Donald L. Woodraska

On-Orbit Degradation of Solar Instruments

We present the lessons learned about the degradation observed in several space solar missions, based on contributions at the Workshop about On-Orbit 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.

SDO-EVE EUV spectrograph optical design and performance

The NASA Solar Dynamics Observatory (SDO), scheduled for launch in 2009, incorporates a suite of instruments including the EUV Variability Experiment (EVE). The EVE instrument package contains grating spectrographs that will measure the solar extreme ultraviolet (EUV) irradiance from 0.1 to 105 nm. The Multiple EUV Grating Spectrograph (MEGS) channels use concave reflection gratings to image solar spectra onto CCDs. MEGS will provide 0.1nm spectral resolution between 5-105nm every 10 seconds with an absolute accuracy of better than 25% over the SDO 5- year mission. MEGS-A utilizes a unique grazing-incidence, off-Rowland circle (RC) design to minimize angle of incidence at the detector while providing ≥ 0.1nm resolution between 5-37 nm. MEGS-B utilizes a double-pass, cross-dispersed double-Rowland circle design while providing ≥ 0.1nm resolution between 35-105 nm. We present the as-built performance of the MEGS optical design, including spectral resolution, wavelength shift, focus and alignment.

TIMED solar EUV experiment: preflight calibration results for the EUV grating spectrograph

The Solar EUV Experiment (SEE) on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite will make measurements of the spectral irradiance of the Sun in the soft x-ray, extreme ultraviolet (EUV), and far ultraviolet (FUV) wavelength range. The EUV Grating Spectrograph (EGS) component of SEE is a 1/4 meter Rowland circle spectrograph with a mechanically-ruled concave grating and a microchannel plate detector with a two-dimensional 1024 x 64 coded anode (CODACON) readout. The EGS covers the wavelength range from approximately 26 to 197 nm. The primary calibration of the instrument was done at the NIST Synchrotron Ultraviolet Radiation Facility (SURF) III on their beam line #2. We will detail the calibration methods and results for the EGS, paying attention to the quantification of sensitivity variations over the instruments large field of view (12.5 degree(s) x 5.3 degree(s)), plus scattered light, second-order, and linearity corrections.

EUV variability experiment (EVE), multiple EUV grating spectrographs (MEGS), radiometric calibrations and results

The NASA Solar Dynamics Observatory (SDO), scheduled for launch in early 2009, incorporates a suite of instruments including the EUV Variability Experiment (EVE). Two channels of EVE, the Multiple EUV Grating Spectrograph (MEGS) A and B channels use concave reflection gratings to image solar spectra onto CCDs to measure the solar extreme ultraviolet (EUV) irradiance from 5 to 105 nm. MEGS provides these spectra at 0.1nm spectral resolution every 10 seconds with an absolute accuracy of better than 25% over the SDO 5-year mission. The calibration of the MEGS channels in order to convert the instrument counts in to physical units of W/m2/nm was performed at the National Institute for Standards and Technology (NIST) Synchrotron Ultraviolet Radiation Facility III (SURF III) located in Gaithersburg, Maryland. Although the final post-environmental calibrations have yet to be performed, preliminary results from the pre-environmental calibrations show very good agreement with the theoretical optical design given by Crotser et al. Further analysis is still needed in regards to the higher order contributions to determine the final first order QT for all channels, but two techniques are currently being analyzed and show promising results.

In-flight calibration and performance of the Solar Extreme ultraviolet Experiment (SEE) aboard the TIMED Satellite

Understanding both the absolute value and time variability of the solar extreme ultraviolet (EUV) spectral irradiance is necessary for understanding the structure and variability of the Earth’s thermosphere and ionosphere. Long-term measurement of the solar EUV irradiance requires a calibration scheme that addresses the following issues: (1) the calibration must be referenced to repeatable radiometric standards; (2) changes in calibration throughout the duration of the measurements must be tracked; and (3) the measurements must be validated with independent instruments and models. The calibration and performance of the TIMED Solar EUV Experiment (SEE), which has been measuring the solar EUV irradiance since early 2002, will be discussed in relation to these calibration objectives. The pre-flight calibrations of SEE are based on calibrated synchrotron sources at the National Institute for Standards and Technology (NIST) Synchrotron Ultraviolet Radiation Facility (SURF). The in-flight calibrations for SEE are based on redundant channels used weekly and annual suborbital rocket flights with the prototype SEE instruments that are calibrated before and after each launch at NIST SURF.

Soft X‐ray irradiance measured by the Solar Aspect Monitor on the Solar Dynamic Observatory Extreme ultraviolet Variability Experiment

The Solar Aspect Monitor (SAM) is a pinhole camera on the Extreme ultraviolet Variability Experiment (EVE) aboard the Solar Dynamics Observatory. SAM projects the solar disk onto the CCD through a metallic filter designed to allow only solar photons shortward of 7 nm to pass. Contamination from energetic particles and out-of-band irradiance is, however, significant in the SAM observations. We present a technique for isolating the 0.01–7 nm integrated irradiance from the SAM signal to produce the first results of broadband irradiance for the time period from May 2010 to May 2014. The results of this analysis agree with a similar data product from EVEs EUV SpectroPhotometer to within 25%. We compare our results with measurements from the Student Nitric Oxide Explorer Solar X-ray Photometer and the Thermosphere Ionosphere Mesosphere Energetics and Dynamics Solar EUV Experiment at similar levels of solar activity. We show that the full-disk SAM broadband results compared well to the other measurements of the 0.01–7 nm irradiance. We also explore SAMs capability toward resolving spatial contribution from regions of solar disk in irradiance and demonstrate this feature with a case study of several strong flares that erupted from active regions on 11 March 2011.