A Post-Conjunction Re-Evaluation of the Calibration and Long-term Evolution of the STEREO-A Heliospheric Imagers

Abstract

AbstractIn two earlier articles (Tappin, Eyles and Davies in Solar Phys. 290, 2143, 2015, and Solar Phys. 292, 28, 2017), we used the stellar photometry to determine the calibration parameters and long-term trends of the Heliospheric Imagers (HI) on board the Solar Terrestrial Relations Observatory (STEREO). In this article we provide an update on these determinations for the ahead spacecraft (STEREO-A) to incorporate the interval after solar superior conjunction (when STEREO-B was non-operational). We describe the modifications needed to our photometry procedures to accommodate the reduced pointing stability following the switch-off of the spacecraft gyros shortly prior to conjunction. We find a small revision to the absolute levels is required. We also show that the very low rates of degradation (less than 0.2% per year) found in the earlier determinations have continued beyond solar conjunction.