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 0.7−0.8 0 /L 0.5 0 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.

Diagnostics of Coronal Heating in Active-Region Loops

“…For example, Ugarte-Urra et al (2005) obtained the filling factor estimates of 0.1-0.7, dependent on the location along the loop and the spectral lines chosen to derive the emission measure. Dudik et al (2011) obtained values from 0.06 to 0.25 in different spectral bands. Moreover, the authors typically assume that the filling factor is the same for all loops which may not in fact be true.…”

“…There is also a generally acknowledged difficulty in simultaneously reproducing X-ray and EUV emission (e.g., Dudik et al 2011), with a suggestion that possibly a majority of EUV loops are in a dynamic state. On the other hand, 3D MHD models assume a Joule heating mechanism (β = 2.0), and present a picture of dynamically evolving corona which looks qualitatively visually convincing but so far do not find support in observational estimates of the power index β.…”

“…This would also allow a derivation of the parameter λ in Equation 4. (c) models of dynamic loops should be incorporated into the diagnostics, as was suggested by Dudik et al (2011), however, without providing sufficient detail in their paper to enable reproducing their results. (d) finally, in parallel to the points above, a growing sophistication of the 3D MHD models may eventually lead to these models accurately reproducing the magnetic structure and the emission of the active regions and enabling a quantitative comparison to observations.…”

“…The EUV emission from these loops in the selected four wavelengths is then modelled by convolving the N e , T e distributions with the temperature responses of SDO/AIA and the pure spectral lines. Here, we do not fit Q a and β to exactly match the entire active region emission in one or more bands, as done for example by Dudik et al (2011). Instead, we use a plausible pair of Q a and β to illustrate the principle of our modelling.…”

“…They confirmed that no more than 28% of loops observed by TRACE and Yohkoh/SXT can be interpreted as static loops. Dudik et al (2011) investigated whether observations in EUV and X-ray filters can be modelled simultaneously under the assumption of steady heating and potential field extrapolations. They used a spatially variable heating rate along the loop, characterised by a heating scale length, calculated hydrostatic models and compared the modelled emission to observed emission simultaneously in SOHO/EIT 171, 195 and 284 bands, and X-ray emission in Hinode/XRT.…”