Benchmarking Fast-to-Alfvén Mode Conversion in a Cold Magnetohydrodynamic Plasma
Abstract
Alfvén waves may be generated via mode conversion from fast magnetoacoustic waves near their reflection level in the solar atmosphere, with implications both for coronal oscillations and for active region helioseismology. 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 magnetohydrodynamic model c → 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 = kh, the magnetic field inclination from the stratification direction θ, and the polarization angle phi 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 phi (80°-90°), 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- September 2011
- DOI:
- 10.1088/0004-637X/738/2/119
- arXiv:
- arXiv:1105.5754
- Bibcode:
- 2011ApJ...738..119C
- Keywords:
-
- magnetohydrodynamics: MHD;
- Sun: oscillations;
- sunspots;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- 14 pages plus supplementary tables. Astrophys J (accepted 25 May 2011). Two ancillary animations (animated gif) attached