Alena Kulinova

Diagnostics of the κ-distribution using Si III lines in the solar transition region

Aims. The solar transition region satisfies the conditions for appearance of the non-thermal κ-distribution. We aim to prove the occurrence of the non-thermal κ-distribution in the solar transition region and diagnose its parameters. Methods. The intensity ratios of Si iii lines observed by SUMER in 1100‐1320 A region do not correspond to the line ratios computed under the assumption of the Maxwellian electron distribution. We computed a set of synthetic Si iii spectra for the electron κ-distributions with different values of the parameter κ. We had to include the radiation field in our calculations to explain the observed line ratios. We propose diagnostics of the parameter κ and other plasma parameters and analyze the effect of the different gradient of differential emission measures (DEM) on the presented calculations. Results. The used line ratios are sensitive to T, density and the parameter κ. All these parameters were determined from the SUMER observations for the coronal hole (CH), quiet Sun (QS) and active region (AR) using our proposed diagnostics. A strong gradient of DEM influences the diagnosed parameters of plasma. The essential contributions to the total line intensities do not correspond to single T but a wider range of T, and they originate in different atmospheric layers. The amount of the contributions from these atmospheric layers depends on the gradient of DEM and the shape of the electron distribution function. Conclusions. The κ-distribution is able to explain the observed Si iii line spectrum in the transition region. The degree of nonthermality increases with the activity of the solar region, it is lower for CH and higher for the AR. The DEM influences the diagnosed T and Ne but it has only little effect on the diagnostics of the parameter κ.

EUV filter responses to plasma emission for the nonthermal κ-distributions

The responses to plasma emission of the TRACE EUV filters are computed by integrating their spectral responses over the synthetic spectra obtained from the CHIANTI database. The filter responses to emission are functions of temperature, electron density, and the assumed electron distribution function. It is shown here that, for the nonthermal κ-distributions, the resulting responses to emission are more broadly dependent on T , and their maxima are flatter than for the Maxwellian electron distribution. The positions of the maxima can also be shifted. Filter reponses to T are density-dependent as well. The influence of the nonthermal κ-distributions on the diagnostics of T from the observations in all three EUV filters is discussed.

The bound-bound and free-free radiative losses for the nonthermal distributions in solar and stellar coronae

Context. The radiative-loss function is an important ingredient in the physics of the solar corona, transition region, and flares. Aims. We investigate the radiative losses due to the bound-bound transitions and bremsstrahlung for nonthermal κ- and n-distributions. Methods. The bound-bound radiative losses are computed by integrating synthetic spectra. An analytical expression is derived for nonthermal bremsstrahlung. The bremsstrahlung is computed numerically using accurate values of the free-free Gaunt factor. Results. We find that the changes in radiative-loss functions due to nonthermal distributions are several times greater than the errors due to the missing contribution of the free-bound continuum or errors in atomic data. For κ-distributions, the radiative-loss functions are in general weaker than for Maxwellian distribution, with a few exceptions caused by the behavior of Fe. The peaks of the radiative-loss functions are in general flatter. The situation is opposite for n-distributions, for which the radiative-loss functions have higher and narrower peaks. Local minima and maxima of the radiative-loss functions may also be shifted. The contribution from bremsstrahlung only changes by a few percent except in the extreme nonthermal case of κ = 2. Stability analysis reveals that the X-ray loops are stable against the radiatively-driven thermal instability.

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

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.

Analytical model of static coronal loops

By solving the energy-equilibrium equation in the stationary case, we derive analytical formulae in the form of scaling laws for non-uniformly heated and gravitationally stratified coronal loops. The heating is assumed to be localized in the chromosphere and to exponentially decrease with increasing distance along the loop strand. This exponential behavior of the heating and pressure profiles implies that we need to use the mean-value theorem, and in turn fit the mean-value parameters of the scaling laws to the results of the numerical simulations. The radiative-loss function is approximated by a power-law function of the temperature, and its effect on the resulting scaling laws for coronal loops is studied. We find that this effect is more important than the effect of varying loop geometry. We also find that the difference in lengths of the different loop strands in a loop with expanding cross-section does not produce differences in the EUV emission of these strands significant enough to explain the observed narrowness of the coronal loops.