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-Energetics-Dynamics (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. © 2005 Springer.