CORONAL MHD PERTURBATION FIELD GENERATED BY LOCALIZED PERTURBERS IN APHOTOSPHERIC ACTIVE-REGION

We investigate the two-dimensional boundary value problem of the linea r wave excitation in the solar corona by a pair of periodic perturbers localized at the photosphere. The physical properties of the corona a llow us to consider a magnetic configuration such as a potential arcad e which is in a magneto-hydrostatic equilibrium with the surrounding p lasma. The model excludes the Alfven mode and since the slow mode is a bsent in a potential arcade, the excited waves are then the fast magne to-acoustic modes. The characteristic magnetic field scale length is t wice the scale height of the coronal plasma, assumed isothermal and th e induced fluid motions are in the direction perpendicular to the magn etic field, in the cross-sectional plane of the arcade. A particular e xample of two localized perturbers in a photospheric active region is given, painting out the effects produced in the perturbed fields by th e variation of the different parameters involved. Such example shows t hat, while a modification of the linear size or the horizontal wave nu mber of the perturbers does not affect significantly the propagating d isturbances, the variation of the perturbers' frequency produces impor tant effects. Such effects can be summarized by saying that low-freque ncy perturbers are able to disturb the whole coronal magnetic configur ation above the active region while high-frequency perturbers only dis turb the coronal region just above them, producing a channeling featur e. Moreover, for high frequencies the perturbations grow with z all th e way from the photosphere while for very law frequencies evanescent w aves are dominant, although thanks to the influence of the medium thei r amplitude starts to grow from some z on.