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Irradiance Variation at Solar Minimum: Real or Instrumental?

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

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Irradiance Variation at Solar Minimum: Real or Instrumental?

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  1. 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.

  2. 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?

  3. 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

  4. Indices of Magnetic Activity Show No Change

  5. 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..

  6. 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… Helioseismological 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…

  7. 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)

  8. 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.

  9. SBI First Results 10 tiles needed to compose a full disk mosaic Each tile is the average of 60 individual frames

  10. SBI bolometric image with limb darkening removed N W 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%

  11. 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.

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