Origin of the Chromospheric Umbral Waves in Sunspots

TL;DR

This study investigates the origin of chromospheric umbral waves in sunspots, revealing that they are likely excited by p-mode oscillations and are associated with specific regions of high oscillation power within the umbra.

Contribution

It provides the first observational evidence linking chromospheric umbral waves to p-mode oscillations through analysis of spiral wave patterns in sunspots.

Findings

Two one-armed spiral wave patterns coexist in sunspot umbrae.

Chromospheric umbral waves originate from high oscillation power regions.

Most umbral waves occur in dark nuclei and strong magnetic field areas.

Abstract

Oscillations are ubiquitous in sunspots and the associated higher atmospheres. However, it is still unclear whether these oscillations are driven by the external acoustic waves (p-modes) or generated by the internal magnetoconvection. To obtain clues about the driving source of umbral waves in sunspots, we analyzed the spiral wave patterns (SWPs) in two sunspots registered by IRIS MgII 2796 {\AA} slit-jaw images. By tracking the motion of the SWPs, we find for the first time that two one-armed SWPs coexist in the umbra, and they can rotate either in the same or opposite directions. Furthermore, by analyzing the spatial distribution of the oscillation centers of the one-armed SWPs within the umbra (the oscillation center is defined as the location where the SWP first appears), we find that the chromospheric umbral waves repeatedly originate from the regions with high oscillation power and most of the umbral waves occur in the dark nuclei and strong magnetic field regions of the umbra. Our study results indicate that the chromospheric umbral waves are likely excited by the p-mode oscillations.