High frequency waves in the solar atmosphere?

TL;DR

This study investigates the reliability of high frequency wave observations in the solar atmosphere, revealing that Doppler signals are influenced by atmospheric response variations and caution is needed when estimating energy fluxes.

Contribution

It compares observational data with 3D simulations to assess the interpretation of high frequency wave signals in the solar atmosphere.

Findings

Doppler signals are affected by rapid height variations in velocity response.

High frequency wave energy flux estimates require careful correction for atmospheric effects.

Observed high frequency signals may not represent true propagating waves.

Abstract

The present study addresses the following questions: How representative of the actual velocities in the solar atmosphere are the Doppler shifts of spectral lines? How reliable is the velocity signal derived from narrowband filtergrams? How well defined is the height of the measured Doppler signal? Why do phase difference spectra always pull to 0 degrees phase lag at high frequencies? Can we actually observe high frequency waves (P<= 70s)? What is the atmospheric MTF of high frequency waves? How reliably can we determine the energy flux of high frequency waves? We address these questions by comparing observations obtained with Hinode/NFI with results from two 3D numerical simulations (Oslo Stagger and CO5BOLD). Our results suggest that the observed high frequency Doppler velocity signal is caused by rapid height variations of the velocity response function in an atmosphere with strong velocity gradients and cannot be interpreted as evidence of propagating high frequency acoustic waves. Estimates of the energy flux of high frequency waves should be treated with caution, in particular those that apply atmospheric MTF corrections.