II) Overview

The SOHO Mission and its Role within the ISTP

SOHO is part of ISTPThe International Solar Terrestrial Physics (ISTP) program is a joint project of the United States, ESA,

and Japan to understand the fundamentalprocesses of the Earth-Sun system. These processesinclude stellar convection, mag-

netic dynamo action, generation of stellar winds, gravity, and fundamental particle physics.The Solar and Heliospheric Obser-

vatory (SOHO) is one of the ISTP system of spacecraft.SOHO's task in the endeavor is to study the Sun from its deep interior

to just before the Earth's magnetosphere. By flying at L1, 1% of the distance to the Sun on the Sun-Earth line,SOHO is ideally

situated continuously to monitor the Sun, the inner heliosphere, and the solar wind particles streaming toward the earth.

ISTP isa Framework forSolar-TerrestrialScienceSpace physicshasa variety of spacecraftto study the Sun-Earth system.

The total system is beginning to function extremely well as is evidenced by our ability to observe and quickly bring together the
totalpictureofmassejectionsfromwhentheyleavetheSuntowhentheyinteractwiththemagnetosphere(cf.http://

sohowww.nascom.nasa.gov/gallery/current). As we move toward solar maximum, expected in 2000 or 2001, this coordi-

nated set of missions will greatly enhance our understanding of the Sun and the reaction of the earth and its environment to solar

variations.

SOHO and Space WeatherWhile space weather monitoring is not a SOHO or ISTP science goal per se, the advanced warning

provided by direct observations of solar disturbances, and the nearly one hour travel time for typical solar wind at L1 before it

reaches the earth, has made both the remote andin situSOHO observations a valuable input component for NOAA's SEC space

weather prediction service.This is only possible because of the timeliness of the release of SOHO data to the public, typically

within a day for snapshotsof the solar images,and literally within minutesof realtime acquisition (and within hoursfor play-

back acquisition) for the solar wind proton parameters.

SOHO's GoalsThe SOHO mission hasthree principalgoals:to gain an understanding of the mechanismsresponsible for the

heating of the Sun's outer atmosphere;to determine where the solar wind originatesand how it is accelerated;and to measure

the properties of, and flows in, the solar interior. The goal of a re-use of SOHO -- the SOHO Solar Maximum Science program

-- is to understand how the processesinside the Sun and itssurrounding atmosphere changeduring the rise to and through an

activity cycle maximum.

SOHO Science Goals:Historical Perspective

Scientific Relevance-Atmospheric HeatingIthasbeen nearly 60 yearssince the discovery thatthe outeratmosphere ofthe

Sun isthree ordersof magnitude hotter than the surface,and nearly 40 yearssince massflow fromthe Sun, a solar wind, was

predicted.In the early 1960'sthe existenceof the solarwind wasconfirmed on the firstNASA/JPLMarinerMission.The

SkylabMission in the early 1970'sestablished thatthe high speed solarwind hasitsorigin in unipolar magnetic regionsthat

have an overlying atmospherethatemitslower than averageEUV and softX-ray flux -- coronalholes.These observations

established a causalconnection between regionson the Sun and geomagnetic variationson the earth thathad been observed

since the middle of the 19th century. A few years later, theOrbiting Solar Observatory-8 (OSO-8)showed that the corona was

NOT heated by acoustic waves, previously thought to be the most likely heating method. TheSolar Maximum Mission(SMM)

in the 1980's established that the Sun was a variable star whose total luminosity changed in phase with the solar magnetic cycle.

The JapaneseYohkohmission has shown that a significantfraction of coronal heating is spatially and temporally localized and

mostprobably resultsfrommagnetic reconnection.Magnetic reconnection, previously thoughtto occur slowly if atall in the

highly conducting outer atmosphere of the Sun, is now observed to occur on scales from the resolution limit of YOHKOH (~ 5

arc seconds) to a solar radius or more.

Scientific Relevance-Solar InteriorIn the 1960's it was discovered that the entire surface of the Sun wasconstantly in motion

and that local areas of the photosphere moved with a period of about five minutes.Initially the motions were thought to be local

responsesto convective plumes below the surface and the five minute period just the natural buoyancy frequency of the atmo-

sphere.In the early 1970'stheoreticianspredicted thatthe oscillations werethe atmosphere'sresponse to globalmodes in the

interior.Within a fewyearsthesepredictionswereverified and thediscipline ofhelioseismology wasborn.By precisely

measuring the frequencies of the modes it is possible to determine temperature, density, equation of state, elemental and isotopic

abundances,interior mixing, interior rotation,and interior flow systemsasa function ofsolar radius,longitude, and latitude.

Therefore,it is possible to verify models of stellar interiors and stellar evolution by direct measurement.A secure understand-

ing of the solar interior isessentialto establishing whether the observed lowerthan expected solarneutrino flux isdue to an

Proposal forSOHO SolarMaximum Science Program-

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