Multi-layer study of wave propagation in sunspots

TL;DR

This study investigates wave propagation in sunspots from the photosphere to the chromosphere, revealing standing and propagating waves, non-linear effects, and differences in line formation heights using spectroscopic data.

Contribution

It provides a multi-layer analysis of wave behavior in sunspots, combining observations with a linear wave propagation model to determine formation heights and non-linear effects.

Findings

Standing waves at frequencies below 4 mHz

Waves propagate and steepen into shocks at higher frequencies

Ca II H forms lower than He I 10830, with a 20 s delay

Abstract

We analyze the propagation of waves in sunspots from the photosphere to the chromosphere using time series of co-spatial Ca II H intensity spectra (including its line blends) and polarimetric spectra of Si I 10827 and the He I 10830 multiplet. From the Doppler shifts of these lines we retrieve the variation of the velocity along the line-of-sight at several heights. Phase spectra are used to obtain the relation between the oscillatory signals. Our analysis reveals standing waves at frequencies lower than 4 mHz and a continuous propagation of waves at higher frequencies, which steepen into shocks in the chromosphere when approaching the formation height of the Ca II H core. The observed non-linearities are weaker in Ca II H than in He I lines. Our analysis suggests that the Ca II H core forms at a lower height than the He I 10830 line: a time delay of about 20 s is measured between the Doppler signal detected at both wavelengths. We fit a model of linear slow magnetoacoustic wave propagation in a stratified atmosphere with radiative losses according to Newton's cooling law to the phase spectra and derive the difference in the formation height of the spectral lines. We show that the linear model describes well the wave propagation up to the formation height of Ca II H, where non-linearities start to become very important.