The Sub-Surface Structure of a Large Sample of Active Regions

TL;DR

This study uses ring-diagram analysis on a large sample of active regions during Solar Cycle 23 to investigate their sub-surface thermal structures, confirming and expanding previous findings about sound speed and adiabatic index variations.

Contribution

It provides a comprehensive statistical analysis of sub-surface thermal perturbations in active regions using a large dataset and PCA, revealing new details about their depth-dependent structure.

Findings

Sound speed decreases at 3-7 Mm depth

Sound speed increases at 11-21 Mm depth

Adiabatic index varies with depth and magnetic field strength

Abstract

We employ ring-diagram analysis to study the sub-surface thermal structure of active regions. We present results using a large number of active regions over the course of Solar Cycle 23. We present both traditional inversions of ring-diagram frequency differences, with a total sample size of 264, and a statistical study using Principal Component Analysis. We confirm earlier results on smaller samples that sound speed and adiabatic index are changed below regions of strong magnetic field. We find that sound speed is decreased in the region between approximately r=0.99R_sun and r=0.995R_sun (depths of 3Mm to 7Mm), and increased in the region between r=0.97R_sun and r=0.985R_sun (depths of 11Mm to 21Mm). The adiabatic index is enhanced in the same deeper layers that sound-speed enhancement is seen. A weak decrease in adiabatic index is seen in the shallower layers in many active regions. We find that the magnitudes of these perturbations depend on the strength of the surface magnetic field, but we find a great deal of scatter in this relation, implying other factors may be relevant.