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Laboratory for Atmospheric and Space Physics

SORCE XPS Level 4 Irradiance Data Product Description

The SORCE XUV Photometer System (XPS) measures the solar XUV irradiance in the 0.1 to 40 nm range in broadbands (7-10 nm resolution). The CHIANTI spectral model is used to generate a higher spectral resolution estimate of the solar XUV irradiance by scaling the model reference spectra to match the XPS photometer currents (signal). The CHIANTI Quiet Sun (QS) spectrum and Active Region (AR) spectrum are used with a scaling of the AR spectrum to match the minimum signal on each day, and then a CHIANTI isothermal flare spectrum is scaled to match the signal above the daily minimum signal. The flare temperature is determined from the ratio of the GOES XRS 0.1-0.8 nm irradiance to the GOES XRS 0.05-0.4 nm irradiance. These scaling factors, flare temperature, and resulting model spectrum in the 0-40 nm range in 0.1 nm intervals are in the XPS Level 4 data product for every measurement made by XPS photometer #1 or #2 (0.1-7 nm band). The daily SORCE XPS Level 4 data products are also merged into a mission data file, both in daily averages and in 5-minute intervals.

Table A-4
SORCE Instrument XPS
Temporal Cadence Daily and 5-minute
Detector XUV Photometers
Accuracy 30%
Long-Term Repeatability 1% per year
Spectral Coverage 0.1 – 40 nm

Data Production

Science data processing is performed automatically within 5 days of data reception from the spacecraft, following receipt of
definitive orbit ephemeris, and data products are immediately made available to the public following automated data quality checks the entire data production and distribution process is automated and runs unattended, with SORCE staff routinely monitoring progress and results.

The XPS Level 2 data products, along with CHIANTI reference spectra, are used to construct the XPS Level 4 data products. The XPS Level 2 data product has the photometer currents (solar, dark, visible signals) and all of the calibration parameters used to convert the solar signal into irradiance units [Woods et al., 2005b]. The XPS integration time was typically 10 seconds until December 2005 (filter wheel anomaly), and then the integration time was changed to 1 minute in January 2006.

Algorithms to produce these products are documented in the SORCE Algorithm Theoretical Basis Document (ATBD) and by Woods, et al. [2005b], and both have been peer reviewed by other members of the solar irradiance community.

Instrument Calibration and Data Quality

On-orbit instrument characterization is an ongoing effort, and the XPS team checks photometer degradation using redundant channels and underflight calibration rockets funded by TIMED SEE. Only the XPS Lyman-a (121.5 nm) channel has shown any degradation, being about 10% the first year and then slowing down with time. The XPS algorithms changed in 2006 in response to the XPS filter anomaly in December 2005, and only minor updates are anticipated for the extended mission. The accuracy of the XPS Level 2 irradiance is 12%-30%, photometer dependent. There is additional uncertainty for applying the spectral model for the XPS Level 4 irradiances, and this estimated accuracy is 30% for the integrated XUV irradiance. The spectral distribution in the XPS Level 4 is from the CHIANTI model and not from direct measurements from XPS, a set of broadband photometers. The spectral distribution above 27 nm has been validated with the TIMED SEE EGS (27-194 nm, 0.4 nm resolution) measurements, so there is good confidence in the spectral distribution shortward of 27 nm.

Data Access and Availability

XPS Level 4 Version 12 data are publicly available from the SORCE web site and are considered the final XPS version for the SORCE mission.

Data Release Notes