Height Variations of Magnetoacoustic Cutoff Frequency in the Solar Atmosphere

TL;DR

This study measures how the magnetoacoustic cutoff frequency varies with height in the solar atmosphere using IRIS spectral data, revealing an increase from the photosphere to the chromosphere.

Contribution

It provides observational measurements of cutoff frequency variations at multiple heights, serving as a benchmark for refining theoretical models.

Findings

Cutoff frequency increases from 3.0 mHz at 0.38 Mm to 8.5 mHz at 1.2 Mm.

Higher chromospheric heights exhibit signs of standing oscillations.

Results compare favorably with previous studies, aiding model refinement.

Abstract

The determination of the cutoff frequency in real solar observations under different local physical conditions is an important and insufficiently explored aspect of waves in solar physics. This work utilizes the near ultraviolet (NUV) spectrum of the QS, observed by the Interface Region Imaging Spectrograph (IRIS) on November 16th, 2013, in sit-n-stare mode. It contains several absorption and emission lines that form at different heights between the photosphere and chromosphere. Cross-wavelet analysis is performed on Doppler velocity time series of pairs of spectral lines sampling different atmospheric layers to estimate the cutoff frequency at six different heights between the photosphere and chromosphere. It is found that the cutoff frequency increases with height from around 3.0 mHz at 0.38 Mm (photosphere) to around 8.5 mHz at 1.2 Mm (chromosphere). Higher chromospheric heights show indications of standing oscillations. The presented observational results are compared with those previously obtained, and serve as a benchmark to refine theoretical models that predict variations of cutoff frequencies in the solar atmosphere.