Solar Ultraviolet Spectral Irradiance Monitor (SUSIM)^*

NASA funded NRL to develop two Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) instruments. Each measures the absolute irradiance of the solar ultraviolet (UV) light in the 115- to 410-nm wavelength range. The ATLAS/SUSIM instrument flew repeatedly on the space shuttle. The UARS/SUSIM instrument was aboard the Upper Atmosphere Research Satellite (UARS) in October 1991 to August 2005. SUSIM covers the wavelength range 110-410 nm with 0.15 and 5.0 nm resolution. 

Why are there two SUSIMs? 

Calibration is an important task for UV instruments  because the measured UV light degrades the responsivity of 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. 

SUSIM/ATLAS

The SUSIM/ATLAS flew on its first Space Shuttle flight, the OSS-1 Mission, in 1982. The instrument has reflown on Spacelab-2 in August 1985, ATLAS-1 in March 1992, ATLAS-2 in April 1993, and ATLAS-3 Missions in November 1994. Calibrations, updates, refurbishment were performed after each of its flights. These solved problems, overcame operational limitations in calibration or flight, and implemented improvements discovered during the previous Space Shuttle missions. The launch of a sister instrument on the Upper Atmospheric Research Satellite (UARS) September 1991 made some of the improvements possible when the spare flight hardware from the UARS SUSIM instrument was no longer needed. Most of the changes were made specifically to decrease uncertainty in the absolute irradiance of the data.

The SUSIM instrument diagram, has two identical double-dispersion scanning spectrometers, seven detectors, entrance and exit slits, filters, and gratings enclosed within a case. Doors seal the case when not making measurements to prevent external contamination from entering the optical area. All materials are specially chosen to minimize any contamination to the optical elements. The electronic modules and a microprocessor for command, control and telemetry surround the instrument are shown in the instrument photograph. SUSIM covers the wavelength range 110-410 nm with 0.15 and 5.0 nm resolution. 

The SUSIM experiment has overcome the difficulty of making high accuracy measurements in the ultraviolet by using contamination control techniques to minimize changes. Remaining changes are tracked with an on board deuterium lamp (D2). Only one of the two spectrometers is used for solar measurements. The sensitometric changes of both spectrometers are monitored with the D2 lamp to make calibration and characterization measurements of all the optical elements. The solar spectrometer is compared to the calibration spectrometer to fully determine its solar UV induced sensitometric changes.

SUSIM/UARS

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The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) is a dual dispersion spectrometer instrument that measured the spectral irradiance of the sun in the ultraviolet (UV). 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. 

Details of the instrument

A. Wavelength encoder

A new electronic module was installed which doubled the measurement resolution of the incremental encoder which is attached to the grating drive arms. This encoder determines the position and eventually the wavelength of the spectrometer measurements. The new module gives 0.013 nm readout precision; a factor of ten more than the 0.15 nm spectral resolution. In addition to the improvement in the encoder electronics, a new technique of determining an absolute position for setting the encoder zero at turn-on was implemented. This new technique was patterned after the UARS/SUSIM instrument and consisted of using a photo diode and light emitting diode pair to determine the position when a mechanical flag attached to the grating arms became centered between them. Measurement of the transition of both sides of the flag during its movement up and down the encoder scale prevents electronic or optical changes from shifting the center determination and hence the zero position for the wavelength scale. As will be seen later, these changes improved the 2 sigma wavelength measurement accuracy to 0.035 nm. The previous flights of the SUSIM instruments had a 2 wavelength accuracy of 0.2 nm. In addition to improvements in the accuracy, spectrometer scans can be made over correct wavelength range without large margins made necessary when there are errors in zeroing. This saving of time is especially important in the flight environment.

B. Photo diode Electrometer

The original SUSIM detector wheel had a high gain electrometer with six gain ranges with the five photo diodes multiplexed to one electrometer due to space limitations. Several SUSIM/ UARS flight spares with an improved design having three gains were available. One was chosen for its stability, time constant and noise characteristic. It was then modified to fit into the SUSIM/ATLAS detector wheel. Three gain unit has greater overlap of the ranges than the six gain unit. The improvement in the signal to noise ratio was nearly a factor of two and the stability and time constant improvements allowed the spectral scans to be completed nearly 20% faster. This was an important time savings in flight operations since data sets could be completed during a single orbit. The new electrometer is responsible for much of the reduction in spectral irradiance uncertainty from 4-8% above 130 nm to 3-6% for ATLAS-3. The improved performance of the electrometer and photo diode system is important during calibration where ratios between gains, detectors, filters, spectrometers and polarization angles are made. Reducing the error in determining the ratio of optical elements and the UV induced degradation during the calibration process substantially improves the overall accuracy.

