Diagnostics of plasma ionisation using torsional Alfvén waves 

Istvan Ballai Submitted: 20200220 02:39
Using the recently observed torsional Alfvén waves in solar prominences, we determine the ionisation state of the plasma by taking into account that Alfvén waves propagate in a partially ionised prominence plasma. We derive the evolutionary equation of waves and compare the analytical solutions to observations to determine the number density of neutrals.Using a single fluid plasma approximation, where the wave damping is provided by the Cowling resistivity, we study the temporal evolution of waves. By comparing the solution of equations with observational data (period, amplitude, propagation speed), we determined the value of the Cowling resistivity that led us to draw a conclusion on the amount of neutrals in the partially ionised plasma, a quantity that cannot be measured directly or indirectly.Our results show that damped torsional Alfvén waves are an ideal diagnostic tool for the ionisation state of the plasma. Using a simple model, we find that at the observational temperature of torsional Alfvén waves, the number of neutrals, is of the order of 5 x 10^{10} cm^{3}.
Authors: Istvan Ballai
Projects:

Publication Status: in press
Last Modified: 20200224 14:13



Mean shear flows generated by nonlinear resonant Alfvén waves 

Istvan Ballai Submitted: 20090715 09:52
In the context of resonant absorption, nonlinearity has two
different manifestations. The first is the reduction in amplitude
of perturbations around the resonant point (wave energy
absorption). The second is the generation of mean shear flows
outside the dissipative layer surrounding the resonant point.
Ruderman et al. [Phys. Plasmas, 4, 75 (1997)] studied
both these effects at the slow resonance in isotropic plasmas.
Clack et al. [Astron. Astrophys., 494, 317 (2009)]
investigated nonlinearity at the Alfvén resonance, however,
they did not include the generation of mean shear flow. In this
present paper, we investigate the mean shear flow, analytically,
and study its properties. We find that the flow generated is
parallel to the magnetic surfaces and has a characteristic
velocity proportional to epsilon^{1/2}, where epsilon is the
dimensionless amplitude of perturbations far away from the
resonance. This is, qualitatively, similar to the flow generated
at the slow resonance. The jumps in the derivatives of the
parallel and perpendicular components of mean shear flow across
the dissipative layer are derived. We estimate the generated mean
shear flow to be of the order of 10 km s^{1} in both the
solar upper chromosphere and solar corona, however, this value
strongly depends on the choice of boundary conditions. It is
proposed that the generated mean shear flow can produce a
KelvinHelmholtz instability at the dissipative layer which can
create turbulent motions. This instability would be an additional
effect, as a KelvinHelmholtz instability may already exist due
to the velocity field of the resonant Alfvén waves. This flow can
also be superimposed onto existing large scale motions in the
solar upper atmosphere.
Authors: Christopher Clack, Istvan Ballai
Projects: None

Publication Status: accepted
Last Modified: 20090715 10:25



On the nature of coronal EIT waves 

Istvan Ballai Submitted: 20051101 10:32
Largescale eruption events in the solar atmosphere can generate global waves, i.e., waves that propagate over
distances comparable to the solar radius. In the low solar corona, global waves observed by SOHO EIT, generated
by coronal mass ejections or flares, are usually referred to as “EIT waves.” The nature of these global waves is
the subject of strong debate, and opinions are divided between different interpretations (e.g., fast magnetohydrodynamic
waves, shock waves, nonwave feature, etc.). In the present Letter, we studied TRACE EUV data
to show that these global coronal disturbances are indeed waves with a welldefined period. Supposing that the
EIT waves transfer all their energy to interacting loops, we also estimate the minimum energy threshold carried
by EIT waves.
Authors: I. Ballai, R. Erdelyi and B. Pinter
Projects: TRACE

Publication Status: ApJL (in press)
Last Modified: 20051101 10:32




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