Surface-focused Seismic Holography of Sunspots: I. Observations

TL;DR

This paper uses surface-focused seismic holography to analyze how p-mode oscillations interact with sunspots, revealing frequency-dependent travel-time shifts and comparing different filtering methods to better understand wave behavior in magnetic regions.

Contribution

It provides a detailed observational analysis of p-mode interactions with sunspots using various filtering techniques, highlighting frequency-dependent effects and differences from previous scattering studies.

Findings

Travel-time shifts vary strongly with frequency within sunspots.

Positive shifts are linked to low-frequency p1 ridge power.

Surface holography results differ from Fourier-Hankel scattering measurements.

Abstract

We present a comprehensive set of observations of the interaction of p-mode oscillations with sunspots using surface-focused seismic holography. Maps of travel-time shifts, relative to quiet-Sun travel times, are shown for incoming and outgoing p modes as well as their mean and difference. We compare results using phase-speed filters with results obtained with filters that isolate single p-mode ridges, and further divide the data into multiple temporal frequency bandpasses. The f mode is removed from the data. The variations of the resulting travel-time shifts with magnetic-field strength and with the filter parameters are explored. We find that spatial averages of these shifts within sunspot umbrae, penumbrae, and surrounding plage often show strong frequency variations at fixed phase speed. In addition, we find that positive values of the mean and difference travel-time shifts appear exclusively in waves observed with phase-speed filters that are dominated by power in the low-frequency wing of the p1 ridge. We assess the ratio of incoming to outgoing p-mode power using the ridge filters and compare surface-focused holography measurements with the results of earlier published p-mode scattering measurements using Fourier-Hankel decomposition.