Unusual Trends in Solar P-Mode Frequencies During the Current Extended Minimum

TL;DR

This study examines intermediate-degree solar p-mode frequency variations during the extended minimum, revealing significant frequency shifts that do not always align with solar activity proxies and indicating early signs of solar cycle 24.

Contribution

It provides new insights into the behavior of solar oscillation frequencies during a prolonged minimum, highlighting discrepancies between different data sources and revealing early cycle signatures.

Findings

Frequency changes exceed activity proxy variations during minimum.

Seismic minimum detected in MDI data but not in GONG mean shifts.

Signs of solar cycle 24 onset observed in GONG data from 2009.

Abstract

We investigate the behavior of the intermediate-degree mode frequencies of the sun during the current extended minimum phase to explore the time-varying conditions in the solar interior. Using contemporaneous helioseismic data from GONG and MDI, we find that the changes in resonant mode frequencies during the activity minimum period are significantly greater than the changes in solar activity as measured by different proxies. We detect a seismic minimum in MDI p-mode frequency shifts during 2008 July--August but no such signature is seen in mean shifts computed from GONG frequencies. We also analyze the frequencies of individual oscillation modes from GONG data as a function of latitude and observe a signature of the onset of the solar cycle 24 in early 2009. Thus the intermediate degree modes do not confirm the onset of the cycle 24 during late 2007 as reported from the analysis of the low-degree GOLF frequencies. Further, both the GONG and MDI frequencies show a surprising anti-correlation between frequencies and activity proxies during the current minimum, in contrast to the behavior during the minimum between cycles 22 and 23.