XUV Photometer System (XPS): Improved Solar Irradiance Algorithm Using CHIANTI Spectral Models

Abstract

Solar soft X-ray (XUV) radiation is highly variable on all time scales and strongly affects Earth's ionosphere and upper atmosphere; consequently, the solar XUV irradiance is important for atmospheric studies and for space weather applications. Although there have been several recent measurements of the solar XUV irradiance, detailed understanding of the solar XUV irradiance, especially its variability during flares, has been hampered by the broad bands measured in the XUV range. In particular, the simple conversion of the XUV photometer signal into irradiance, in which a static solar spectrum is assumed, overestimates the flare variations by more than a factor of two as compared to the atmospheric response to the flares. To address this deficiency in the simple conversion, an improved algorithm using CHIANTI spectral models has been developed to process the XUV Photometer System (XPS) measurements with its broadband photometers. Model spectra representative of quiet Sun, active region, and flares are combined to match the signals from the XPS and produce spectra from 0.1 to 40 nm in 0.1-nm intervals for the XPS Level 4 data product. The two XPS instruments are aboard NASA's Solar Radiation and Climate Experiment (SORCE) and Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED) satellites. In addition, the XPS responsivities have been updated for the latest XPS data processing version. The new XPS results are consistent with daily variations from the previous simple conversion technique used for XPS and are also consistent with spectral measurements made at wavelengths longer than 27 nm. Most importantly, the XPS flare variations are reduced by factors of 2 -4 at wavelengths shorter than 14 nm and are more consistent, for the first time, with atmospheric response to solar flares. Along with the details of the new XPS algorithm, several comparisons to dayglow and photoelectron measurements and model results are also presented to help verify the accuracy of the new XUV irradiance spectra.

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“…Woods et al, 2008]. Note that the forecast times are greater than the column header values (i.e., 0, 1, 3, and 7 days) by approximately 16 h. data.…”

“…Note within Table 1 that some of the Solomon-Qian bands seemingly overlap (bands 11 through 18); however, these bands are actually subdivided relative to the absorption cross section (referred to as low, middle, and high) of molecular nitrogen [see Solomon and Qian, 2005]. From the SEE/XPS Level 4 product data [Woods et al, 2008] and SEE/EGS Level 3 data, the time series of the daily means (with flares removed) of the individual bands are generated using the EUV Variability Experiment analysis procedures provided in the Solar Software library [Freeland and Handy, 1998]. Note that spectral bands 1 through 8 are not explicitly measured by TIMED/XPS but are instead inferred from spectral modeling [see Woods et al, 2008].…”

“…From the SEE/XPS Level 4 product data [Woods et al, 2008] and SEE/EGS Level 3 data, the time series of the daily means (with flares removed) of the individual bands are generated using the EUV Variability Experiment analysis procedures provided in the Solar Software library [Freeland and Handy, 1998]. Note that spectral bands 1 through 8 are not explicitly measured by TIMED/XPS but are instead inferred from spectral modeling [see Woods et al, 2008]. With TIMED/SEE observations beginning in early 2002, the comparison period spans 2002 until 2011.…”

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

“…Measurements shortward of 27 nm are obtained by XPS that makes broad-band photodiode observations. These observations are converted to spectra by assuming spectral shapes within the regions covered by the photodiodes (Woods et al, , 2008.…”

Sensitivity of upper atmospheric emissions calculations to solar/stellar UV flux

“…Finally, the data in the 1-27 nm range come from the XUV Photometer System (XPS) instrument on board SORCE, which is not a spectrometer but a radiometer using several broad bandpasses. The XUV data are then processed with an algorithm using the CHIANTI spectra models (Woods et al 2008). Although ideal, this approach relies on good calibration of the instruments, which are largely debated in the solar community as discussed by these two papers (Haigh et al 2010;Lean et al 2011).…”