Irradiance Variation at Solar

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Irradiance Variation at Solar Minimum Real or
Instrumental? David Rust, JHU/APL Space Physics
Group

• A Cooperative Research Project for the
  International
• Heliospheric Year (IHY) - 2007
• Includes Solar Imaging, Helioseismology,
  Radiometers and Spectrometers at the next sunspot
  minimum (expected in 2007)
• Broad objective is to measure secular trends in
  solar irradiance
• (if any) and find their sources on the Sun.

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Irradiance at Solar Minimum Project Objectives

• Does irradiance vary from one solar minimum to
  the next?
• Why does the correlation with magnetic features
  seem to change from one cycle to the next?
• Are all variations associated with magnetic
  features?
• What is the irradiance when there are no magnetic
  features?
• Are there large-scale, subtle irradiance
  variations associated with torsional waves, giant
  convection cells, meriodional flows, etc.?
• Do the large-scale features revealed by
  time-distance helioseismology cause some
  irradiance variations?
• What is the baseline spectral distribution at
  solar minimum?

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Evidence that the Total Solar Irradiance at
Sunspot Minimum Has Increased
Using a 25-year series of satellite
experiments, a minima-to-minima total solar
irradiance trend of 0.04/decade is resolved
during solar cycles 21 to 23. -- R. Wilson,
Columbia University
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Indices of Magnetic Activity Show No Change

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Brightness of Fraunhofer Lines Shows a Secular
Increase Since the Last Sunspot Minimum

These results and the apparent increase in solar
irradiance at sunspot minimum may be
instrumental, but if they are real, the
implications for climate change are profound..
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Participants in the Campaign to
Measure Irradiance Variations
Solar Imagers SOHO, STEREO, RISE (Radiative
Inputs of the Sun to Earth) Network, SBI (Solar
Bolometric Imager), CSUN (Cal. State University
at Northridge), ground-based observatories Helios
eismological Imagers Global Oscillations
Network Group, Michelson-Doppler
Imager Magnetographs Global Oscillations
Network Group, Michelson-Doppler Imager, SOLIS
Solar Irradiance Monitors ACRIM, SOHO
(Virgo),TIMED, SORCE, SOLIS, NSO Calcium Line
Monitor
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New Instrument Solar Bolometric Imager (SBI)

• SBI is a bolometric telescope it images
  radiant energy of the sun.
• SBI may help resolve the present controversy
  about the reality of
• long-term variations in solar irradiance.
• On September 1, 2003, SBI had a successfu 9-hour
  flight over New Mexico aboard the JHU/APL
  balloon-borne gondola.
• Experiment science team P. N. Bernasconi, H. A.
  C. Eaton,
• D. M. Rust (all JHU/APL) and P. Foukal
  (Heliophysics)

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Imaging Bolometer Characteristics

• 320x240 Barium Strontium Titanate (BST)
  ferroelectric detector elements
• Pixel size 50 x 50 mm.
• On-chip thermal regulation to BST Curie
  temperature (30 C).
• Senses by pyroelectric and dielectric effects.
• AC coupled ? Frame rate 30 images per second.
• Detector array covered with a thin film of gold
  black .
• Spectral absorptance of gold black films vary
  less than 1.
• Absorbed radiation of different wavelengths is
  totally absorbed and redistributed by the gold
  black in form of thermal emission.
• The thin uniform coating retains 70 of the
  original detector MTF.

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SBI First Results
10 tiles needed to compose a full
disk mosaic Each tile is the average of 60
individual frames
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SBI bolometric image with limb darkening removed
This is the first full-disk total light image of
the Sun ever obtained. It is a composite of 10
separate images recorded by the SBI on September
1, 2003. Faculae, spots and enhanced network
smaller than 10 arcsec are clearly
resolved. Pixel-to-pixel RMS noise is about 0.2
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Irradiance Variation at Solar Minimum Real or
Instrumental? Conclusions

• Coordinated Solar Imaging, Helioseismology,
  Radiometry and Spectrometry will be required.
• Broad objective is to measure variations in solar
  irradiance
• and find their sources on the Sun.
• New instruments will enable unprecedented
  precision measurements at a crucial phase of the
  sunspot cycle.