Spectral analysis of highly inhomogeneous chromospheric flares

Analysis of a hydrogen flare spectrum is carried out, assuming that the flare radiation is diluted due to a highly inhomogeneous space distribution of the flare elements in the chromosphere. It is shown that one obtains the correct physical parameters in the flare elements, irrespective of the extent of dilution, if all the elements are optically thin in the spectral regions considered. However, if this is not true for all the elements, the parameters deduced are in error, which increases with the extent of dilution. The worst affected parameter is the linear thickness of the flaring region which is found to be too low by a factor which may be as high as 10. Errors also enter into electron and atom densities which are found to be too high and into the column density of hydrogen atoms which is underestimated. On the other hand, the deduced source function, and in consequence of it the electron temperature as well, remain unaffected, if the analysis is properly carried out. All the errors may increase strikingly if the analysis is performed for optically thick regions. Several kinds of observation are mentioned which indicate that dilution of radiation may occur in flares. However, reasonable estimates of the flare parameters are still possible provided that one avoids spectral regions with 1.Senior Visiting Scientist to ESTEC, 1971–1972.