Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999apj...521..851r&link_type=abstract
The Astrophysical Journal, Volume 521, Issue 2, pp. 851-858.
Astronomy and Astrophysics
Astronomy
18
Conduction, Magnetohydrodynamics: Mhd, Plasmas, Sun: Corona, Waves
Scientific paper
Two scenarios of coronal loop heating by directly driven torsional Alfvén waves are considered. In the first scenario the driving is assumed to be harmonic. In the steady state of oscillations, wave energy dissipation mainly occurs in a narrow dissipative layer embracing an ideal resonant magnetic surface. The wave motion in the dissipative layer is characterized by very large amplitudes. It is assumed that the radiative and thermoconductive losses from loops are exactly covered by wave energy dissipation. This assumption allows expression of the maximum value of the velocity in the dissipative layer in terms of the energy losses and the loop parameters. It turns out that this maximum velocity is proportional to R^1/3, where R is the total Reynolds number accounting for both viscosity and resistivity. For typical coronal loops and R=10^6, the maximum velocity in the dissipative layer is between 800 and 1000 km s^-1. In the second scenario the driving is assumed to be a stationary stochastic process. Once again it is assumed that energy losses from the loop are covered by wave energy dissipation. The maximum value of the mean square velocity turns out to be proportional to R^1/6. This value is also very sensitive to the width of the driver frequency spectrum. In two considered examples, one with a narrow spectrum and another with a wide spectrum, the maximum values of the mean square velocity between 300 and 400 km s^-1 and between 150 and 200 km s^-1 were obtained, respectively, for typical coronal loops and R=10^6. Since the observed nonthermal velocities in coronal loops never exceed a few tens of km s^-1, these results lead to the conclusion that both scenarios do not satisfy the observational constraints.
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