Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996mnras.281..761l&link_type=abstract
Monthly Notices of the Royal Astronomical Society, Volume 281, Issue 3, pp. 761-780.
Astronomy and Astrophysics
Astronomy
6
Mhd, Sun: Corona, Sunspots, Sun: Transition Region, Stars: Magnetic Fields, Stars: Oscillations
Scientific paper
Numerical solutions are obtained for three-dimensional compressible magnetohydrodynamic (MHD) perturbations in a spherically symmetric, isothermal, static stellar atmosphere embedded with a radial magnetic field. Sufficiently far above the stellar surface, we impose analytic solutions for radially outward propagation of either magneto-acoustic waves or magnetic waves. Results of sample numerical calculations are presented to reveal global properties of magnetic waves and magneto-acoustic-gravity waves as well as MHD wave interactions in the mid-atmosphere. We emphasize, in particular, that magneto-gravity waves in the low-frequency regime can be effectively blocked in the mid-atmosphere either as a result of the systematic decrease of the Brunt-Vaisala buoyancy frequency N_BV with increasing height or as a result of the magnetic cut-off mechanism; this trapping process generally involves an MHD wave transformation in the mid-atmosphere from magneto-gravity waves near the stellar surface to magneto-acoustic waves sufficiently far away. We provide convenient conversion formulae for relevant physical parameters of compressible MHD waves in magnetized stellar atmospheres. Based on our theoretical and numerical analyses, we propose several applications for compressible MHD waves in stellar atmospheres: (1) the interpretation of the discrete low-frequency spectral modes recently detected in the solar wind within the scenario of discrete magneto-gravity modes trapped in the lower solar corona and above the transition region; (2) the possible roles of compressible MHD waves, especially magnetic waves, in the context of stellar wind acceleration and coronal heating; and (3) the re-examination of the physical nature of 5-min photospheric oscillations of sunspot umbrae in terms of trapped magneto-gravity waves in the local isothermal approximation.
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