C. Filter Wheel

A fixed MgF2 entrance window which required instrument disassembly in order to measure its transmission before and after flight was removed and a new filter wheel was designed and installed behind the entrance slits. This wheel can position each of the entrance filters listed in Table I behind either of the two spectrometers thus providing a method for periodically measuring each filter's spectral characteristic. During calibration, the intense short wavelength radiation in the beam at the Synchrotron Ultraviolet Radiation Facility (SURF) at the National Institute of Standards and Technology (NIST) facility in Gaithersburg, Md., causes extensive aging of the SUSIM spectrographs. By using a "working" filter during much of the SURF calibration, the magnitude of the aging correction of the "flight" filters between pre-flight and post-flight calibration can be reduced. The UV degradation is largest on the MgF2 filters which are used below 250 nm. Three MgF2 filters were put in the filter wheel to provide a filter for periodic comparison to the flight and work filters to give an additional measure of their aging.

D. D2 lamp System

A program to produce a stable and repeatable D2 lamp was undertaken during the design and fabrication of the UARS SUSIM instrument. It was hoped that a new D2 lamp could be developed that would not have the tendency to periodically jump to new level of output. These jumps of up to 8% prevented detailed monitoring of the instruments spectroradiometric condition throughout the pre- to post-calibration period. The United Kingdom's National Physical Laboratory (NPL) together with its industry partners made several changes to the end-on, MgF2 lensed lamp used on the earlier SUSIM flights. They felt that stabilizing the source of electron generation on the filament could make the arc more stable and repeatable. First they supported the lamp filament with a ceramic rod so that its could withstand the vibration levels of launch and landing for the ATLAS instrument. In addition to mechanically improving the stability of the filament, a new power supply with increased accuracy in its 250 milliwatt current limited, greater stability and the controls to accommodate a new operating technique was developed. By reducing the filament current after the lamp was operating rather than turning the filament off, the source of the electrons would be more distributed. With the filament off, the source would tend to be a single spot and this spot could shift to a new location on the filament if the cathode material became aged. NPL also found a new operating technique. This technique was refined at NRL and became the standard operating procedure for the SUSIM D2 lamps. The lamp is operated for 20 minutes and then cooled for one hour to condition the lamp. This short conditioning and then again warming the lamp for 20 minutes prior to collecting data improves the accuracy of the irradiance output. The initial 20 minutes on and one hour off conditioning is done twice whenever the lamp has been shipped or stored for longer than a day. Repeated striking, burn-in, aging and launch level vibration testing was done then two sets of four flight lamps for the UARS SUSIM and two lamps for the SUSIM ATLAS were selected. A new power supply was developed for the SUSIM ATLAS instrument using the UARS design. The accuracy of the lamp selected for their high stability and repeatability has been demonstrated by the UARS SUSIM and SUSIM ATLAS-3 flight D2 lamps.

E. Microprocessor

 The instrument command and control microprocessor used for the first four flights had become had to maintain. A number of the components such as the UV proms had become unavailability since the unit had been built in 1979. Therefore a new unit was purchased and flight qualified for the ATLAS-3 Mission. New software was developed which incorporated the new command and telemetry mentioned in the wavelength section above and to incorporate the changes in control required by the encoder, electrometer, filter wheel, and D2 power supply. A by-product of software effort was an addition of stored commands for the instrument so that canned procedures could be up-linked to the instrument and executed. These stored command sequences provided new opportunities for operations in flight. D2 conditioning and calibration operations could be started at the preprogramed times even when the space shuttle was out of ground command contacts. Calibrations at NRL and NIST went much faster and the calibration team could concentrate on the data quality rather than the need to operate the instrument.  

SUSIM Objectives: 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:

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. 

* This content is summarized from http://wwwsolar.nrl.navy.mil/susim.html.

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"Response Degradation in the SOHO CELIAS SEM Experiment and Variations in Solar He 30.4~nm Flux," L.E. Floyd, Proceedings of 1st TIGER Symposium, Frieburg, Germany IPM, 17, 1998.

"Instrument responsivity evolution of SUSIM UARS," L.E. Floyd, L.C. Herring, D.K. Prinz, and P.C. Crane, Proceedings of SPIE, 3427, 445-456, 1998.

"Filter responsivity degradation caused by solar UV exposure," L. Floyd, Adv. Space Res., 23(8), 1459-1462, 1999.

"Variations in Total Solar and Spectral Irradiance as Measured by the VIRGO Experiment on SOHO", J. Pap, M. Anklin, C. Frohlich, C. Wehrli, F. Varadi, and L. Floyd, Adv. Space Res., 24(2), 215-224, 1999.

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"The SOLAR2000 EMPIRICAL solar irradiance model and forecast tool," W.K. Tobiska, T. Woods, F. Eparvier, R. Viereck, L. Floyd, D. Bouwer, G. Rottman, O.R. White, and R.F. Donnelly, Journal of Atmospheric and Solar-Terrestrial Physics, 62, 1233-50, 2000.

