Variations of the Solar Acoustic High-Degree Mode Frequencies over Solar Cycle 23

TL;DR

This study investigates how solar acoustic mode frequencies vary with the solar activity cycle 23 using high-degree mode analysis from SOHO/MDI data, confirming known frequency shift behaviors and discussing instrumental effects.

Contribution

It provides detailed analysis of high-degree solar acoustic mode frequency variations over solar cycle 23, including effects of instrumental defocusing and validation against medium-degree results.

Findings

High-degree mode frequency shifts correlate with solar activity.

Frequency shift scaled by mode inertia depends on frequency, not degree.

Instrumental defocusing significantly affects high-degree mode estimates.

Abstract

Using full-disk observations obtained with the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO) spacecraft, we present variations of the solar acoustic mode frequencies caused by the solar activity cycle. High-degree (100 < l < 900) solar acoustic modes were analyzed using global helioseismology analysis techniques over most of solar cycle 23. We followed the methodology described in details in Korzennik, Rabello-Soares and Schou (2004) to infer unbiased estimates of high-degree mode parameters (see also Rabello-Soares, Korzennik and Schou, 2006). We have removed most of the known instrumental and observational effects that affect specifically high-degree modes. We show that the high-degree changes are in good agreement with the medium-degree results, except for years when the instrument was highly defocused. We analyzed and discuss the effect of defocusing on high degree estimation. Our results for high-degree modes confirm that the frequency shift scaled by the relative mode inertia is a function of frequency and it is independent of degree.