XUV Photometer System (XPS): Overview and Calibrations

Abstract

The solar soft X-ray (XUV) radiation is highly variable on both short-term time scales of minutes to hours due to flares and long-term time scales of months to years due to solar cycle variations. Because of the smaller X-ray cross sections, the solar XUV radiation penetrates deeper than the extreme ultraviolet (EUV) wavelengths and thus influences the photochemistry and ionization in the mesosphere and lower thermosphere. The XUV Photometer System (XPS) aboard the Solar Radiation and Climate Experiment (SORCE) is a set of photometers to measure the solar XUV irradiance shortward of 34 nm and the bright hydrogen emission at 121.6 nm. Each photometer has a spectral bandpass of about 7 nm, and the XPS measurements have an accuracy of about 20%. The XPS pre-flight calibrations include electronics gain and linearity calibrations in the laboratory over its operating temperature range, field of view relative maps, and responsivity calibrations using the Synchrotron Ultraviolet Radiation Facility (SURF) at the National Institute of Standards and Technology (NIST). The XPS in-flight calibrations include redundant channels used weekly and underflight rocket measurements from the NASA Thermosphere-Ionosphere-Mesosphere-EnergeticsDynamics (TIMED) program. The SORCE XPS measurements have been validated with the TIMED XPS measurements. The comparisons to solar EUV models indicate differences by as much as a factor of 4 for some of the models, thus SORCE XPS measurements could be used to improve these models.

Early Observations by the GOES-13 Solar Extreme Ultraviolet Sensor (EUVS)

“…XPS on-board SORCE makes measurements similar to SEE/XPS (Woods, Rottman, and Vest, 2005;. SORCE/XPS data are used to construct spectra from 1 to 40 nm that are then integrated over EUVS's reporting intervals for Channels A and B.…”

“…Two of these are the abovecited SOHO/SEM and TIMED/SEE sensors, which were launched in December 1995 and December 2001, respectively. Others are the XUV sensor on-board the Student Nitric Oxide Explorer (SNOE) launched in February 1998 (Bailey et al, 2000(Bailey et al, , 2001) and a similar sensor on-board SORCE that was launched in January 2003 (Woods, Rottman, and Vest, 2005;. The SNOE, SORCE, and short wavelength SEE measurements (< 27 nm) are made with broad-band photometers (a version of XPS on each satellite).…”

Next generation x-ray sensor (XRS) for the NOAA GOES-R satellite series

“…This approach was used for the XPS calibrations as described by Woods et al 5,10 using Beamline-2 at SURF. Using this approach, the XRS directly views the synchrotron radiation (without a monochromator).…”

“…This design, which features multiple channels containing simple bandpass limiting filters and stable Si photodiodes 2 , has significant heritage from multiple space-based platforms for observing the solar soft X-ray irradiance over the past 14 years. These instruments include the X-Ray Photometer System (XPS) which is part of the Solar EUV Experiment (SEE) 3 onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite, as well as similar XPSs that have flow on the Student Nitric Oxide Experiment (SNOE) 4 and are currently flying on the SOlar Radiation and Climate Experiment (SORCE) 5 . Other simple enhancements were made to the design to allow the additional capability of determining the location on the solar disk where flares occur.…”

The Solar Radiation and Climate Experiment (SORCE) Mission for the NASA Earth Observing System (EOS)

“…The Total Irradiance Monitor (TIM) measures the TSI (Kopp and Lawrence, 2005). The XUV Photometer System (XPS) measures the soft X-ray (XUV) SSI in the 0.1-34 nm range (Woods, Rottman, and Vest, 2005). The Solar Stellar Irradiance Comparison Experiment measures the far ultraviolet and middle ultraviolet SSI in the 115-320 nm range (McClintock, Rottman, and Woods, 2005).…”