \to 0$ . This provides a benchmark for fast-to-Alfvén mode conversion in more complex atmospheres. Assuming a uniform inclined magnetic field and an exponential Alfvén speed profile with density scale height $h$ , the Alfvén conversion coefficient depends on three variables only; the dimensionless transverse-to-the-stratification wavenumber $κ = k h$ , the magnetic field inclination from the stratification direction $θ$ , and the polarization angle $ϕ$ of the wavevector relative to the plane containing the stratification and magnetic field directions. We present an extensive exploration of mode conversion in this parameter space and conclude that near-total conversion to outward-propagating Alfvén waves typically occurs for small $θ$ and large $ϕ$ ( $8 0^{\circ}$ -- $9 0^{\circ}$ ), though it is absent entirely when $θ$ is exactly zero (vertical field). For wavenumbers of helioseismic interest, the conversion region is broad enough to encompass the whole chromosphere.

14 pages plus supplementary tables. Astrophys J (accepted 25 May 2011). Two ancillary animations (animated gif) attached

Benchmarking Fast-to-Alfven Mode Conversion in a Cold MHD Plasma

Abstract

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