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University of Southern California

SEH-2 (Solar EUV Hitchhiker)



Submitted by: Darrell L. Judge. P-I

Animated Spectral Sunrise!

The SEH extreme ultraviolet instrumentation aboard the space Shuttle Discovery (STS-85) has produced excellent full disk absolute solar flux data throughout the mission. All our primary objectives were met or exceeded. The observations consisted of thirteen subsolar data sets, in addition to three at sunset, one at sunrise and one data set running from sunrise through sunset.

The SEH instrumentation consists of :

  • A Ti coated Si photodiode plus an Al free standing film for observing the full solar disk shortward of 200 A (Angstroms).
  • An Al coated Si photodiode plus an Al free standing film located on the GLO 6 scan platform, to observe the full solar disk in the wavelength region shortwards of 800 A.
  • A helium double ionization cell measuring the photoionization rate of helium.
  • A neon ionization cell which measures the absolute solar flux at wavelengths shortward of 575 A.
  • A normal incidence vacuum ultraviolet spectrometer which measures the solar spectrum from 250 -1700 A.

Examples of the raw data obtained in real time during the flight are given in the following figures.

The power of spectrally resolved planetary atmosphere occultation data may be seen in the real time spectra obtained during Shuttle sunrise, August 11, 1997. A sequence of twelve spectra showing the altitude dependence of the atmospheric column density in the Earth's upper atmosphere is shown in Figure 1. This same technique can be readily applied to the atmosphere of Pluto, for example, to measure atmospheric composition and temperature. Such data can be realistically obtained using low weight and low power instrumentation meeting the resources likely to be available for a Pluto mission, unlike airglow observations.

Figure 1. A sequence of spectra showing the altitude dependence of the atmospheric column density in the wavelength range shortward of 1600 A.

The dynamic nature of the solar flux in the extreme ultraviolet (EUV) can be seen in Figure 2. During the eleven day mission the flux shortward of 200 A changed by more than 30%! IT is clear from such data that atmospheric observations should be accompanied by simultaneous observations of the EUV which is driving much of the photochemistry of planetary atmospheres. The observed variability is consistent with that observed in the EUV region being continuously monitored by our CELIAS/SEM instrument on SOHO.

Figure 2. The full disk solar flux in the spectral region shortward of 200 A. Changes in flux greater than 10% are seen to occur in a matter of hours.

Figure 3. Raw Solar EUV Intensity Spectrum without a spectral filter. The large peak is HI Lyman-a at 1216 A. The small peak left of 350 A is the He II 304 A emission.

Figure 4. Raw Solar EUV Intensity Spectrum observed through a MgF filter. The dominant feature shown is the HI Lyman-aresonance emission.

Figure 5. Raw Solar EUV Intensity Spectrum with an Al filter. The nominal Al band-pass is between 170 to about 800 A. Radiation counts longward of 800 A are due to multiple order photons. The sharp cutoff near 950 A is due to physical blockage of the incident radiation by the Al filter mount. Emissions due to helium and oxygen are evident in the spectral region near 550 A. The far left peak is the intense He II (304 A) resonance emission.

Figure 6. Raw Solar EUV Intensity Spectrum as observed through an Indium Filter. The nominal wavelength band-pass is between 750 and 1100 A. The two lines on the long wavelength side of the spectrum are the CIII (977 A) and HI Lyman-b (1025 A) emissions. Also shown is the hydrogen continuum beginning at the ionization limit at 911 A.

August 11, 1997 Sunrise/Sunset - Coordinated Science Observations

STS-85 Shuttle Mission, Orbit 67

Simultaneous solar EUV irradiance data were obtained by the IEH/SEH Shuttle instrumentation on Discovery and by the USC sounding rocket instrumentation launched from the White Sands Missile Range (WSMR) at MET 04/03:37 simultaneous measurements were obtained for about 10 min. The coordinated SEH observations were successfully performed on August 11, 1997 between MET 04/03:06 and MET 04/04:04. The SEH and the sounding rocket measurements provided a set of complementary data. Examples of the spectral data obtained by the SEH spectrometer during the mission are shown in the preceeding figures. The following two figures show the integrated solar soft X-ray flux shortward of about 200 A, corresponding to the use of a Ti coated photodiode plus a free standing Al filter.


Figure 7. Ti coated Si photodiode data obtained during the sunrise solar occultation on August 11, 1997.

Figure 8. Ti coated Si photodiode data obtained during the sunset solar occultation on August 11, 1997.






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