Verification of SOHOCELIASSEM EUV flux calibration based on seven sounding rocket flights

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Verification of SOHO/CELIAS/SEM EUV flux
calibration based on seven sounding rocket
flights
Joint CELIAS/PLASTIC/SEPT Workshop April 6-9, 2009

• Darrell L. Judge, Seth R. Wieman, Leonid V.
  Didkovsky
• Univ. of Southern Calif. Space Sciences Center
  SHS-274 UPC Los Angeles, CA 90089

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Introduction

• An updated, verified version of the
  SOHO/CELIAS/SEM database was released Dec. 2008
  available at www.usc.edu/dept/space_science/sem_d
  ata/sem_data.html
• SEM clone and Ne rare gas ionization cell (RGIC)
  measurements from seven sounding rocket flights
  were analyzed to complete this version
• Calibrated flux values are very similar to those
  of the previous (2000) version

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Background

• SOHO/CELIAS/SEM is a highly stable transmission
  grating spectrometer with high photometric
  accuracy
• Measures solar flux in two bandpasses 26-34
  nm(/- 1st order) and 0.1-50 nm (0-order)
• Data is used for studies in
• solar variability
• space weather
• earth atmosphere
• and is the basis for the S10 index in the
  Jacchia-Bowman thermospheric density model
  Bowman, et. al, 2008

Al coated Si photodiodes
-1 order (26-34 nm)
Transmission Grating
0 order (0.1-50 nm)
Freestanding Al Filter
1 order (26-34 nm)
SOHO Solar EUV Monitor (SEM)
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Background

• How we determine SOHO/SEM flux values from
  measured photocurrent

• NIST measured instrument efficiencies
• SOLERS22 composite reference spectrum Woods, et.
  al, 1998
• Degradation model based on build-up of carbon
  contamination layer

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Equation for determining flux values
26-34 nm flux
where k1 a correction for SEM 1st order
sensitivity band extending slightly beyond 26-34
nm (accounts primarily for sensitivity to 17.4
nm radiation diffracted in the second
order) bkgrd background signal due to
diode/electrometer dark current and residual
light leaks DNSEMch data channel raw count
rate A entrance aperture area ? SEM channel
efficiency from NIST calibration ?S22 Solar
flux from SOLERS22 reference spectrum
all functions of ? fcarbon-trans transmission
through carbon contamination layer, f(time) f1AU
correction for 1 AU
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Sounding rocket underflight measurements

• Current SOHO/SEM calibrated fluxes are based on
  measurements from seven sounding rocket
  underflights (alt. 300km)
• Solar flux measurements used for the calibration
  are primarily from
• SEM clone
• Neon rare gas ionization cell (RGIC)

Instrument suite aboard the SOHO/SEM underflight
calibration sounding rocket
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SEM Clone

• Flux values determined from DN in a manner
  similar to SOHO/SEM, with some differences
  related to
• Correction for atmospheric absorption (based on
  NRLMSIS plus N2, O2, O total absorption cross
  sections)
• Background subtraction (both visible leakage and
  EUV leakage now considered)

180 roll
off-pointing maneuvers
Typical SEM clone sounding rocket flight data
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SEM Clone
SEM clone 4-axis vacuum manipulator next to the
test chamber on the end of NIST SURF BL-9

• SEM Clone sensitivity profiles are measured
  before/after each rocket flight at NIST-SURF
  BL-9, a synchrotron beam line equipped with a
  high resolution monochromator and an absolute
  radiometric standard
• Calibration includes efficiency measurements at
  wavelengths between 15 and 49 nm in 1 nm
  increments with several instrument orientations
  (on and off-pointing, horizontal and vertical
  beam polarizations)

SEM clone instrument response profiles measured
at NIST compared to the SOLERS22 EUV reference
spectrum
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Ne RGIC

• Absolute detector similar to the radiometric
  standard previously used at NIST Carlson, et.
  al, 1984
• Flat response profile from 5-57.5nm (defined by
  Ne photoionization cross-sections)

Ne RGIC response profile compared to SEM 1st
order channel profile and the SOLERS22 reference
spectrum
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Ne RGIC

• Operates optically thick
• Windowless no optical surfaces to degrade
• Target gas is periodically cycled through the cell

Schematic of the Ne rare gas ionization cell and
gas supply system
Typical Ne RGIC sounding rocket data . Periodic
gas pulses give rise to peaks in ionization
current. Peaks are highest near flight apogee (
300 sec after lift-off) where atmospheric EUV
absorption is at a minimum
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Ne RGIC

• Solar flux is determined from ionization vs.
  pressure curve from each purge-fill cycle (linear
  portion of curve is extrapolated back to zero
  pressure current corresponding to current with no
  contribution from secondary ionization)
• Geometry of cell (long along optic axis with
  small radius) makes it optically thick for EUV
  but optically thin for ejected photoelectrons

