SUSIM UARS: An Ongoing Satellite Experiment Measuring the Spectral Composition of Solar Ultraviolet Light


Experiment Overview

The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) is a dual dispersion spectrometer instrument which measures from near-Earth orbit the absolute irradiance of the sun in the ultraviolet (UV) wavelength range of 115 nm to 410 nm.

Here is a plot of the latest available daily solar spectrum from SUSIM UARS:

SUSIM UARS is one experiment aboard the Upper Atmosphere Research Satellite. Its daily solar observations began on October 11, 1991 and continue to this day. A sister instrument, SUSIM ATLAS, has made similar measurements while flying five times aboard the Space Shuttle.

The SUSIM UARS principal investigator is Dr. John Cook. The SUSIM Program is part of the Solar Physics Branch of the Space Science Division and the E. O. Hulburt Center for Space Research at the Naval Research Laboratory in Washington, DC, USA.

Solar Measurements

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 roughly correlated. Solar UV light can only be accurately measured from outside the Earth's atmosphere because this is where most of it is absorbed. The most practical way to observe the sun in the UV over an entire 11 year solar cycle is through a satellite-based instrument.

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 careful and accurate 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.

Atmospheric Measurements

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 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.

Data Products

The data products described above, i.e. the 1 nm gridded irradiances and the solar indices, are available to the general public from the GSFC DAAC or from NRL via anonymous ftp from The Data Page for SUSIM UARS provides more information and access instructions for these data.

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Hits since December 16, 1998:
Last revised July 7, 2005 - AEE
Certifying Official: Dr. H Gursky, Superintendent, Space Science Division

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