Variations of Solar Luminosity

RadiometryDetermine spectral and total irradiance variations and

the spectral and spatial distribution ofradiation, to provide

accurate inputfor heating of the solarouter atmosphere and for terrestrialclimate modeling.Two decadesof observationsof

solar irradiance showthatthe solarenergy flux variesovera widerange ofperiodicities:fromminutesto the 11-yearsolar

activity cycle. Since we only observe the Sun's irradiance from one direction in space, we must determine whether the observed

irradiancevariationsrepresentchangesin the luminosity, or area resultof the angulardistribution of the radiation fromthe

structured photosphere.The solar cycle related long-termirradiance changesrepresentrealluminosity changes,whereasthe

short-term variations from days to months caused by active regions via the combined effect of dark sunspots and bright faculae

may have a largecomponentdue to the distribution of the radiation fromthese localized sources.Since variationsin the solar

energy flux - persistent over long periods of time - may trigger climate changes, it is fundamental to understand the underlying

physical mechanisms.

Empirical models of totalirradiance, solely based on surface magnetic field effects, underestimate the observed changes at the

maxima of solar cycles 21 and 22. The origin of the discrepancy may be temporal changes in differential rotation in the interior,

the magnetic fields near the base of the convective zone, large scale mixing flows, or large scale convective cells.The VIRGO

totalandspectralirradianceobservationswith thehigh resolutionsolarimagesandmagnetogramsfromMDIandthelow

resolution LOI images will clarify which solar irradiance are related to the localized magnetic phenomena such as sunspots and

faculae and diffuse bands with slight temperature differences.While the helioseismology data from GOLF, VIRGO, and MDI

will determine how temperature structures propagate through the solar interior.

The spectral distribution of total irradiance variability is not yet known.Since the Sun's energy input is the main driver of the

physicalprocesseswithintheEarth'satmosphere,understandingtheunderlyingphysicalmechanismsanddeterminingthe

contributionofvariousspectralbandstothetotalfluxvariabilityareatthecenterofstudyingtheclimateimpactofsolar

variability.The VIRGO integrated-light and spectral irradiance observations in the near-UV, visible, and near-IR, together with

the UV observationsof the UARS, NOAA-9,and NOAA-11 satellitesand the EUV observationsof SOHO'sCELIAS/SEM,

provide the first real opportunity to estimate the spectraldistribution of the changes in the solar energy flux.The ultimate goal

isto understand how,why, and on whattime scales,the mechanismgoverning the solar energy flux varies.Fromthis recon-

struction and prediction of the solar induced climate changes might be possible.

EUV RadiometryDetermine, with more precise photometry than ever before achievable, the variation in the solar EUV bands

most responsible for changes in the terrestrial thermosphere. The CELIAS SEM is a well calibrated EUV sensor that to date has

measured the short-term (including flare-related) variations in the EUV irradiance, and the rotational modulation in that signal;

as active regions rotate onto and off the visible hemisphere.Continued observations into the next phase of the solar cycle would

be of extreme interest due to the expected increase in solar activity, both because most of the total luminosity variations over the

solar cycle are believed (but notyet conclusively proven) to occur in the UV, and because very little is known of the solar EUV

irradiance variation during the rising phase of the solar cycle.

Figure of the LimbStudy long-termvariations in the figure of the limb.

The expected accuracy of limb shape determination has been realized.Results from the first SOHO roll observations produced

the most precise determination of solar oblateness.There is no excess surface oblateness.Thep-mode ridges are clearly visible

in the limb data with individual mode amplitudes measurable in micro-arc-seconds(corresponding to a fewmeters).

Initial analyses of the shape of the Solar limb have been surprising.The data show evidence for a larger new component of the

Solar figure - "mountains" on the sun with horizontal scales several times supergranule scalesand height scales of hundredsof

meters. Simultaneously we have seen evidence for similar scale structuresin both directDoppler data and in line-depth obser-

vations.We have notyetcombined the three typesof observationsto see if this new componentof structure isthe tailof the

supergranular spatial and temporal spectrumor if it is an indication of a previously unobserved phenomenon.

Interior and Surface Flows

SupergranulationStudy the evolution of supergranular convection cells and magnetic network reorganization. The distribution

of active region magnetic fields over the surface are thought to be due to the interaction ofsupergranules and magnetic fields.

Proposal forSOHO SolarMaximum Science Program-

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