Cyclic Changes of the Sun's Seismic Radius

TL;DR

This study analyzes 21 years of helioseismology data to reveal cyclic variations in the Sun's seismic radius, showing it shrinks during solar maxima mainly beneath the surface, likely due to magnetic field changes.

Contribution

It provides the first detailed measurement of subsurface seismic radius variations over two solar cycles using high-precision helioseismology data.

Findings

Seismic radius decreases by 1-2 km during solar maxima.

Most significant radius variations occur about 5 Mm beneath the surface.

Changes are likely caused by vertical magnetic fields of ~10 kG.

Abstract

The questions whether the Sun shrinks with the solar activity and what causes this have been a subject of debate. Helioseismology provides means to measure with high precision the radial displacement of subsurface layers, co-called "seismic radius", through analysis of oscillation frequencies of surface gravity (f) modes. Here, we present results of a new analysis of twenty one years of helioseismology data from two space missions, Solar and Heliospheric Observatory (SoHO) and Solar Dynamics Observatory (SDO), which allow us to resolve previous uncertainties and compare variations of the seismic radius in two solar cycles. After removing the f-mode frequency changes associated with the surface activity we find that the mean seismic radius is reduced by 1-2 km during the solar maxima, and that most significant variations of the solar radius occur beneath the visible surface of the Sun at the depth of about 5 Mm, where the radius is reduced by 5-8 km. These variations can be interpreted as changes in the solar subsurface structure caused by predominately vertical ~10 kG magnetic field.