Three-Minute Oscillations in Sunspot's Penumbrae and Superpenumbrae. Alfv\'{e}nic or Sound?

TL;DR

This study investigates 3-minute oscillations in sunspot penumbrae and superpenumbrae, concluding they are mainly magnetoacoustic waves with potential Alfvénic wave presence in the photosphere, based on multi-height solar observations.

Contribution

The paper provides evidence that 3-minute LOS velocity oscillations are primarily magnetoacoustic, clarifying their nature and suggesting Alfvénic waves may exist in the photosphere.

Findings

Oscillations are mainly magnetoacoustic in nature.

Alfvénic waves may be present in the photosphere.

Further multi-height observations are needed.

Abstract

In the immediate sunspots' vicinity -- their superpenumbra -- 3-minute line-of-sight (LOS) velocity oscillations dominate in the photosphere and chromosphere. Oscillations of similar periods are also registered in the transition region and lower corona above active regions. The goal of the work is to clarify whether these LOS velocity oscillations are manifestations of Alfv\'enic waves in the lower solar atmosphere. The study is based on three sunspots with the use of the Solar Dynamics Observatory data. Additional observations of a sunspot were carried out at ground-based Automated Solar Telescope. We use narrow-band frequency filtration (5.6--5.8 mHz) of the LOS velocity, magnetic field, and intensity signals of the Fe i 6173 A spectral line. For the analysis, we used 90-minute long series. We come to the conclusion that the 3-minute oscillations in the LOS velocity signals result from magnetoacoustic waves rather than from Alfv\'enic waves. However, oscillations registered in magnetic field signals indicate that Alfv\'enic waves may be present already in the photosphere. Further research requires simultaneous observations of LOS velocity, magnetic field strength, spectral line width, and intensity carried out at two heights of the solar atmosphere.