The Interface Region Imaging Spectrograph (IRIS) has revealed a wealth of dynamic, fine-scale transition region structures; these structures may account for the so-called “unresolved fine structure” in the solar transition region and the unexplained excess of transition region emission. In particular, small scale “loops” are observed in active region cores when there is strong magnetic shear. Typical ionization times for the transition region lines observed by IRIS are tens of seconds, which are approximately the same as the lifetimes of the structures, and drives the population of emitting ions out of thermal equilibrium with the plasma. This makes understanding the evolution of these loops impossible using standard observational diagnostic techniques. However, because of the high quality IRIS data and the existence of several transition region lines in the different Solar Dynamics Observatory / Atmospheric Imaging Assembly (SDO/AIA) channels, we have the information required to constrain models of the ionization state and ultimately determine the energy deposition in these structures.

Working in collaboration with Dr. Amy Winebarger (NASA Marshall Space Flight Center), we are investigating the fine-scale structure of the transition region by:

1. characterizing the evolution of transition region loops using both IRIS and SDO/AIA data;
2. modeling the loops with a hydrodynamic code, solving for the time-dependent ionization state, finding their temperature and density evolution, and consequently their energy requirements; and
3. building time-dependent 3D models of the loops using a new forward modeling code that combines hydrodynamic solutions with magnetic field extrapolations to synthesize observable quantities for direct comparison with our IRIS and AIA observations.

By quantitatively assessing the ability of our models to reproduce the observations we gain insights into the underlying physical processes and will thereby achieve our science goal to understand the energy balance of transition region structures observed by IRIS in non-equilibrium emission.

In a second project with Dr. Paola Testa (Smithsonian Astrophysical Observatory) we are addressing fundamental questions that lie at the heart of coronal heating research. We are analyzing IRIS imaging and spectroscopic data from the transition region foot-points of coronal loops, in non-flaring active regions, to investigate the effects of non-equilibrium ionization on the emission observed during impulsive heating localized in the corona. IRIS provides unprecedented spatial and temporal resolution for spectral observations in the transition region and by combining data analysis with numerical and forward modeling we

• develop diagnostics to determine when we should be concerned about non-equilibrium ionization; and
• show how to account for non-equilibrium ionization when diagnosing the properties of coronal heating from transition region emission.