First direct observation of a torsional Alfvén oscillation at coronal heights

Abstract

Context. Torsional Alfvén waves are promising candidates for transport of energy across different layers of the solar atmosphere and have been theoretically predicted for decades. Previous detections of Alfvén waves so far have however mostly relied on indirect signatures. Aims. We present a first direct observational evidence of a fully resolved torsional Alfvén oscillation of a large-scale structure occurring at coronal heights. Methods. We analyse IRIS imaging and spectral observation of a surge resulting from magnetic reconnection between active region prominence threads and surrounding magnetic fieldlines. Results. The IRIS spectral data provides clear evidence of an oscillation in the line-of-sight velocity with a 180 • phase difference between the oscillation signatures at opposite edges of the surge flux tube. This together with an alternating tilt in the Si IV and Mg II k spectra across the flux tube and the trajectories traced by the individual threads of the surge material provides clear evidence of torsional oscillation of the flux tube. Conclusions. Our observation shows that magnetic reconnection leads to the generation of large-scale torsional Alfvén waves.

Diagnostics of plasma ionisation using torsional Alfén waves

“…Interestingly, this value is almost identical with the VAL-C model prediction (Vernazza et al 1981). Although the observations made by Kohutova et al (2020) constitute the first observation of torsional Alfvén waves in the solar prominence plasma, the observed values for damping time, period, amplitude, and phase speed do not contain the error range that usually comes with observationally determined parameters. That is why we would like to investigate what effect a 10% error range in physical parameters would have on the determined value of neutral number density.…”

“…The large-scale torsional wave observed by Kohutova et al (2020) was detected in both the imaging and spectral data. The helical motion is clearly visible in the temporal evolution of the Doppler velocity as an anti-phase oscillation at the opposite edges of the flux tube.…”

“…Arregui et al 2012) . However, this wavelength is larger than the gravitational scale-height corresponding to T = 10 4 K, meaning that for a more accurate description of torsional Alfvén waves observed by Kohutova et. al.…”

“…In a recent study, Kohutova et al (2020) report on the first detection of large-scale torsional Alfvén waves that propagate in the solar chromosphere. These waves were initiated by a reconnection event, which was followed by a surge of cool plasma that was previously confined in the prominence flux rope.…”

“…The present Letter is intended to exploit the propagation characteristics of recently observed torsional Alfvén waves in the solar prominences (Kohutova et al 2020) to infer information about the ionisation degree of the partially ionised prominence plasma. Using a very simple mathematical model to describe waves, we derived an evolutionary equation and its characteristics are compared to observed values.…”