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During a coordinated campaign which took place in May 2001, a C-class flarewas observed both with SOHO instruments and with the Dunn Solar Telescope ofthe National Solar Observatory at Sacramento Peak. In two previous papers wehave described the observations and discussed some dynamical aspects of theearlier phases of the flare, as well as the helium line formation in the activeregion prior to the event. Here we extend the analysis of the helium lineformation to the later phases of the flare in two different locations of theflaring area. We have devised a new technique, exploiting all availableinformation from various SOHO instruments, to determine the spectraldistribution of the photoionizing EUV radiation produced by the coronaoverlying the two target regions. In order to find semiempirical modelsmatching all of our observables, we analyzed the effect on the calculatedhelium spectrum both of A(He) (the He abundance) and of the uncertainties inthe incident EUV radiation (level and spectral distribution). We found that theabundance has in most cases (but not in all) a larger effect than the coronalback-radiation. The result of our analysis is that, considering the error ofthe measured lines, and adopting our best estimate for the coronal EUVillumination, the value A(He)=0.075 +/- 0.010 in the chromosphere (for T>6300K) and transition region yields reasonably good matches for all the observedlines. This value is marginally consistent with the most commonly acceptedphotospheric value: A(He)=0.085.

Helium line formation and abundance during a C-class flare