Hi-C Observations of Sunspot Penumbral Bright Dots

TL;DR

This study uses high-resolution Hi-C data to observe and analyze bright dots in a sunspot penumbra, revealing their sizes, motions, interactions, and likely transition region temperatures, which enhances understanding of small-scale solar phenomena.

Contribution

First high-resolution analysis of penumbral bright dots using Hi-C, providing detailed measurements and temperature estimates that improve understanding of their nature and behavior.

Findings

BDs are about 1 arcsecond in size, smaller than AIA resolution.

Most BDs move laterally along penumbral striations.

BDs likely have transition region temperatures between 100,000 and 400,000 K.

Abstract

We report observations of bright dots (BDs) in a sunspot penumbra using High Resolution Coronal Imager (Hi-C) data in 193 \AA\ and examine their sizes, lifetimes, speeds, and intensities. The sizes of the BDs are on the order of 1\arcsec\ and are therefore hard to identify in the Atmospheric Imaging Assembly (AIA) 193 \AA\ images, which have 1.2\arcsec\ spatial resolution, but become readily apparent with Hi-C's five times better spatial resolution. We supplement Hi-C data with data from AIA's 193 \AA\ passband to see the complete lifetime of the BDs that appeared before and/or lasted longer than Hi-C's 3-minute observation period. Most Hi-C BDs show clear lateral movement along penumbral striations, toward or away from the sunspot umbra. Single BDs often interact with other BDs, combining to fade away or brighten. The BDs that do not interact with other BDs tend to have smaller displacements. These BDs are about as numerous but move slower on average than Interface Region Imaging Spectrograph (IRIS) BDs, recently reported by \cite{tian14}, and the sizes and lifetimes are on the higher end of the distribution of IRIS BDs. Using additional AIA passbands, we compare the lightcurves of the BDs to test whether the Hi-C BDs have transition region (TR) temperature like that of the IRIS BDs. The lightcurves of most Hi-C BDs peak together in different AIA channels indicating that their temperature is likely in the range of the cooler TR ($1-4\times 10^5$ K).