Trace and Ground-Based Observations of Small-Scale Dynamics in Active Regions

Abstract

Observations of solar active regions made with the Transition Region and Coronal Explorer (TRACE) have revealed finely textured, low-lying extreme ultraviolet (EUV) emission, called “moss”: a bright, highly dynamic pattern with dark inclusions, structured on spatial scales of 1 to 3 Mm. It was identified as the conductively heated, classical upper transition region above active region plage and below relatively hot and high pressure loops, coexisting with chromospheric jets at similar low heights (< 10, 000 km). This renders the moss into an excellent proxy for studies of the fine-structure of the transition region, and its coupling with chromosphere and corona in active regions. In this paper we review the known properties of the upper transition region above active region plage, focusing on its relationship to other temperature regimes, as well as what is behind the highly dynamic nature of this emission. We describe recent results quantifying much of the earlier qualitative work, and detailing the role of the chromospheric jets in the energy and momentum balance of the transition region. We also discuss how or whether the moss emission could be used as a proxy for coronal heating studies.