Magnetosonic wave

A magnetosonic wave (also magnetoacoustic wave) is a longitudinal wave^[1] of ions (and electrons) in a magnetized plasma propagating perpendicular to the stationary magnetic field. The wave is dispersionless with a phase velocity ω/k given by

$\frac{\omega^2}{k^2}=c^2\,\frac{v_s^2+v_A^2}{c^2+v_A^2}$,

where v_s is the speed of the ion acoustic wave, v_A is the speed of the Alfvén wave, and c is the speed of light in vacuum.

In the limit of low magnetic field (v_A→0), the wave turns into an ordinary ion acoustic wave. In the limit of low temperature (v_s→0), the wave becomes a modified Alfvén wave. Because the phase velocity of the magnetosonic mode is almost always larger than v_A, the magnetosonic wave is often called the "fast" hydromagnetic wave.

Both fast and slow magnetoacoustic waves have been recently discovered in the solar corona^[2], which created an observational foundation for the novel technique for the coronal plasma diagnostics, coronal seismology.

See also

• Waves in plasmas
• Alfvén wave
• Ion acoustic wave
• Coronal seismology

References

1. ^ The wave is longitudinal in the perturbation of the fluid velocity, although the perturbation of the magnetic field is transverse. See Schmidt, Physics of High Temperature Plasmas, p.101.
2. ^ Nakariakov, V.M., Verwichte, E. (2005). "Coronal waves and oscillations". Living Rev. Solar Phys. 2: 3. http://www.livingreviews.org/lrsp-2005-3.