Observations of Running Penumbral Waves emerging in a Sunspot

TL;DR

This study investigates 5-minute umbral oscillations and their relation to running penumbral waves in a sunspot, revealing new insights into wave propagation and emergence patterns across atmospheric layers.

Contribution

It presents the first observation of 3-minute umbral oscillatory events propagating from the umbral boundary, linked to running penumbral waves, and suggests twisted waveguides in sunspot magnetic structures.

Findings

Detected 5-minute oscillations as running waves outside the umbral center.

First observation of 3-minute oscillatory events propagating from umbral boundaries.

Most umbral oscillatory events develop into running penumbral waves.

Abstract

We present results from the investigation of 5-min umbral oscillations in a single-polarity sunspot of active region NOAA 12132. The spectra of TiO, H$\alpha$, and 304 \AA{} are used for corresponding atmospheric heights from the photosphere to lower corona. Power spectrum analysis at the formation height of H$\alpha$ - 0.6 \AA{} to H$\alpha$ center resulted in the detection of 5-min oscillation signals in intensity interpreted as running waves outside the umbral center, mostly with vertical magnetic field inclination $>15\deg$. A phase-speed filter is used to extract the running wave signals with speed $v_{ph}> 4$ km s$^{-1}$, from the time series of H$\alpha$ - 0.4 \AA{} images, and found twenty-four 3-min umbral oscillatory events in a duration of one hour. Interestingly, the initial emergence of the 3-min umbral oscillatory events are noticed closer to or at umbral boundaries. These 3-min umbral oscillatory events are observed for the first time as propagating from a fraction of preceding Running Penumbral Waves (RPWs). These fractional wavefronts rapidly separates from RPWs and move towards umbral center, wherein they expand radially outwards suggesting the beginning of a new umbral oscillatory event. We found that most of these umbral oscillatory events develop further into RPWs. We speculate that the waveguides of running waves are twisted in spiral structures and hence the wavefronts are first seen at high latitudes of umbral boundaries and later at lower latitudes of the umbral center.