The new version of the PH code we have presented, achieves an adequately high spatial resolution all along the loop from the corona to the chromosphere.
Beyond the obvious advantage of an accurate calculation of the plasma evolution even during very fast and dynamic phases such as the beginning of a flare, we now have the possibility to diagnose and/or predict plasma conditions in the transition region, and reliably derive line or band emission even in the EUV band. In this respect the resolution of the transition region, ensuring a fractional jump of temperature and density below 10%, also implies a high sampling of the ranges of temperatures of practically all the transition region lines of interest, an important factor for accurate line synthesis.
The potential for new diagnostics now unfolds, therefore, and it is possible to compare model predictions with EUV observations directly. This is particularly promising in view of the upcoming data from the CDS, SUMER, EIT instruments on board the SOHO mission. As for coronal data, and more specifically, X-ray observations, comparison with the results of the previous version are comforting: while certainly some details of the evolution appear different, the global features and plasma characteristics remain qualitatively unaltered. The results developed with this new version of the code show that previous applications of the PH code, being limited in scope to the dynamics of the plasma in the corona were essentially correct.
The previous PH code has been extensively used to simulate and study stellar flares; a fortiori the new version can be applied to stellar flares as well. Solar X-ray flares are typically observed with fast flares modes and/or hard filters or lines, while stellar flares are observed with the same wide band instruments used for standard observations. The implication is that ROSAT/PSPC and EINSTEIN/IPC observations of stellar flares cover down to 0.1 keV and 0.16 keV respectively, and are sensitive to plasma at a few million degrees, i.e. in the transition region of the flare. The high resolution of the new code allows us to synthesize more accurately the emission in all of the instrumental band.
In conclusion this improved version of the PH code allows us to obtain reliable diagnostics in a very wide spectral band, and appears to be adequate to model observational data taken with new generation instruments, as we plan to do in the near future.
Acknowledgements
We acknowledge partial support from Ministero dell'Università e della Ricerca Scientifica e Tecnologica, and Consiglio Nazionale delle Ricerche.