Bibcode
Khomenko, E.; Cally, P. S.
Bibliographical reference
The Astrophysical Journal, Volume 746, Issue 1, article id. 68 (2012).
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2
2012
Journal
Citations
98
Refereed citations
89
Description
We study the conversion of fast magnetoacoustic waves to Alfvén
waves by means of 2.5D numerical simulations in a sunspot-like magnetic
configuration. A fast, essentially acoustic, wave of a given frequency
and wave number is generated below the surface and propagates upward
through the Alfvén/acoustic equipartition layer where it splits
into upgoing slow (acoustic) and fast (magnetic) waves. The fast wave
quickly reflects off the steep Alfvén speed gradient, but around
and above this reflection height it partially converts to Alfvén
waves, depending on the local relative inclinations of the background
magnetic field and the wavevector. To measure the efficiency of this
conversion to Alfvén waves we calculate acoustic and magnetic
energy fluxes. The particular amplitude and phase relations between the
magnetic field and velocity oscillations help us to demonstrate that the
waves produced are indeed Alfvén waves. We find that the
conversion to Alfvén waves is particularly important for strongly
inclined fields like those existing in sunspot penumbrae. Equally
important is the magnetic field orientation with respect to the vertical
plane of wave propagation, which we refer to as "field azimuth." For a
field azimuth less than 90° the generated Alfvén waves
continue upward, but above 90° downgoing Alfvén waves are
preferentially produced. This yields negative Alfvén energy flux
for azimuths between 90° and 180°. Alfvén energy fluxes
may be comparable to or exceed acoustic fluxes, depending upon geometry,
though computational exigencies limit their magnitude in our
simulations.
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Magnetic fields are at the base of star formation and stellar structure and evolution. When stars are born, magnetic fields brake the rotation during the collapse of the mollecular cloud. In the end of the life of a star, magnetic fields can play a key role in the form of the strong winds that lead to the last stages of stellar evolution. During
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