Solar-cycle variation of sound speed near the solar surface

TL;DR

This study reveals a two-layer pattern of sound-speed variation near the solar surface that correlates with solar activity cycles, using helioseismic data to analyze changes from the solar minimum to maximum.

Contribution

It provides the first detailed characterization of the two-layer sound-speed variation pattern associated with solar activity, based on helioseismic observations.

Findings

Sound speed is smaller at solar maximum near the surface (~5.5 Mm).

The difference in sound speed between solar maximum and minimum varies with depth, reversing sign around 7-10 Mm.

The variation pattern is consistent with a two-layer structure linked to solar activity.

Abstract

We present evidence that the sound-speed variation with solar activity has a two-layer configuration, similar to the one observed below an active region, which consists of a negative layer near the solar surface and a positive one in the layer immediately below the first one. Frequency differences between the activity minimum and maximum of solar cycle 23, obtained applying global helioseismology to the Michelson Doppler Imager (MDI) on board SOHO, is used to determine the sound-speed variation from below the base of the convection zone to a few Mm below the solar surface. We find that the sound speed at solar maximum is smaller than at solar minimum at the limit of our determination (5.5 Mm). The min-to-max difference decreases in absolute values until ~7 Mm. At larger depths, the sound speed at solar maximum is larger than at solar minimum and their difference increases with depth until ~10 Mm. At this depth, the relative difference is less than half of the value observed at the lowest depth determination. At deeper layers, it slowly decreases with depth until there is no difference between maximum and minimum activity.