Detection of Fast-Moving Waves Propagating Outward along Sunspots' Radial Direction in the Photosphere

TL;DR

This study detects a fast-moving magnetoacoustic wave propagating outward along sunspots' radial direction, revealing new insights into sunspot dynamics and internal structure through helioseismic observations.

Contribution

It reports the first observation of a high-speed wave traveling along sunspots' radial direction, suggesting a new method to probe sunspot interior structures.

Findings

Detected a wave with phase velocity of 45.3 km/s

Wave propagates from 5 Mm beneath the surface to beyond the sunspot boundary

Wave frequency range is 2.5 - 4.0 mHz

Abstract

Helioseismic and magnetohydrodynamic waves are abundant in and above sunspots. Through cross-correlating oscillation signals in the photosphere observed by the SDO/HMI, we reconstruct how waves propagate away from virtual wave sources located inside a sunspot. In addition to the usual helioseismic wave, a fast-moving wave is detected traveling along the sunspot's radial direction from the umbra to about 15 Mm beyond the sunspot boundary. The wave has a frequency range of 2.5 - 4.0 mHz with a phase velocity of 45.3 km/s, substantially faster than the typical speeds of Alfven and magnetoacoustic waves in the photosphere. The observed phenomenon is consistent with a scenario of that a magnetoacoustic wave is excited at approximately 5 Mm beneath the sunspot, and its wavefront travels to and sweeps across the photosphere with a speed higher than the local magnetoacoustic speed. The fast-moving wave, if truly excited beneath the sunspot's surface, will help open a new window to study the internal structure and dynamics of sunspots.