Observations and Modeling of Long, Cool, and Overdense Loops in Active Region 11575

Long coronal loops at the periphery of active regions have been observed to be steady over intervals greater than a radiative cooling time, overdense, near-isothermal at approximately 1.5 MK, and have flat filter ratios. These relatively steady, high-density structures cannot be explained by either hydrostatic equilibrium or simple post-nanoflare radiative cooling and thus pose a challenge to current models of quiescent active region heating. To address these ambiguities, we analyze observations of active region 11575 as observed on 29 September 2012 by the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) and the Extreme Ultraviolet Imaging Spectrometer (EIS) onboard the Hinode spacecraft. We manually isolate a single long loop near the periphery of the active region in SDO/AIA 171 A and calculate the density, emission measure distribution, and filter ratio. Additionally, we analyze the time variability of this structure in the EUV channels of AIA over a 12 h interval and compute cross-correlations between these channels. We then model the hydrodynamic evolution of this loop structure using the field-aligned Hydrodynamics and Radiation (HYDRAD) model for several different heating scenarios, including steady uniform heating as well as steady and time-dependent stratified footpoint heating. From our model results, we derive density and temperature diagnostics, emission measure distributions, and cross-correlations between synthetic SDO/AIA light curves in order to compare with our observations and thus constrain the parameter space of feasible heating models. While stratified, fully-asymmetric footpoint heating greatly increases the density of a 1.5 MK loop over hydrostatic equilibrium, we find that the modeled densities for all heating scenarios are significantly lower than those we derive from the EIS observations. Furthermore, we find that impulsive heating as well as thermal non-equilibrium, as induced by symmetric stratified footpoint heating, lead to emission measure distributions that are much broader than the observed distributions.