into the much longer livedg-modes.

The principle difficulty in identifying theg-modesisthe interference fromincoherentsolar processeswhich produce theob-

servedbackgroundspectrum.Thedominantprocessintheg-modefrequencybandisthoughttobethesupergranulation.

BecauseGOLFobservesthe entireSun,rotation and limb darkening cause differentpartsof thesolar disk to contribute by

variableamountstotheobservedDopplersignal.Consequently,theinstrumentdoesnotresponduniformlytoallofthe

supergranuleson thesurface.MDI providesseparate velocitiesforeach partofthe solarimage and thisspatialinformation

providesa mechanism to reduce the amplitude of the supergranule component of the noise signals.VIRGO produces data from

12 pixels so that the MDI derived correction can be verified if GOLF and VIRGOg-mode frequencies are identical.

Internal Rotation-Determine the rotation rate as a function of radius and latitude.Internal rotation causes splitting of theg- and

p-modes.Inversion techniquesusing the measured splittings yield the radialand latitude dependence ofthe internalrotation.

Thisknowledge isessentialfor understanding solar and stellar evolution.The interaction between rotation and convection is

criticalto understanding solar activity,the generation of magnetic fields,and the nature of the solar dynamo.

Shown in Figure 4 is the spherically symmetric solar interior rotation determined from the SOI medium-ldata. The figure shows

a shear layer in the region thatseparatesthedifferentially rotating convection zoneand the morerigidly rotating core.This

resultcombined with the observation of greaterturbulencein thislayer isevidenceforthe site of the generation of the solar

cycle dynamo. From low degreep-mode analysis there are cur-

rently no indications that the core region is spinning extremely

rapidly; the most probable value is near the surface rotation rate.

Excitation and DampingExplore the couplingof acoustic-grav-

ity wavesto turbulentconvection.Determine mode lifetimes,

and elucidatethe driving mechanisms. Itis now agreed thatp-

modes are excited by turbulent convection. Not only the depen-

dence of the amplitude upon the frequency, but also the tempo-

ralmodulation of solarp-modescanbewelldescribed asan

ensemble ofstochastic oscillators.The asymmetry in oscilla-

tory line profilesprovidesinformation aboutthe location and

couplingofthesourceturbulencetowaves.Thelifetimesof

modes are indicated by their line widths in the power spectrum.

The damping ofp-modes, however, depends on several effects:

non-adiabatic interaction due to radiative transfer, coupling with

the turbulentconvection and scattering ofthe acoustic waves

by turbulentflow fields.Asthe formofaline maybeinflu-

enced by the way the time seriesare analyzed thismethod has

yielded controversial results.The same may apply to the analy-

sisof the amplitude modulation of modes.

Figure4. Thesolar rotation rateinferred from two months of

SOHO-MDImedium-l(l< 300) solarp-mode observations, as

a function of radius at three latitudes (0º, 30º, and 60º).

Analysis of high frequency ridges and p-mode asymmetries observed in both velocity and intensity indicate that at this time in

thesolarcyclethesourcesoftheacousticwavesareabout50 kmbelowthesurface.Therearesomeindicationsthatthe

excitation level varieswith latitude currently peaking near 40 degrees north and south.

Comparison ofMDI,GOLF,and Virgo data haveverified thatthetime seriesand spectraoflow degreesmodesobserved

simultaneously in each quantity are virtually identical.Thismeans that the spectral details observed by the instruments are due

to the propertiesof the stochastic, excited,damped oscillationsin the Sun rather than instrumentaleffects.Thus, analysiscan

proceed with confidencethatsolar sourcesand damping are being evaluated.

Solar BackgroundDeterminesthe globalcharacteristicsof smallscale solar surfacephenomena.

The shape,time variation

and amplitude of the solar noise signalis directly dependent on the flux parametersof solar surface structures.From the study

of thissignalitispossible to deduce theglobalcharacteristicsof granulation,mesogranulation,supergranulation and active

regions.This information is necessary to estimate how surface velocity structures affect line profiles and line profile asymme-

tries on the Sun and other stars.Resolution of controversies on the existence of planets are dependent on the understanding of

line profile shapes.

From low to highlthe solar noise level has been reached and very careful analysis has begun to separate small systematic noise

sources.