Concept & Operation
SOLSTICE consists of a two-channel grating spectrometer capable of being pointed at the Sun or at selected stars. It measures spectral irradiance from 115 nm to 320 nm with a 0.1 nm resolution for the Sun and between 1.1 and 2.2 nm for the stars. The stellar targets, observed with the same optics and detectors employed for the solar measurements, are essential because they establish long-term corrections to the instrument calibration. Intrinsic variability in irradiance from early-type stars is less than one part in ten thousand over thousands of years; therefore, the ensemble average flux from 18 of them provides a standard source for in-flight calibration. This stellar theory shows that the unique SOLSTICE calibration technique thereby establishes the instrument response as a function of time throughout the SORCE mission and yields a time series of solar data corrected for instrumental effects to an accuracy of about 1 percent. Moreover, the SOLSTICE technique provides the unique ability to directly compare solar irradiance measurements made during the SORCE mission with previous and future observations. Since the stellar irradiance remains essentially constant, stellar measurements provide a stable in-flight reference that is readily transferred between missions, provided each mission makes observations of the same stars.
If the SOLSTICE instrument was limited to a single star for its calibration, the technique could be suspect; therefore, it is the ensemble average of many stars that is used to derive the trends in the instrument response. During the UARS mission, the final selection of stable stars was based on repeated observations throughout the eleven-year (1991 to the present) operation, and stars that were found to vary excessively compared to the other stars in the ensemble have been eliminated. This same set of stars will be used for the SORCE mission.
The SOLSTICE measurements are made using a pair of identical spectrometers. Each spectrometer is independently housed and mounted on the spacecraft optical bench and can operate in either of two modes. These instruments utilize traditional techniques of scanning spectrometers to make ultraviolet spectral measurements in two intervals: 115-180 nm and 170-320 nm. Two instruments are required to simultaneously measure the two intervals, but each of the two SOLSTICE spectrometers is capable of measuring either interval. During normal operations, one of the spectrometers operates in the short-wavelength mode and the other operates in the long-wavelength mode. On a low duty-cycle, the instruments are reconfigured to observe the other wavelength interval for added capability of monitoring changes in instrument performance. Moreover, in the event of either a full or partial instrument failure, one instrument can serve the dual role of observing both spectral intervals but at a lower frequency.
The SORCE spacecraft operates in a 645 km, 40° orbit providing about 15 orbits per day. Each orbit consists of a daytime portion when the Sun is visible to the spacecraft and a nighttime portion when the Sun is out of view behind the Earth. During any given orbit, the SOLSTICE instruments make solar spectral measurements during the daytime and stellar observations during the night. This provides solar spectra at least every 6 hours and multiple stellar observations each calendar day.
The SOLSTICE instrument is quite versatile operationally, but most often operates in a high-precision solar scan mode that provides a complete solar spectrum from 115-320 nm in approximately 30 minutes, or a quick solar scan mode that provides a complete solar spectrum (with a lower signal-to-noise ratio) in about 5 minutes. Stellar observations are typically performed using long (10-15 minutes) fixed-wavelength integrations on individual stellar targets.