Acoustic Mode Frequencies of the Sun during the Minimum Phase between Solar Cycles 23 and 24

TL;DR

This study analyzes the variations in solar acoustic mode frequencies during the extended minimum between cycles 23 and 24, revealing weak correlations with magnetic activity and suggesting an earlier minimum epoch than traditional indices.

Contribution

It provides detailed analysis of high-degree mode frequency shifts during a solar minimum, highlighting spatial and temporal variations and their relation to solar activity indices.

Findings

Weak correlation between frequency shifts and magnetic field strength during low activity.

Maximum correlation of about 72% between frequency shifts and activity indices.

Minimum epoch of solar cycle 24 likely occurred in February 2008, earlier than activity indices suggest.

Abstract

We investigate the spatial and temporal variations of the high-degree mode frequencies calculated over localized regions of the Sun during the extended minimum phase between solar cycles 23 and 24. The frequency shifts measured relative to the spatial average over the solar disk indicate that the correlation between the frequency shift and magnetic field strength during the low-activity phase is weak. The disk-averaged frequency shifts computed relative to a minimal activity period also reveal a moderate correlation with different activity indices, with a maximum linear correlation of about 72%. From the investigation of the frequency shifts at different latitudinal bands, we do not find a consensus period for the onset of solar cycle 24. The frequency shifts corresponding to most of the latitudes in the northern hemisphere and 30 degree south of the equator indicate the minimum epoch to be February 2008, which is earlier than inferred from solar activity indices.