Determining Absorption, Emissivity Reduction, and Local Suppression Coefficients inside Sunspots

TL;DR

This study quantifies how absorption, emissivity reduction, and local suppression contribute to the reduction of solar acoustic wave power inside sunspots, revealing dominant mechanisms at different travel distances.

Contribution

It provides the first comprehensive measurement of multiple power-reduction coefficients inside sunspots using unfiltered SOHO/MDI data across 47 active regions.

Findings

Absorption dominates at short travel distances.

Local suppression is the main cause at large travel distances.

Emissivity reduction accounts for a smaller fraction of power loss.

Abstract

The power of solar acoustic waves is reduced inside sunspots mainly due to absorption, emissivity reduction, and local suppression. The coefficients of these power-reduction mechanisms can be determined by comparing time-distance cross-covariances obtained from sunspots and from the quiet Sun. By analyzing 47 active regions observed by SOHO/MDI without using signal filters, we have determined the coefficients of surface absorption, deep absorption, emissivity reduction, and local suppression. The dissipation in the quiet Sun is derived as well. All of the cross-covariances are width corrected to offset the effect of dispersion. We find that absorption is the dominant mechanism of the power deficit in sunspots for short travel distances, but gradually drops to zero at travel distances longer than about 6 degrees. The absorption in sunspot interiors is also significant. The emissivity-reduction coefficient ranges from about 0.44 to 1.00 within the umbra and 0.29 to 0.72 in the sunspot, and accounts for only about 21.5% of the umbra's and 16.5% of the sunspot's total power reduction. Local suppression is nearly constant as a function of travel distance with values of 0.80 and 0.665 for umbrae and whole sunspots respectively, and is the major cause of the power deficit at large travel distances.