Excess heating of corona and chromosphere above magnetic regions by non-linear Alfvén waves

✍ Scribed by Yutaka Uchida; Osamu Kaburaki

• Publisher: Springer
• Year: 1974
• Tongue: English
• Weight: 866 KB
• Volume: 35
• Category: Article
• ISSN: 0038-0938
• DOI: 10.1007/bf00151968

✦ Synopsis

Excess heating of the active region solar atmosphere is interpreted by the decay of MHD slow-mode waves produced in the corona through the non-linear coupling of Alfv6n waves supplied from subphotospheric layers, it is stressed that the Alfv6n-mode waves may be very efficiently generated directly in the convection layer under the photosphere in magnetic regions, and that such magnetic regions, at the same time, provide the 'transparent windows' for Alfv6n waves in regard to the Joule and frictional dissipations in the photospheric and subphotospheric layers. Though the Alfvdn waves suffer considerable reflection in the chromosphere and in the transition layer, a certain fraction of this large flux is propagated out to the corona, and a large velocity amplitude exceeding the local Alfv6n velocity is attained during the propagation along the magnetic tubes of force into a region of lower density and weaker magnetic field. The otherwise divergence-free velocity field in Alfv6n waves gets involved in such a case with a compressional component (slow-mode waves) which again is of considerable velocity amplitude relative to the local acoustic velocity when estimated by using the formulation for non-linear coupling between MHD wave modes derived by Kaburaki and Uchida (1971). Therefore, the compressional waves thus produced through the non-linear coupling of Alv6n waves will eventually be thermalized to provide a heat source. The introduction of this nonlinear coupling process and the subsequent thermalization of thus produced slow-mode waves may provide means of converting the otherwise dissipation-free Alfv6n mode energy into heat in the corona. The liberated heat will readily be redistributed by conduction along the magnetic lines of force, with higher density as a consequence of increased scale height, and thus the loop-like structure of the coronal condensations (or probably also the thread-like feature of the general corona) may be explained in a natural fashion.