Modeling of AR 12760 with GX Simulator and Evidence for the Extended Transition Region in Peripheral Active Region Loops

In order to understand solar atmospheric heating, it is important to test heating models against spatially resolved data from solar active regions. Here, we model a small active region, AR 12760, observed on 2020 April 28, with the GX Simulator package by fitting the extreme-ultraviolet (EUV) intensities in wave bands observed by the Solar Dynamics Observatory’s Atmospheric Imaging Assembly. We assume the temporally and spatially averaged heating rate along a loop has a power-law dependence on loop length, L, and average magnetic field strength along the loop, B_avg. We find that the best-fit heating model for the 211 Å band is ⟨Q⟩ ≈ 7 × 1 0⁻³ (B_avg / 100 G)^1.5 (L / 10⁹ cm)⁻¹ erg cm⁻³ s⁻¹, but that there is a range of parameters that give qualitatively reasonable fits, which we conclude is due to a correlation between B_avg and L. In addition, we find that the models of the bands including cooler emission (131 and 171 Å) greatly underestimate the extent of the emission in the legs of the longer loops at the peripheries of the active region that are the strongest contributors of the emission in those bands. We conclude that this is because the modeling assumes that all transition-region emission is confined to the loop footpoints, but in reality the upper transition region of longer loops extends significantly farther into the loop. It will be important to consider this aspect of the transition region in future efforts to model EUV emission.