||The upper transition region at the footpoints of the hottest loops in active regions is known as moss, highly structured and dynamic 1 MK plasma that is formed at the same heights as dynamic chromospheric jets emanating from the underlying plage regions. Moss provides an excellent laboratory to disentangle the complex interface between chromosphere and corona and to study how chromospheric and coronal heating mechanisms are spatio-temporally correlated (if at all). This is because moss is very sensitive to changes in the local heating rate and, since it is formed in a thin, corrugated layer, avoids the confusion introduce by line-of-sight superposition taht affects optically thin coronal diagnostics. Previous results based on lower-resolution instruments (e.g., TRACE, SDO/AIA) suggested a puzzling mismatch between low chromospheric and upper TR emission.
We will present results based on analysis of a unique coordinated dataset from IRIS and the sounding rocket HiC. The HiC 2.1 flight took place in 2018 and obtained several minutes of sub-arcsecond resolution images of the upper TR in Fe IX 171A, while IRIS obtained high-resolution rasters in the Mg II h & k lines at high cadence. Our analysis will focus on spatio-temporal correlations between the properties of the optically thick Mg II h & k lines, and the intensities of the HiC 2.1 images. We will also exploit the recently developed IRIS2 database to invert the Mg II h & k profiles and study correlations between the derived chromospheric temperature, density, and micro-turbulence (as a function of height in the chromosphere) and the overlying upper TR and coronal emission.
Our analysis provides insight and constraints on the nature and (dis)similarities of the heating mechanisms in both the chromosphere and corona.