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B. De Pontieu (Lockheed Martin Solar and Astrophysics Lab), P.C.H. Martens (Physics Department, Montana State University), H.S. Hudson (SPRC/ISAS)
We study the damping of Alfvén mode oscillations on coronal loops and in the chromosphere. First we consider damping of standing waves on coronal loops, such as those observed in the aftermath of a flare with the Transition Region and Coronal Explorer (TRACE). We calculate the leakage of wave energy from the coronal volume through the footpoints of a coronal loop, assuming constant Alfvén speed vA in the corona and vA varying exponentially with height in the photosphere/chromosphere at both ends of the loop. We study analytically the influence of chromospheric damping of standing waves on a coronal loop and find that, for a moderate amount of chromospheric damping, the footpoint leakage can be enhanced. The damping in the partially ionized chromosphere is mostly due to collisions between ions and neutrals.
In a second part we calculate the damping time for Alfvén waves of a given frequency, propagating through (model) chromospheres of various solar structures such as active region plage, quiet sun and the penumbra and umbra of sunspots. For a given wave frequency, the maximum damping always occurs at temperature minimum heights and in the coldest structure(s), i.e. the umbra of a sunspot. Energy dissipation due to ion-neutral damping of Alfvén waves could play a considerable role in the energy balance of umbrae, quiet sun and plage for wave periods of the order, respectively, 100, 10 and 1 s.
The author(s) of this abstract have provided an email address for comments about the abstract: bdp@lmsal.com