Seismic sensitivity to sub-surface solar activity from 18 years of GOLF/SoHO observations

TL;DR

This study analyzes 18 years of solar oscillation data to understand how the Sun's sub-surface structure and magnetic activity responded to the unusual low activity during Solar Cycle 24, revealing that deeper layers remained similar while upper layers weakened.

Contribution

It provides a detailed analysis of solar acoustic mode frequency shifts over 18 years, highlighting differences between Cycles 23 and 24 in various sub-surface layers.

Findings

Deeper layers showed similar structural and magnetic properties in Cycles 23 and 24.

Higher-frequency modes indicate a weaker magnetic field in the upper layers during Cycle 24.

The quasi-biennial oscillation signature is visible in low-frequency modes.

Abstract

Solar activity has significantly changed over the last two Schwabe cycles. After a long and deep minimum at the end of Cycle 23, the weaker activity of Cycle 24 contrasts with the previous cycles. In this work, the response of the solar acoustic oscillations to solar activity is used in order to provide insights on the structural and magnetic changes in the sub-surface layers of the Sun during this on-going unusual period of low activity. We analyze 18 years of continuous observations of the solar acoustic oscillations collected by the Sun-as-a-star GOLF instrument onboard the SoHO spacecraft. From the fitted mode frequencies, the temporal variability of the frequency shifts of the radial, dipolar, and quadrupolar modes are studied for different frequency ranges which are sensitive to different layers in the solar sub-surface interior. The low-frequency modes show nearly unchanged frequency shifts between Cycles 23 and 24, with a time evolving signature of the quasi-biennial oscillation, which is particularly visible for the quadrupole component revealing the presence of a complex magnetic structure. The modes at higher frequencies show frequency shifts 30% smaller during Cycle~24, which is in agreement with the decrease observed in the surface activity between Cycles 23 and 24. The analysis of 18 years of GOLF oscillations indicates that the structural and magnetic changes responsible for the frequency shifts remained comparable between Cycle 23 and Cycle 24 in the deeper sub-surface layers below 1400 km as revealed by the low-frequency modes. The frequency shifts of the higher-frequency modes, sensitive to shallower regions, show that Cycle 24 is magnetically weaker in the upper layers of Sun.