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Instrument: SUSIM : Solar Ultraviolet Spectral Irradiance Monitor
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Associated Platforms

Spectral/Frequency Information
Wavelength Keyword: Ultraviolet
Spectral/Frequency Coverage/Range: 115 nm and 411 nm

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The main objective of the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) experiment was to measure with high precision full-disk solar fluxes and their changes over a solar activity cycle in the 120-400 nm wavelength region. The SUSIM experiment on all three Atmospheric Laboratory for Applications and Science (ATLAS) missions flew concurrently with another SUSIM experiment onboard the Upper Atmosphere Research Satellite (UARS) and was able to provide correlative measurements and calibration checks. The goals of the SUSIM experiment are (1) to improve the absolute accuracy of solar continuum irradiance measurements in the 140-400 nm region to plus or minus 6 to 10 % (wavelength-dependent), (2) improve the absolute accuracy of solar emission line irradiance measurements in the 120-400 nm region to plus or minus 6 to 10% (wavelength-dependent), (3) measure with high accuracy the intensities of the continuum below 208 nm relative to the intensities of the continuum above 208 nm to plus or minus 1%, (4) measure with high accuracy the intensities of solar emission lines below 208 nm relative to the stable solar continuum above 208 nm to plus or minus 1 to 5 % (wavelength-dependent), (5) measure the degree of correlation of the solar flares in the 120-400 nm region with ground observations. The instrument consists of two identical double-dispersion scanning spectrometers, seven detectors (five photodiodes and two photon counters), a UV calibration source, and a Sun sensor. The spectrometers and detectors are sealed in a canister filled with 1.1 atm of argon gas to eliminate contamination of the optics. One spectrometer is used continuously to measure the solar intensities while the other monitor is used once a day to track the stability of the first spectrometer. The two photon counters obtain a spectral resolution of 0.15 nm while the photodiodes obtain a spectral resolution of 5 nm. A deuterium lamp transfer standard is built into the instrument for calibration of the seven detectors.

The Space Shuttle Discovery carrying UARS was launched on September 12, 1991 from Kennedy Space Flight Center. UARS was released to orbit on September 15, 1991, and the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) began scientific observations of the earth's upper atmosphere October 11, 1991.

For centuries, the number of sunspots has been observed to vary on an 11 year cycle. Measurements during the last two solar cycles have shown that sunspot numbers and the magnitude of solar UV light are correlated. Solar UV light can only be accurately measured from outside the Earth's atmosphere because this is where most of it is absorbed. To observe the sun in the UV over an entire 11 year solar cycle, a satellite-based experiment is the most practical means. SUSIM UARS makes measurements over its 115-410 nm wavelength range daily at 1 and 5 nm resolutions and weekly at 0.15 nm resolution. It is hoped through elaborate and painstaking calibrations made both before and during flight that the calibration of the instrument can be maintained to an absolute accuracy of 6% and a relative accuracy of 2% for the duration of a solar cycle.

SUSIM UARS also observes occultations of the sun by the Earth's atmosphere. Through comparison of the amount of UV light of selected wavelengths that penetrate the atmosphere as a function of altitude, densities of upper atmosphere UV light absorbers molecular oxygen and ozone are measured.

The experiment is operated as a part of the SUSIM Program by the NRL SUSIM UARS team through daily command streams sent via the NASA GSFC UARS Payload Operations Control Center. The raw SUSIM data is downlinked through the NASA Tracking and Data Relay Satellite, the NASA Communications Network, and the GSFC Data Capture Facility, to the UARS Central Data Handling Facility (CDHF). The CDHF processes the data based on algorithms and calibrations developed by the SUSIM UARS team to produce absolutely calibrated irradiances at instrument resolution for each solar scan. A subset of these data are processed further each day to produce 1 nm integrated irradiances at 1 nm intervals and the values of 7 solar indices.

SUSIM UARS Principal Investigator: Guenter E. Brueckner

Brueckner, G. E. et al., 'The Solar Ultraviolet Spectral Irradiance
Monitor Experiment Onboard the Upper Atmosphere Research Satellite',
J. Geophys. Res. 98, D6, 10695-10711, 1993.

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