We present the lessons learned about the degradation observed in several space solar missions, based on contributions at the Workshop about OnOrbit Degradation of Solar and Space Weather Instruments that took place at the Solar Terrestrial Centre of Excellence (Royal Observatory of Belgium) in Brussels on 3 May 2012. The aim of this workshop was to open discussions related to the degradation observed in Sun-observing instruments exposed to the effects of the space environment. This article summarizes the various lessons learned and offers recommendations to reduce or correct expected degradation with the goal of increasing the useful lifespan of future and ongoing space missions.

The Extreme ultraviolet SpectroPhotometer (ESP) is one of five channels of the Extreme ultraviolet Variability Experiment (EVE) onboard the NASA Solar Dynamics Observatory (SDO). The ESP channel design is based on a highly stable diffraction transmission grating and is an advanced version of the Solar Extreme ultraviolet Monitor (SEM), which has been successfully observing solar irradiance onboard the Solar and Heliospheric Observatory (SOHO) since December 1995. ESP is designed to measure solar Extreme UltraViolet (EUV) irradiance in four first-order bands of the diffraction grating centered around 19 nm, 25 nm, 30 nm, and 36 nm, and in a soft X-ray band from 0.1 to 7.0 nm in the zeroth-order of the grating. Each band's detector system converts the photo-current into a count rate (frequency). The count rates are integrated over 0.25-second increments and transmitted to the EVE Science and Operations Center for data processing. An algorithm for converting the measured count rates into solar irradiance and the ESP calibration parameters are described. The ESP pre-flight calibration was performed at the Synchrotron Ultraviolet Radiation Facility of the National Institute of Standards and Technology. Calibration parameters were used to calculate absolute solar irradiance from the sounding-rocketThe Solar Dynamics Observatory

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During the extended solar minimum from 2007 to 2009 anomalously low densities were observed in the Earth's thermosphere. Solar activity for the minimum of Solar Cycles 23 and 24 was lower than for Solar Cycles 22 and 23. However, a comparison of global daily averaged Total Electron Content (TEC) for these two solar minima does not show any significant difference. Why these significant changes between the 2008 and 1996 minima were observed in the He II EUV absolute irradiance and not in the global TEC for which EUV irradiance is one of the primary drivers is not well understood and may be interpreted as an effect of EUV instrument degradation. In this study, we analyze TEC data from 1995 to 2013 from the Center for Orbit Determination in Europe (CODE) database and compare global daily averaged TEC signal with TEC sectorial spherical harmonics which reflect a narrower temporal and spatial integration of the TEC in the Earth's ionosphere. Our analyses of both TEC data and errors show significant decreases of sectorial TECs for the 2008 and 2009 minimum compared to the minimum of 1996. We also compare the most recent version of Solar and Heliospheric Observatory/Solar EUV Monitor (SOHO/SEM) He II absolute EUV irradiance with the measurements from the Solar Dynamics Observatory/Extreme Ultraviolet Variability Experiment channels. This comparison shows that the EUV He II irradiance was about 12 ± 4% lower for the minimum of 2008 and 2009 compared to the minimum of 1996 when a low-pass filter with a 365 day window was used.

The Solar EUV Monitor (SEM) onboard SOHO has measured absolute extreme ultraviolet (EUV) and soft X-ray solar irradiance nearly continuously since January 1996. The EUV Variability Experiment (EVE) on SDO, in operation since April of 2010, measures solar irradiance in a wide spectral range that encompasses the band passes (26 – 34 nm and 0.1 – 50 nm) measured by SOHO/SEM. However, throughout the mission overlap, irradiance values from these two instruments have differed by more than the combined stated uncertainties of the measurements. In an effort to identify the sources of these differences and eliminate them, we investigate in this work the effect of reprocessing the SEM data using a more accurate SEM response function (obtained from synchrotron measurements with a SEM sounding-rocket clone instrument taken after SOHO was already in orbit) and time-dependent, measured solar spectral distributions – i.e., solar reference spectra that were unavailable prior to the launch of the SDO. We find that recalculating the SEM data with these improved parameters reduces mean differences with the EVE measurements from about 20 % to less than 5 % in the 26 – 34 nm band, and from about 35 % to about 15 % for irradiances in the 0.1 – 7 nm band extracted from the SEM 0.1 – 50 nm channel.