Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...468..907k&link_type=abstract
Astrophysical Journal v.468, p.907
Astronomy and Astrophysics
Astronomy
12
Magnetohydrodynamics: Mhd, Sun: Magnetic Fields
Scientific paper
We investigate the importance of the slow continuum (from linear, ideal magnetohydrodynamics [MHD]) for hot, evacuated, and strongly magnetic fibrils with nonnegligible radial structure. The radial structure allows for both slow and Alfvén resonant absorption of acoustic power (in linear, visco-resistive MHD). When calculating how efficiently the acoustic power is absorbed by such "hot magnetic fibrils," embedded in a uniform compressible medium, as a function of the real driving frequency, it is found that the axisymmetric component of the acoustic excitation is absorbed quite strongly for frequencies within the range of the slow continuum.
Additionally, for these one-dimensional hot magnetic fibrils, a sequence of absorption maxima accumulates in real driving frequency above the range of the slow continuum, still within the Alfvén continuum. The maximal absorption coefficients reach 80% and more. We identify the complex optimal driving frequencies and the associated complex leaky eigenmodes responsible for these absorption maxima.
The leaky eigenmodes relate to the well-known tube speed modes of a uniform, hot, and evacuated flux tube. The complex eigenfrequencies of the leaky eigenmodes of the radially structured fibrils are calculated from the impedance criterion that these eigenfrequencies satisfy.
We define the generally complex optimal driving frequencies to be those driving frequencies at which total (100%) absorption of the incoming wave field takes place. They also obey an impedance criterion, similar to the one that defines the eigenfrequencies. Both impedance criteria demonstrate clearly the connection between optimal driving frequencies and leaky eigenmodes. This also calls for a reevaluation of the results of Goossens & Hollweg, in which optimal and total resonant absorption for real driving frequencies and the complex leaky eigenmodes was discussed.
For network and plage magnetic elements in the solar atmosphere, our results may be relevant for wave interactions within a layer situated at a geometrical height of about 400 km above photospheric τ = 1.
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