Volume 635, March 2020
|Number of page(s)||4|
|Section||Letters to the Editor|
|Published online||02 March 2020|
Letter to the Editor
Diagnostics of plasma ionisation using torsional Alfén waves
Plasma Dynamics Group, School of Mathematics and Statisitics, University of Sheffield, Hounsfield Road, Hicks Building, Sheffield S3 7RH, UK
Accepted: 17 February 2020
Aims. 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.
Methods. 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.
Results. 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 × 1010 cm−3.
Key words: Sun: filaments, prominences / magnetohydrodynamics (MHD) / Sun: oscillations / Sun: magnetic fields
© ESO 2020
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