Hinode EIS: updated in-flight radiometric calibration

TL;DR

This paper updates the in-flight radiometric calibration of the Hinode EUV Imaging Spectrometer (EIS) by analyzing observations from 2007 to 2022, ensuring accurate spectral measurements for solar studies.

Contribution

The study provides a revised calibration method for EIS using emission measure analyses and cross-calibration with SDO/AIA, extending previous work with improved accuracy and validation.

Findings

Calibration uncertainty reduced to about +/- 20%

Good agreement within +/- 15% between measured and expected line intensities

Validated calibration through flare line ratios and density-sensitive line ratios

Abstract

We present an update to the in-flight radiometric calibration of the Hinode EUV Imaging Spectrometer (EIS), revising and extending our previous studies. We analyze full-spectral EIS observations of quiet Sun and active regions from 2007 until 2022. Using CHIANTI version 10, we adjust the EIS relative effective areas for a selection of dates with emission measure analyses of off-limb quiet Sun. We find generally good agreement (within typically +/- 15%) between measured and expected line intensities. We then consider selected intensity ratios for all the dates and apply an automatic fitting method to adjust the relative effective areas. To constrain the absolute values from 2010 and later we force agreement between EIS and Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA) 193 A observations. The resulting calibration, with an uncertainty of about +/- 20%, is then validated in various ways, including flare line ratios from Fe XXIV and Fe XVII, emission measure analyses of cool active region loops, and several density-dependent line ratios.