Comparisons of FUV Solar Irradiance Measurements
by SORCE, TIMED-SEE, and UARS
F.G. Eparvier1,*, T.N. Woods1, G.J. Rottman1,
L.E. Floyd2, W.E. McClintock1, and M. Snow1
• Measurements of the
solar spectral
irradiance in the Far
Ultraviolet (FUV) have
been made almost
continuously since
1991, spanning a full
solar cycle.
• There are currently
three satellites (UARS,
TIMED, and SORCE)
observing the solar
FUV.
TIMED - SEE (SOLAR EUV EXPERIMENT)
• Launched: Dec. 7, 2001
• Data available: Jan. 22, 2002 - present
• Two solar XUV-FUV instruments:
• EGS (EUV Grating Spectrograph)
• All instruments compare extremely well
at the bright Hydrogen Lyman-α line at
121.6 nm. (Note: SEE EGS exhibits
somewhat larger rotational variations
which may be due to an uncorrected
satellite beta angle dependence).
• In the 130-145 nm range SEE and
SORCE agree to better than 20%, but
SUSIM is higher than SEE by as much
as a factor of 2. (This difference was
also noted with UARS SUSIM and
UARS SOLSTICE comparisons.)
• 1/4 m Spectrograph w/ MCP detector
• 27 - 195 nm at 0.4 nm resolution
• XPS (XUV Photometer System)
• Silicon photodiodes with broadband filters
• 0.1 - 34 nm at ~ 7 nm resolution
• Plus HI-Ly-α (121.6 nm)
• Filter wheel problem limits XPS to 0-10, 27-34 nm since July 24, 2002.
• SEE observes the Sun for 3 min. of every 97 min.
• SEE has redundant channels and periodic rocket
underflights for calibration purposes.
•SEE Version 6 data is used in this study.
• At longer wavelengths the SUSIM and
SORCE irradiances show closer
agreement.
• However, the SEE irradiances are larger
than both SORCE and SUSIM by as much
as 40% at 180 nm.
SORCE-SOLSTICE
UARS
• Launched: Sep. 11, 1991
• Data available Oct. 1991 - present
• Two solar UV instruments:
• SOLSTICE
• Grating Spectrometer with PMT detectors
• 119 - 420 nm at 0.1 - 0.35 nm resolution
• SOLSTICE calibrates against stars
• UARS SOLSTICE Version 18 data is used in this
study
• SUSIM
• Dual Dispersion Spectrometer
• 115 - 410 nm at 0.15 - 5 nm resolution
• SUSIM calibrates against an on-board lamp
• UARS-SUSIM preliminary version 22 data is used in
this study
SORCE
• Launched: Jan. 25, 2003
• Data available: Mar. 2, 2003 - present
• Two solar XUV-UV instruments:
• SOLSTICE
• Grating Spectrometer with PMT detectors
• 115 - 320 nm at 0.1 nm resolution
• Calibrates against stars
• SOLSTICE preliminary Version 3 data is used in this study
• XPS
• Silicon photodiodes with broadband filters
• 0.1 - 34 nm at ~ 7 nm resolution
• Plus HI-Ly-α (121.6 nm)
• XPS has redundant diodes for calibration purposes
• XPS Version 4 data is used in this study.
University of Colorado - Laboratory for Atmospheric & Space Physics, Boulder, CO; 2 Interferometrics, Inc. &
Naval Research Laboratory, Washington, DC; * Contact: eparvier@colorado.edu
1
SORCE-XPS
• The longer wavelength SEE irradiances
also show a gradual overall rise with time
due to a known instrument gain increase
which will be addressed when the latest
sounding rocket calibration is applied in
Version 7 (due out in Dec. 2003).