EIT and TRACE responses to flare plasma

TL;DR

This study analyzes how active region and flare plasmas contribute to the EUV channels of SOHO/EIT and TRACE during a solar flare, using spectral synthesis and observations to understand emission sources.

Contribution

It provides a detailed comparison of observed and predicted EUV emissions during a flare, highlighting the dominance of Fe XXIV and identifying discrepancies in count rates.

Findings

EIT and TRACE $ m u$195 channels are dominated by Fe XXIV during flares.

Predicted count rates are within 25% for outer regions but overestimate for flare regions.

Fe XXIV emission significantly influences EUV observations during solar flares.

Abstract

Aims: To understand the contribution of active region and flare plasmas to the $\lambda$195 channels of SOHO/EIT (Extreme-ultraviolet Imaging Telescope) and TRACE (Transition Region and Coronal Explorer). Methods: We have analysed an M8 flare simultaneously observed by the Coronal Diagnostic Spectrometer (CDS), EIT, TRACE and RHESSI. We obtained synthetic spectra for the flaring region and an outer region using the differential emission measures (DEM) of emitting plasma based on CDS and RHESSI observations and the CHIANTI atomic database. We then predicted the EIT and TRACE count rates. Results: For the flaring region, both EIT and TRACE images taken through the $\lambda$195 filter are dominated by Fe ${\rm XXIV}$ (formed at about 20 MK). However, in the outer region, the emission was primarily due to the Fe${\rm XII}$, with substantial contributions from other lines. The average count rate for the outer region was within 25% the observed value for EIT, while for TRACE it was a factor of two higher. For the flare region, the predicted count rate was a factor of two (in case of EIT) and a factor of three (in case of TRACE) higher than the actual count rate. Conclusions: During a solar flare, both TRACE and EIT $\lambda$195 channels are found to be dominated by Fe ${\rm XXIV}$ emission. Reasonable agreement between predictions and observations is found, however some discrepancies need to be further investigated.