Is it possible to model observed active region coronal emission simultaneously in EUV and X-ray filters?

Abstract

Aims: We investigate the possibility of modeling the active region coronal emission in the EUV and X-ray filters using one, universal, steady heating function, tied to the properties of the magnetic field.
Methods: We employ a simple, static model to compute the temperature and density distributions in the active region corona. The model allows us to explore a wide range of parameters of the heating function. The predicted EUV and X-ray emission in the filters of EIT/SOHO and XRT/Hinode are calculated and compared with observations. Using the combined improved filter-ratio (CIFR) method, a temperature diagnostic is employed to compare the modeled temperature structure of the active region with the temperature structure derived from the observations.
Results: The global properties of the observations are most closely matched for heating functions scaling as B_0^{0.7-0.8/L_0^{0.5}} that depend on the spatially variable heating scale-length. The modeled X-ray emission originates from locations where large heating scale-lengths are found. However, the majority of the loops observed in the 171 and 195 filters can be modeled only by loops with very short heating scale-lengths. These loops are known to be thermally unstable. We are unable to find a model that both matches the observations in all EUV and X-ray filters, and contains only stable loops. As a result, although our model with a steady heating function can explain some of the emission properties of the 171 and 195 loops, it cannot explain their observed lifetimes. Thus, the model does not lead to a self-consistent solution. The performance of the CIFR method is evaluated and we find that the diagnosed temperature can be approximated with a geometric mean of the emission-measure weighted and maximum temperature along the line of sight.
Conclusions: We conclude that if one universal heating function exists, it should be at least partially time-dependent.