"An Analysis of the EUV Time Series from the First Order Channel of the SOHO CELIAS SEM Experiment", Floyd, L. E., and L.C. Herring, Phys. Chem. Earth (C), 25(5/6), 421-424, 2000.

"Using Precise Solar Limb Shape Measurements to Study the Solar Cycle", J.R. Kuhn, L. Floyd, C. Frohlich, and J.M. Pap, Space Science Reviews, 94(1/2), 169-176, 2000.

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"11 Years of Solar UV Irradiance Measurements from UARS", Linton Floyd, Gary Rottman, Matthew DeLand, and Judit Pap, Proceedings of the ISCS 2003 Symposium, Solar Variability as an Input to the Earth's Environment, 23-28 June 2003, Tetranska Lomnica, Slovak Republic, ESA SP-535 195-204, 2003.

"Status of Solar UV Irradiance Data", M.T. DeLand, L.E. Floyd, G.J. Rottman, and J.M. Pap, Adv. Space Res., 34, 243-250, 2004.

"Solar Irradiance Spectra for Two Solar Activity Levels", Gerard Thuillier, Linton Floyd, Thomas N. Woods, Richard Cebula, Ernest Hilsenrath, Michel Herse, and Dietrich Labs, Adv. Space Res., 34, 256-261, 2004.

"The Center-to-Limb Behavior of Solar Active Regions at Ultraviolet Wavelengths", P.C. Crane, L.E. Floyd, J.W. Cook, L.C. Herring, E.H. Avrett, and D.K. Prinz, Astronomy and Astrophysics, 419, 735-746, 2004.

"Measurement of the solar ultraviolet irradiance", Gary Rottman, Linton Floyd, and Rodney Viereck, in Solar Variability and its Effect on the Earth's Atmosphere and Climate System, edited by J. M. Pap, P. Fox, C. Frohlich, H.S. Hudson, J. Kuhn, J. McCormack, G. North, W. Sprigg, S.T. Wu, American Geophysical Union, Washington, DC, 111-125, 2004.

"Solar Irradiance Reference Spectra", Gerard Thuillier, Linton Floyd, Thomas N. Woods, Richard Cebula, Ernest Hilsenrath, Michel Herse, and Dietrich Labs, in Solar Variability and its Effect on the Earth's Atmosphere and Climate System, edited by J. M. Pap, P. Fox, C. Frohlich, H.S. Hudson, J. Kuhn, J. McCormack, G. North, W. Sprigg, S.T. Wu, American Geophysical Union, Washington, DC, 171-194, 2004.

"A Composite Mg II Index Spanning from 1978 to 2003", Rodney Viereck, Linton E. Floyd, Patrick C. Crane, Thomas N. Woods, Barry G. Knapp, Gary Rottman, Mark Weber, Lawrence C. Puga, Matthew T. DeLand, Space Weather, 2, S10005, doi:10.1029/2004SW000084, 2004.

"Solar EUV and UV Spectral Irradiances and Solar Indices", Linton Floyd, Jeff Newmark, John Cook, Lynn Herring, Don McMullin, Journal of Atmospheric and Solar-Terrestrial Physics, 67, 3-15, 2005.

"The October 28, 2003 Extreme EUV Solar Flare and Resultant Extreme Ionospheric Effects: Comparison to Other Halloween Events and the Bastille Day Event", B. T. Tsurutani, D. L. Judge, F. L. Guarnieri, P. Gangopadhyay, A. R. Jones, J. Nuttall, G.A. Zambon, L. Didkovsky, A.J. Mannucci, B. Iijima, R. R. Meier, T.J. Immel, T. N. Woods, S. Prasad, L. Floyd, J. Huba, S. C. Solomon, P. Straus, R. Viereck, Geophysical Research Letters, 32, L03S09, doi:10.1029/2004GL021475, 2005.

"Two Unusual Episodes of ~13-day Variations II. Implications for the Solar Radio Flux Density, F10.7cm", Patrick C. Crane, Solar Physics, 32, 127-133, 2005.

"Reconstruction of solar UV irradiance in cycle 23", N.A. Krivova, S.K. Solanki, and L. Floyd, Astronomy and Astrophysics, 452, 631, 2006.

"Measurements of thermospheric molecular oxygen from the Solar Ultraviolet Spectral Irradiance Monitor", J. D. Lumpe, L. E. Floyd, L. C. Herring, S. T. Gibson, and B. R. Lewis, J. Geophys. Res., 112, D16, D16308 10.1029/2006JD008076, 2007.

"SUSIM Reference Solar Minimum Irradiance Spectrum", J.W. Cook, D.K. Prinz, L.E. Floyd, P.C. Crane, and L.C. Herring, Astrophysical Journal, in review, 2007.