Incompressible magnetohydrodynamic modes in the thin magnetically twisted flux tube

Context. Observations have shown that twisted magnetic fields naturally occur, and indeed are omnipresent in the Sun’s atmosphere. It is therefore of great theoretical interest in solar atmospheric waves research to investigate the types of magnetohydrodynamic (MHD) wave modes that can propagate along twisted magnetic flux tubes. Aims. Within the framework of ideal MHD, the main aim of this work is to investigate small amplitude incompressible wave modes of twisted magnetic flux tubes with m ≥ 1. The axial magnetic field strength inside and outside the tube will be allowed to vary, to ensure the results will not be restricted to only cold plasma equilibria conditions. Methods. The dispersion equation for these incompressible linear MHD wave modes was derived analytically by implementing the long wavelength approximation. Results. It is shown, in the long wavelength limit, that both the frequency and radial velocity profile of the m = 1 kink mode are completely unaffected by the choice of internal background magnetic twist. However, fluting modes with m ≥ 2 are sensitive to the particular radial profile of magnetic twist chosen. Furthermore, due to background twist, a low frequency cut-off is introduced for fluting modes that is not present for kink modes. From an observational point of view, although magnetic twist does not affect the propagation of long wavelength kink modes, for fluting modes it will either work for or against the propagation, depending on the direction of wave travel relative to the sign of the background twist

Slow magnetoacoustic-like waves in post-flare loops

We investigate the stability to the development of plasma waves in the preflare situation of a loop structure at the chromospheric part of a current circuit of a loop. We investigate the conditions under which low-frequency plasma instabilities can develop, assuming the absence of beam instabilities.The large-scale quasi-static electric field in the loop circuit is assumed to be “subdreicer" and weak. Thus the percentage of “runaway" electrons is very small and their influence on the process of instability development is negligible. The pair Coulomb collisions are described by a BGK-model integral. We consider the situation when the plasma at the surface layer of a loop has a spatial gradient of density. In accordance with Heyvaerts-Priest-Rust theory, such a preflare situation would typically exist when the amplitude of the weak electric field $\left| \vec{E}_{0} (t) \right|$ in the circuit of an “old" loop in an active region begins to increase when “new" magnetic flux emerges from under the photosphere. We have found that two types of waves are generated in such a plasma due to the growth of instabilities: the “kinetic Alfven-like" waves and new type of waves, in the range of magnetoacoustic ones. The instability of these latter waves has a clear threshold and it can be considered as an “indicator" of the development of a preflare situation in an active region