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#### Magneto-acoustic wave propagation in the solar atmosphere: vertical magnetic field, disturbance $\delta B$

Author: Christian Nutto
Affiliation: Kiepenheuer-Institut für Sonnenphysik, Freiburg

Co-authors: M. Roth, O. Steiner

Main category: Natural Sciences (Astronomy)

Caption:
Magnetic field perturbations, $\delta B/B_0$, for different time steps and for the scenario of a vertical magnetic field. The magneto-acoustic wave propagates from the source (piston) located at (x; z) = (1000; -750) km. The horizontal line shows the equipartition level, where the sound speed equals the Alfven speed, $c_\mathrm{s}=c_\mathrm{A}$. The vertical lines represent magnetic field lines.

Further information:
The wave is launched below the equipartition level by the piston as a predominantly acoustic wave. Upon reaching the equipartition level, where the Alfven speed reaches the sound speed, $c_\mathrm{s}=c_\mathrm{A}$, the acoustic wave is partially transmitted, and changes from the fast to the slow branch of the magneto-acoustic waves. This slow mode is then guided along the magnetic field lines into the higher layers of the atmosphere . The part of the wave energy that is not transmitted, is converted into the fast magnetic mode. This fast mode emerges from the equipartition level and is then refracted due to the high gradient of the Alfven speed.

Further Reading: Dissertation: Numerical experiments on the propagation of waves in a convectively unstable magneto-atmosphere: A new interpretation of the measured reduction of wave travel-times in magnetic regions of the Sun. (Figure 4.5)

DOI: 10.18147/smn.2015/video:1