High-resolution observations of active region moss and its dynamics

Abstract: The \textit{High resolution Coronal Imager (Hi-C)} has provided the sharpest view of the EUV corona to date. In this paper we exploit its impressive resolving power to provide the first analysis of the fine-scale structure of moss in an active region. The data reveal that the moss is made up of a collection of fine threads, that have widths with a mean and standard deviation of $440\pm190$~km (Full Width Half Maximum). {The brightest moss emission is located at the visible head of the fine-scale structure and the fine structure appears to extend into the lower solar atmosphere.} The emission decreases along the features implying the lower sections are most likely dominated by cooler transition region plasma. These threads appear to be the cool, lower legs of the hot loops. In addition, the increased resolution allows for the first direct observation {of physical displacements of the moss fine-structure in a direction transverse to its central axis. Some of these transverse displacements demonstrate periodic behaviour, which we interpret as a signature of kink (Alfv\'enic) waves. Measurements of the properties of the transverse motions are made and the wave motions have} means and standard deviations of $55\pm37$~km for the transverse displacement amplitude, $77\pm33$~s for the period and $4.7\pm2.5$~km/s for the velocity amplitude. The presence of waves in the transition region of hot loops could have important implications for the heating of active regions.