Irradiance Variation at SolarIrradiance Variation at SolarMinimum: Real or Instrumental?Minimum: Real or Instrumental?

David Rust, JHU/APL Space Physics GroupDavid Rust, JHU/APL Space Physics Group

A Cooperative Research Project for the International A Cooperative Research Project for the International

Heliospheric Year (IHY) - 2007Heliospheric Year (IHY) - 2007

• Includes Solar Imaging, Helioseismology, Radiometers and Includes Solar Imaging, Helioseismology, Radiometers and Spectrometers at the next sunspot minimum (expected in 2007)Spectrometers at the next sunspot minimum (expected in 2007)

• Broad objective is to measure secular trends in solar irradiance Broad objective is to measure secular trends in solar irradiance

(if any) and find their sources on the Sun.(if any) and find their sources on the Sun.

Irradiance at Irradiance at Solar Minimum:Solar Minimum:

Project ObjectivesProject Objectives• Does irradiance vary from one solar minimum to the next?Does irradiance vary from one solar minimum to the next?

• Why does the correlation with magnetic features seem to change Why does the correlation with magnetic features seem to change from one cycle to the next? from one cycle to the next?

• Are all variations associated with magnetic features?Are all variations associated with magnetic features?

• What is the irradiance when there are no magnetic features?What is the irradiance when there are no magnetic features?

• Are there large-scale, subtle irradiance variations associated with Are there large-scale, subtle irradiance variations associated with torsional waves, giant convection cells, meriodional flows, etc.?torsional waves, giant convection cells, meriodional flows, etc.?

• Do the large-scale features revealed by time-distance Do the large-scale features revealed by time-distance helioseismology cause some irradiance variations?helioseismology cause some irradiance variations?

• What is the baseline spectral distribution at solar minimum?What is the baseline spectral distribution at solar minimum?

Evidence that the Total Evidence that the Total Solar Irradiance at SunspotSolar Irradiance at Sunspot

Minimum Has IncreasedMinimum Has Increased

Using a 25-year series of satellite experiments, a minima-to-minima total solar irradiance 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.trend of + 0.04%/decade is resolved during solar cycles 21 to 23.

-- R. Wilson, Columbia University-- R. Wilson, Columbia University

Indices of MagneticIndices of MagneticActivity Show No ChangeActivity Show No Change

Brightness of Brightness of Fraunhofer Lines ShowsFraunhofer Lines Showsa Secular Increase Sincea Secular Increase Since

the Last Sunspot Minimumthe Last Sunspot Minimum

These results and the apparent increase in solar irradiance at sunspot minimumThese results and the apparent increase in solar irradiance at sunspot minimummay be instrumental, but if they are real, the implications for climate change are profound..may be instrumental, but if they are real, the implications for climate change are profound..

Participants in theParticipants in theCampaign to MeasureCampaign to MeasureIrradiance VariationsIrradiance Variations

Solar Imagers:Solar Imagers: SOHO, STEREO, RISE (Radiative Inputs of the SOHO, STEREO, RISE (Radiative Inputs of the Sun un to Earth) Network, SBI (Solar Bolometric Imager), CSUN (Cal. to Earth) Network, SBI (Solar Bolometric Imager), CSUN (Cal. State University at Northridge), ground-based observatories…State University at Northridge), ground-based observatories…

Helioseismological ImagersHelioseismological Imagers: Global Oscillations Network Group, : Global Oscillations Network Group, Michelson-Doppler ImagerMichelson-Doppler Imager

MagnetographsMagnetographs: Global Oscillations Network Group, Michelson-: Global Oscillations Network Group, Michelson-Doppler Imager, SOLISDoppler Imager, SOLIS

Solar Irradiance Monitors:Solar Irradiance Monitors: ACRIM, SOHO (Virgo),TIMED, ACRIM, SOHO (Virgo),TIMED, SORCE, SOLIS, NSO Calcium Line Monitor…SORCE, SOLIS, NSO Calcium Line Monitor…

New Instrument: Solar New Instrument: Solar Bolometric Imager (SBI)Bolometric Imager (SBI)

SBI is a bolometric telescope = it images radiant energy of the sun.SBI is a bolometric telescope = it images radiant energy of the sun.

SBI may help resolve the present controversy about the reality ofSBI may help resolve the present controversy about the reality of

long-term variations in solar irradiance.long-term variations in solar irradiance.

On September 1, 2003, SBI had a successfu 9-hour flight over New On September 1, 2003, SBI had a successfu 9-hour flight over New Mexico aboard the JHU/APL balloon-borne gondola.Mexico aboard the JHU/APL balloon-borne gondola.

Experiment science team: Experiment science team: P. N. Bernasconi, H. A. C. Eaton, P. N. Bernasconi, H. A. C. Eaton,

D. M. Rust (all JHU/APL) and D. M. Rust (all JHU/APL) and P. Foukal (Heliophysics)P. Foukal (Heliophysics)

Imaging Bolometer CharacteristicsImaging Bolometer Characteristicso 320x240 Barium Strontium Titanate (BST) ferroelectric detector elements320x240 Barium Strontium Titanate (BST) ferroelectric detector elements

o Pixel size: 50 x 50 Pixel size: 50 x 50 m.m.o On-chip thermal regulation to BST Curie temperature (~30 °C).On-chip thermal regulation to BST Curie temperature (~30 °C).o Senses by pyroelectric and dielectric effects.Senses by pyroelectric and dielectric effects.o AC coupled AC coupled Frame rate 30 images per second. Frame rate 30 images per second.

o Detector array covered with a thin film of Detector array covered with a thin film of gold blackgold black . .

o Spectral absorptance of gold black films vary less than 1%.Spectral absorptance of gold black films vary less than 1%.o Absorbed radiation of different wavelengths is totally absorbed and redistributed by the Absorbed radiation of different wavelengths is totally absorbed and redistributed by the

gold black in form of thermal emission.gold black in form of thermal emission.o The thin uniform coating retains ~ 70% of the original detector MTFThe thin uniform coating retains ~ 70% of the original detector MTF..

[%]

SBI First ResultsSBI First ResultsSBI First ResultsSBI First Results

10 tiles needed to 10 tiles needed to

compose a full diskcompose a full disk

mosaicmosaic

Each tile is the Each tile is the

average of 60average of 60

individual framesindividual frames

WN

SBI bolometric image with limb darkening removedSBI bolometric image with limb darkening removed

This is the first full-disk This is the first full-disk total light image of the total light image of the Sun ever obtained.Sun ever obtained.

It is a composite of 10 separate It is a composite of 10 separate images recorded by the SBI on images recorded by the SBI on September 1, 2003.September 1, 2003.

Faculae, spots and Faculae, spots and enhanced network smaller enhanced network smaller than 10than 10 arcsec are clearly arcsec are clearly resolved.resolved.

Pixel-to-pixel RMS noise Pixel-to-pixel RMS noise is about 0.2%is about 0.2%

Irradiance Variation at SolarIrradiance Variation at SolarMinimum: Real or Instrumental?Minimum: Real or Instrumental?

ConclusionsConclusions

• Coordinated Solar Imaging, Helioseismology, Radiometry and Coordinated Solar Imaging, Helioseismology, Radiometry and Spectrometry will be required.Spectrometry will be required.

• Broad objective is to measure variations in solar irradiance Broad objective is to measure variations in solar irradiance

and find their sources on the Sun.and find their sources on the Sun.

• New instruments will enable unprecedented precision New instruments will enable unprecedented precision measurements at a crucial phase of the sunspot cycle.measurements at a crucial phase of the sunspot cycle.