Helical and rotating plasma structures in the solar atmosphere
Department of Physics, Faculty of Sciences, University of Birjand, Shahid Aviny Street, Birjand, Iran
2 Department of Physics, Tafresh University, Tafresh, 39518 79611 Iran
Accepted: 9 July 2018
Aims. We model helical or rotating signatures in the solar atmosphere to further understand the efficiency of the equilibrium conditions, for example magnetic twist, rotation, plasma-β, and viscous effects on the life of solar helical structures.
Methods. Solar rotating structures, such as tornadoes, spirals, and whirls are modelled by considering a rotating and twisted magnetic cylinder residing in an environment with a straight magnetic field. A macroscopic approach proves adequate for working on the phase speed and damping of waves in solar atmospheric structures; as such, the magnetohydrodynamic theory is implemented. In this way the second order thin flux tube approximation is used for obtaining expressions for the frequency, deceleration, and damping of torsional waves in solar plasma structures in the presence of equilibrium rotation, magnetic twist, viscosity, and gravity.
Results. The dependency of the dissipation effects regarding the torsional wave in the linear regime is highlighted. The dispersion relation for axisymmetric oscillations propagating along a rotating and twisted solar cylindrical plasma structure in the presence of plasma viscosity and gravity is obtained. In this way we present explicit expressions for the oscillation and damping of torsional waves. The explicit expressions shed light on the influence of the equilibrium and environmental conditions on the speed deceleration, frequency, and damping of the torsional wave that exists in various layers of the solar atmosphere. The dispersion of the torsional wave is highly controlled by the combined effects of the rotation and the plasma-β, where when both are zero, the magnetic twist becomes significant only when the plasma resistivity comes into play. Regarding damping, the dominant actor for coronal conditions is the magnetic twist. However, since the damping time is highly dependent on the plasma-β, for photospheric conditions, the rotation becomes very significant. The damping of torsional waves is inversely proportional to the elevation of the rotating structure. This means that if the torsional wave survives through the photosphere and chromosphere, the chance for it to extend through the corona and solar wind is very high by gradually dissipating energy, which gives more opportunity for it to be observed.
Key words: magnetohydrodynamics (MHD) / waves / Sun: corona / Sun: activity
© ESO 2018