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MHD wave interaction with coronal active region plasmas
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Vasheghani Farahani, Soheil (2011) MHD wave interaction with coronal active region plasmas. PhD thesis, University of Warwick.

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Official URL: http://webcat.warwick.ac.uk/record=b2491296~S15
Abstract
Interaction of magnetohydrodynamic (MHD) waves with various structures in a
magnetised plasma was considered theoretically in the context of the interpretation
of recently observed phenomena in the corona of the Sun. The main emphasis
was put on the development of analytical models, utilising various asymptotic techniques
based upon the presence of a small parameter. In the consideration of waves
guided by fieldaligned plasma nonuniformity, such as coronal jets and plumes, the
small parameter was the ratio of the diameter of the guiding nonuniformity to the
wavelength, the approach known as the \thin
uxtube approximation". In the consideration
of nonlinear effects, the wave amplitude was taken to be finite, but small,
and hence could be treated as a small parameter too.
In the thesis, we addressed several specific timely problems of modern solar physics:
the interpretation of recently discovered transverse waves on soft Xray coronal jets
in terms of a kink fast magnetoacoustic wave; modelling of enigmatic torsional waves
(also known as twisting waves or waves of the electric current) guided by cylindrical
coronal structures, such as loops, plumes, filaments and jets, accounting for the effects
of the magnetic twisting and rotation of the equilibrium plasma configuration;
weakly nonlinear effects appearing during the propagation of the torsional waves
along coronal magnetic waveguides, concentrating on the nonlinearly induced compressible
perturbations; and nonlinear steepening of fast magnetoacoustic waves in
the vicinity of a magnetic nullpoint, in the context of the possible triggering of
magnetic reconnection by the deposition of currentdriven anomalous resistivity.
In the first Chapter, we give an overview of the solar atmosphere and dynamical
processes observed there such as MHD waves and plasma
ows. Also, the set of
MHD equations is introduced, and the main modes of a basic coronal plasma structure,
a magnetic cylinder, are considered by the method of dispersion relation.
In Chapter 2, we considered the longwavelength limit in the magnetic cylinder dispersion
relations, and derived explicit expressions, which link the phase and group
speeds for linear kink magnetoacoustic waves guided by hot plasma jets surrounded
by a static plasma. With the use of the derived expressions, we showed that transverse
waves recently discovered by Hinode/XRT on coronal jets are the kink waves.
In the observationally determined range of parameters, the waves are not found to
be subject to either the KelvinHelmholtz instability or negative energy wave instabilities,
and hence they are likely to be excited at the source of the jet. We also
carried out forward modelling of the observables, and demonstrated its consistency
with XRT observations.
In Chapter 3 we considered long wavelength axisymmetric magnetohydrodynamic
waves, and derived asymptotic dispersion relations linking phase speeds with the
plasma parameters using the second order thin
ux tube approximation. We showed
that when uniform twist and rotation are both present, the phase speed of torsional
waves depends upon the direction of the wave propagation. In addition, the twist
and rotation causes compressibility of the torsional waves. The phase relations show
that in a torsional wave the density and azimuthal magnetic field perturbations are
in phase with the axial magnetic field perturbations and antiphase with tube crosssection
perturbations. In a zeroβ nonrotating plasma cylinder confined by the
equilibrium twist, the density perturbation is found to be about 66 percent of the
amplitude of the twist perturbation in torsional waves.
In Chapter 4, we considered the nonlinear phenomena accompanying longwavelength
torsional waves in an untwisted and nonrotating magnetic
ux tube. We showed
that propagating torsional waves induce compressible perturbations by nonlinear forces, these compressible perturbations oscillate with double the frequency of the
torsional waves. In contrast with plane shear Alfvén waves, the amplitude of compressible
perturbations is independent of the plasmaβ. But, as in the shear Alfvén
wave, the amplitude of compressible perturbations are proportional to the torsional
wave amplitude square. It was also shown that standing torsional waves induce
compressible perturbations of two kinds, those which grow with the characteristic
time inversely proportional to the sound speed, and those which oscillate at double
the frequency of the inducing torsional wave. The growing density perturbation
saturates at the level, inversely proportional to the sound speed.
In Chapter 5, we studied the generation of fast magnetoacoustic shocks in the vicinity
of a magnetic nullpoint. In the weakly nonlinear limit, we derived a simple wave
evolutionary equation, which provided us with the qualitative information about the
nonlinear evolution of the fast wavepulse: formation of the shock and deformation
of the initial shape of the perturbation depending upon the polar angle. We compared
our analytical solutions with numerical solutions and found that the speed of
the fast magnetoacoustic pulse depends on the initial amplitude of the pulse. In our
parametric studies we showed that although the initial amplitude of the magnetoacoustic
pulse is responsible for the time the pulse overturns, the initial width of the
pulse should not be ignored. We showed that narrower and higher amplitude pulses
overturn at larger distance from the nullpoint. In the context of the sympathetic flaring a stronger initial pulse does not guarantee a stronger effect.
Item Type:  Thesis or Dissertation (PhD)  

Subjects:  Q Science > QB Astronomy  
Library of Congress Subject Headings (LCSH):  Magnetohydrodynamic waves, Sun  Corona  
Official Date:  March 2011  
Dates: 


Institution:  University of Warwick  
Theses Department:  Department of Physics  
Thesis Type:  PhD  
Publication Status:  Unpublished  
Supervisor(s)/Advisor:  Nakariakov, Valery M. ; Verwichte, Erwin  
Sponsors:  Overseas Research Students Awards Scheme (ORSAS)  
Extent:  viii, 124 leaves : ill., charts  
Language:  eng 
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