The acoustic cut-off frequency of the Sun and the solar magnetic activity cycle

TL;DR

This study measures the Sun's acoustic cut-off frequency using SOHO data and finds it varies with the solar magnetic activity cycle, providing insights into the solar atmosphere's dynamics.

Contribution

It introduces a method to determine the acoustic cut-off frequency through coherence analysis of velocity and intensity data, revealing its variation over the solar cycle.

Findings

The acoustic cut-off frequency varies with the solar magnetic activity cycle.

A new coherence-based method effectively measures the acoustic cut-off frequency.

The variation correlates with solar activity levels.

Abstract

The acoustic cut-off frequency -the highest frequency for acoustic solar eigenmodes- is an important parameter of the solar atmosphere as it determines the upper boundary of the p-mode resonant cavities. At frequencies beyond this value, acoustic disturbances are no longer trapped but traveling waves. Interference amongst them give rise to higher-frequency peaks -the pseudomodes- in the solar acoustic spectrum. The pseudomodes are shifted slightly in frequency with respect to p modes making possible the use of pseudomodes to determine the acoustic cut-off frequency. Using data from GOLF and VIRGO instruments on board the SOHO spacecraft, we calculate the acoustic cut-off frequency using the coherence function between both the velocity and intensity sets of data. By using data gathered by these instruments during the entire lifetime of the mission (1996 till the present), a variation in the acoustic cut-off frequency with the solar magnetic activity cycle is found.