Conversion from magnetoacoustic to Alfven waves

In active regions this reflection typically occurs high enough that the Alfven 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/Alfven 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-Alfven mode conversion in more complex atmospheres. Assuming a uniform inclined magnetic field and an exponential Alfven speed profile with density scale height h, the Alfven conversion coefficient depends on three variables only: the dimensionless transverse-to-the-stratification wavenumber {kappa}=kh, the magnetic field inclination from the stratification direction {theta}, and the polarization angle {phi} of the wavevector relative to the plane containing the stratification and magnetic field directions.