33 research outputs found
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
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