Abstract
The properties of axisymmetric MHD wave modes are examined for the structured model of coronal magnetic tube, consistingof a central cord with homogeneouse axial magnetic field, surrounded by an annulus with twisted magnetic field. Thederived dispersion relation is solved numerically. A number of limiting cases are examined analytically. The two familiesof axisymmetric modes are found to exist in the model. The first one includes an infinite number of fast-sausage modes,modified by the twist and the second one is a set of modes with frequencies, lying in a narrow band, closed to Alfvénfrequency of a twisted layer ω A0 . The fundamental fast-sausage mode (the mode of the lowest radial order) exists as atrapped mode for the entire range of axial wavenumbers. Its phase speed is always below the Alfvén speed of a magneticallytwisted layer. This mode has a weak dispersion in the range of long and intermediate wavelengths. The application of themain results of the study to coronal seismology is considered.
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