ccurate pre-flight and in-flight calibrations of the SOLSTICE are both crucial to the solar irradiance measurements. The pre-flight calibrations provide the absolute values of the irradiances that are traceable to the National Institute of Standards and Technology (NIST) radiometric standards. The in-flight calibrations provide the relative knowledge on how irradiances on any day can be compared precisely to the irradiances on any other day during the mission.
System Level Pre-Flight Calibrations:
Due to the nature the SOLSTICE design, continued exposure to ultraviolet
solar radiation causes a gradual decay in the instrument sensitivity. We
believe two distinct processes share responsibility for this decay - reduction
in photocathode efficiency of our photomultiplier tubes with prolonged exposure
and polymerization of the optical elements from exposure to hard x-rays.
In order to model the resulting decay in instrument sensitivity, SOLSTICE observes an ensemble of stars at specific pre-selected wavelengths over the duration of the mission. General information about this technique is also available.
Prior to launch, SOLSTICE personnel selected approximately 30 different stars of spectral classes O, B, and A to observe during portions of the spacecraft orbit when the instrument does not observe the sun. Stars were selected according to visual magnitude, spectral class, and any known variability in apparent brightness. Only the brightest stars with no known variability were selected. After launch, the instrument began observing each star at every selected wavelength whenever possible. Certain stars were removed from the list upon determination of their pathological nature relative to others.
The data from all these observations exists as a time-series for each star at each calibration wavelength. The stellar data analysis algorithms filter the stellar data according to the intensity of each stellar measurement, as well as geophysical effects, such as the South Atlantic Anomaly and others. Modeling of changes in instrument sensitivity occurs by fitting the entire ensemble of stars at each wavelength using a multi-variate non-linear least squares algorithm. The resulting calibration curves are then folded into SOLSTICE production processing algorithms to correct the solar observations.
Application of the Calibration Parameters:
In calculating the irradiances, the solar data are corrected for scattered light, detector linearity, detector dark counts, detector gain changes, instrument sensitivity and degradation. The stellar irradiances undergo similar processing, but the degradation factors are treated as free parameters and are adjusted to make the mean stellar irradiance invariant in time. The resulting degradation factors are then the same ones applied to the solar data.
The wavelength scale is referenced in vacuum wavelength units to high resolution solar spectra above 200 nm and to atomic or ionic transition levels below 200 nm. Each spectrum's wavelength scale is also adjusted to the SOLSTICE reference wavelength scale to account for small wavelength shifts related to temperature changes and to pointing offsets.
A detailed description of the SOLSTICE instrument calibration is given by Woods et al. ("Solar Stellar Irradiance Comparison Experiment 1: 2. instrument calibration", J. Geophys. Res. 98, 10679-10694, 1993) and by Woods et al. ("Validation of the UARS Solar Ultraviolet Irradiances: Comparison with the ATLAS-1, -2 Measurements", J. Geophys. Res., in press, 1996).