Modeling Flare Continuum Emission Observed by Hinode/EIS: Instrument Calibration and Element Composition Results

Continuum emission from a solar flare observed with the Extreme ultraviolet Imaging Spectrometer (EIS) on board the Hinode satellite is used to obtain the radiometric calibration of the instrument. The flare had a GOES class of M8, and peaked at 23:59 UT on 2024 September 30. The continuum is modeled by computing a differential emission measure curve using EIS emission lines and atomic data from the CHIANTI database. The ratio of the observed continuum to model continuum yields effective area curves for the instrument. The new curves confirm earlier findings that the EIS long-wavelength channel has degraded by a factor of 2 compared to the short-wavelength channel. However, no evidence is found for the fine-scale structure in the effective area curves that has been presented by previous authors. In order to reproduce both the emission-line intensities and the continuum, it is found that the plasma must be depleted in elements with low first ionization potentials (FIPs), i.e., the so-called inverse-FIP effect. In particular, the Fe/H relative abundance is found to be a factor of 0.57 below the photospheric value at a temperature of 10 MK. This is confirmed by an analysis of soft X-ray spectra from the Solar X-ray Monitor on Chandrayaan-2, which yields an Fe/H FIP bias of 0.55 averaged over the entire flare.