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Ne RGIC

• Flux values for the downward leg (lower
  background) measurements are fit to a modeled
  5-57 nm atmospheric absorption profile based on
  NRLMSIS and O2, N2 and O cross sections.
• The above atmosphere flux (i.e. zero absorption
  flux, referred to as I0 in plot at right) is the
  value which provides the best fit between RGIC
  measurements and the modeled profile

time
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Sounding rocket timeline

• Seven sounding rocket flights and 9 NIST
  calibrations of the SEM clone have been completed
  since the launch of SOHO

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Results
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Results
Values for 26-34 nm solar flux are obtained from
the Ne RGIC measurements based on the SOLERS22
reference spectra (i.e. by multiplying the 5-57nm
values by the ratio
where FS22 is the SOLERS22 flux as a function of
?).
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Results
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Conclusions

• Both the SEM clone values and the RGIC values are
  in good agreement with the published SOHO/SEM
  values suggesting that the degradation model
  established (in Dec. 2000) based on the first two
  underflights is still valid.

Future Work

• Ongoing work related to the SOHO/SEM database
  includes
• Releasing versions in which SOHO/SEM absolute
  flux and sounding rocket fluxes (and accordingly
  the degradation model) are determined based on
  reference spectra other than SOLERS22, such as
  Solar2000 Tobiska, http//www.spacewx.com/solar20
  00.html, NRLEUV Warren, 2006 or FISM
  Chamberlin, et. al, 2008.
• Continue maintaining calibrated flux values to
  provide overlap with other EUV missions (e.g.
  SDO, GOES/EUVS).
• Provide reconstructed EUV data for solar flares
  during which SOHO/SEM measurements were either
  saturated or contaminated by x-rays or energetic
  particles.

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Acknowledgements

• This work was supported by NASA grants NNG05WC09G
  and NNX08AM94G
• We would like to thank Rob Vest, Charles Tarrio,
  Mitch Furst and the rest of the staff of the NIST
  SURF facility for their tremendous contribution
  to the calibration effort throughout the SOHO/SEM
  program. We would also like to thank Don McMullin
  for his many contributions to the continuing
  success of the SOHO/SEM mission, as well as his
  management of sounding rocket flights providing
  periodic calibration of the on-orbit SOHO/SEM.

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References

• Judge, D.L., Wieman, S., Didkovsky, L.,
  Gangopadhyay, P., 13 years of SOHO/CELIAS/SEM
  Calibrated Solar Extreme Ultraviolet Irradiance
  Data, AGU Fall Meeting 2008, SH23A-1629 San
  Francisco
• Judge, D.L., et al , First Solar EUV Irradiances
  Obtained from SOHO by the CELIAS/SEM, Solar
  Physics Volume 177, Numbers 1-2 January, 1998
  Pages 161-173
• Ogawa, H., D. Judge, D. McMullin, P.
  Gangopadhyay, A. Galvin (1998), First-year
  continuous solar EUV irradiance from SOHO by the
  CELIAS/SEM during 1996 solar minimum, J. Geophys.
  Res., 1-6.
• Gangopadhyay, P.L. Didkovsky, H. Ghadimi, S.
  Wieman, and D. L. Judge, UltraLow Frequency
  Solar He ii 30.4 nm Pulsations, The
  Astrophysical Journal, 67014141419, 2007
  December 1
• Tsurutani, B.T., Judge, D.L., Guarnieri, F.L.,
  Gangopadhyay, P., Jones, A.R., Nuttall, J.,
  Zambon, G.A., Didkovsky, L., A.J. Mannucci, B.
  Iijima, R. R. Meier, T.J. Immel, , T. N. Woods,
  S. Prasad , J. Huba, S. C. Solomon, P. Straus, R.
  Viereck, The October 28, 2003 extreme EUV solar
  flare and resultant extreme ionospheric effects
  Comparison to other Halloween events and the
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• Viereck, R., L. Puga, D. McMullin, D. Judge, M.
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  Astrodynamics Specialist Conference 18-21 August
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  Solers 22 Workshop, Solar Physics, 511, 1998.
• http//physics.nist.gov/MajResFac/SURF/SURF/index.
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  solar cycle 23 minimum, J. Geophys. Res., Vol.
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• Hedin, A. E., MSIS-86 Thermospheric Model, J.
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• http//www.spacewx.com/solar2000.html
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  Space Weather, 6, S05001

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Additional slides
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0-order saturation and 1st-order contamination
during solar events
SEM count-rate (CR) measurements (thick lines)
and GOES 8 16 MeV particle flux (thin line) for
the BD solar flare event. The SEM Ch 1 (top
panel) is contaminated by the rise in SEP flux
after the time indicated by the vertical dotted
line. The SEM Ch 0 (bottom panel) is saturated by
the soft x-ray and EUV flux. The unsaturated
decay portion of the SEM Ch 0 profile is only
weakly affected by the increased SEP flux. L. V.
Didkovsky, D. L. Judge, A. R. Jones, S. Wieman,
B. T. Tsurutani, and D. Mcmullin, Correction of
SOHO CELIAS/SEM EUV Measurements Saturated by
Extreme Solar Flare Events Astronomische
Nachrichten, Volume 328, Issue 1, 2007, Pages
36-40
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