CHIANTI - An atomic database for emission lines. XI. EUV emission lines of Fe VII, Fe VIII and Fe IX observed by Hinode/EIS

TL;DR

This paper presents new observations and identifications of EUV emission lines from Fe VII, Fe VIII, and Fe IX, revealing discrepancies with existing atomic models and emphasizing the need for improved atomic data for solar plasma diagnostics.

Contribution

It introduces new line identifications and compares observed spectra with atomic models, highlighting discrepancies and updating models for solar EUV emission lines.

Findings

New line identifications for Fe IX at 176.959 A and 177.594 A.

Discrepancies between observed and predicted line ratios for Fe VII and Fe VIII.

Updated atomic data significantly affect solar EUV imaging response models.

Abstract

A detailed study of emission lines from Fe VII, Fe VIII and Fe IX observed by the EUV Imaging Spectrometer on board the Hinode satellite is presented. Spectra in the ranges 170-212 A and 246-292 A show strongly enhanced lines from the upper solar transition region (temperatures 5.4 <= log T <= 5.9) allowing a number of new line identifications to be made. Comparisons of Fe VII lines with predictions from a new atomic model reveal new plasma diagnostics, however there are a number of disagreements between theory and observation for emission line ratios insensitive to density and temperature, suggesting improved atomic data are required. Line ratios for Fe VIII also show discrepancies with theory, with the strong 185.21 and 186.60 lines under-estimated by 60-80 % compared to lines between 192 and 198 A. A newly-identified multiplet between 253.9 and 255.8 A offers excellent temperature diagnostic opportunities relative to the lines between 185-198 A, however the atomic model under-estimates the strength of these lines by factors 3-6. Two new line identifications are made for Fe IX at wavelengths 176.959 A and 177.594 A, while seven other lines between 186 and 200 A are suggested to be due to Fe IX but for which transition identifications can not be made. The new atomic data for Fe VII and Fe IX are demonstrated to significantly modify models for the response function of the TRACE 195 A imaging channel, affecting temperature determinations from this channel. The data will also affect the response functions for other solar EUV imaging instruments such as SOHO/EIT, STEREO/EUVI and the upcoming AIA instrument on the Solar Dynamics Observatory.