Slow magnetosonic waves and instabilities in expanded flux tubes anchored in chromospheric/photospheric regions

In this paper some aspects of the continuous spectrum of magnetic flux tubes with end points anchored in chromospheric/photospheric regions are investigated. For the isothermal plasmas that are considered the variation of the slow magnetosonic continuum frequencies across the flux surfaces is very modest for the lowest frequency branches, even for higher averaged beta cases. An important feature of resonant absorption is that frequencies can be an order of magnitude lower than can be expected from a line-tied model of the low-density part of the tube. The slow magnetosonic continuum can become unstable and it is investigated how the growth rates depend on expansion of the flux tubes and how they depend on the pressure scale length.