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SUSIM: An NRL Program to Measure Solar Ultraviolet Irradiance

NASA sponsors the two Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) instruments in the SUSIM program of the Solar Physics Branch in the Space Science Division at the Naval Research Laboratory in Washington, DC, USA. Each measures the absolute irradiance of the solar ultraviolet (UV) light in the wavelength range 115 to 410 nm. The SUSIM ATLAS instrument has flown repeatedly on the Space Shuttle. The SUSIM UARS instrument has taken data aboard the Upper Atmosphere Research Satellite (UARS) since October 1991.

More information is found on the pages for SUSIM:


Why Are There Two SUSIMs?

Calibration is an important task for UV instrument scientists because the measured UV light degrades the responsivity of their instruments by an uncertain amount. The calibrations of both SUSIM UV instruments along with many others were carried out using the Beamline 2 Spectrometer Calibration Facility at NIST SURF. SUSIM ATLAS is calibrated both before and after each flight the last three of which have occurred approximately yearly. SUSIM UARS was also calibrated before its flight and carries four deuterium lamps to help to maintain its calibration. As a result, the SUSIM ATLAS is expected to better measure the absolute solar UV intensity while the SUSIM UARS should be able to track relative solar variability. Further, the results from SUSIM ATLAS are expected to help to maintain the calibration of the SUSIM UARS during a mission we hope will last through an 11 year solar cycle.

Why measure solar ultraviolet light?

There are three general areas of scientific interest for which measurements of the solar UV spectrum would help our understanding:

  1. Solar Physics, i.e. the mechanisms responsible for the workings of and changes in the Sun, our nearest star,
  2. the Earth's Upper Atmosphere, i.e. changes resulting from the UV light absorption in the stratosphere, mesosphere, and thermosphere, and
  3. the Earth's Climate, e.g. changes in tropospheric temperatures.
The sun is the primary driving force behind the Earth's climate. Both the sun itself and the climate are changing and evolving consistent with known physical laws. However, due to the very complicated nature of these laws, predictions of their overall effects are uncertain. To understand and properly model the evolution of the state of the upper atmosphere or climate systems, it is necessary to know the spectral distribution of solar light and the energies and fluxes of incoming particles.

Solar Physics

The variability of the sun's radiation in the UV spectrum is far greater than in its aggregate output. The UV light measured by SUSIM originates in the upper photosphere, chromosphere, and corona of the sun. Generally, the longer the wavelength, the lower in the solar atmosphere it originates. Strong absorption lines (such as Ca II) are exceptions to this rule, originating at much higher altitudes than the surrounding spectrum. Various ground based indices of solar activity such as sunspot number and fluxes at various radio frequencies have been shown to correlate with aspects of the UV spectrum. Measurements of the solar UV spectrum through time help our understanding of the processes at the various altitudes in the solar atmosphere and their connection to other solar indices and events.

Earth's Upper Atmosphere

Solar UV light is primarily responsible for both creation and destruction of ozone in the earth's stratosphere and mesosphere. Ozone is the molecular form of oxygen which shields the Earth's surface from solar UV-B radiation through their absorption. The same process also causes the temperature in the stratosphere to be higher than in the upper troposphere. Stratospheric ozone densities are known to vary with the 11 year solar cycle. Solar variability over the solar cycle causes expansion and contraction of the outward extension of the Earth's atmosphere into space. Scientists will use SUSIM data along with constituent, dynamical, and other radiation measurements made by UARS instruments to better model the processes occurring in the earth's upper atmosphere particularly involving the creation and destruction of ozone.

Earth's Climate

The connection of solar UV light and its variability to climate change is controversial among scientists. Recent measurements of the sun's total irradiance show that it varied by about 0.1% during the recent 11 year solar cycle. Computational models indicate that this level of variation is insufficient to significantly modulate the climate. However, the models do not include subtle feedback mechanisms (e.g. enhanced cloud formation) which could magnify the impact of this tiny variation. It is also possible that changes in the Earth's upper atmosphere induced by solar UV light could similarly affect the surface climate. Yet, skeptics point out that the energy per unit volume stored in the tropopause (the boundary between the troposphere and the stratosphere) is 100 times greater than in the upper atmosphere indicating that such causality is unlikely. Numerous correlations between solar activity and climatic events have been claimed in the past, many of which were abandoned when their statistical significance could not be convincingly established. A dramatic example of a connection which remains credible occurred during the extended seventeenth century period known as the Little Ice Age which was characterized by Earth surface temperatures much colder than normal and which coincided with a very unusual period of low solar activity and no sunspots known as the Maunder Minimum.

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Certifying Official: Dr. H Gursky, Superintendent, Space Science Division