UVCS images of streamers show striking differences between ions.Quantitative analysis yields abundances in the outer layers

of the streamers in agreement with those observed in the slow solar wind.This supports a picture in which the slow solar wind

originatesfromthe sidesof streamersasinferred fromthe LASCO tracer observations.

Coronal Holesand HighSpeed SolarWind.Observing the corona over the Sun'spolesisa significantchallenge.

Here the

coronaconsistsof faintplumesalignedalong theopen magnetic field linesextending fromthepolar coronalhole.UVCS

measurementsof coronalhole outflowsshow speedsashigh as200 kms ^{- 1}at2 R_Sun.Time sequencesof LASCO imagesshow

individual features in plumes with speeds of 350-500 kms ^{- 1}compared to 100-150 km s^{- 1} at the same height in coronal streamers.

SOHO isnow poised to exploit these breakthrough observations.Asthe mission continues, combining LASCO observations

withthoseofMDI,EIT,CDS,SUMER,andUVCSshouldmakeitpossibletolearnindetailwhatmaintainsthecoronal

temperature aswellaswherethe slow wind originatesand how it is related to the Sun's magnetic field.

3. IN SITUSOLAR WIND

Solar Wind PlasmaThe CELIAS Proton Monitor (PM) measures ionsin situin the range 0.3 to 6 keV/e and generates values of

the solar wind proton bulk speed, density, thermal speed, and north/south flow direction with a 30-second temporal resolution.

With SOHO'sposition atL1,the Proton Monitor samplessolar wind thathasnotbeen modified by the presenceof the earth.

(Thesolarwindplasmaisdeceleratedanddeflectedinthepresenceofdiffuseion eventsintheearth'sforeshockregion.)

Correlations of the "pristine" L1 solar wind with the near-earth ISTP solar wind data study the affect of the foreshock region on

the solar wind.Spatial structures in the solar wind are also being studied using multiple spacecraft (SOHO, IMP, INTERBALL,

WIND).

PMmeasurementsof probableinterplanetaryshock waveswere

observed in early 1997.Preliminary evidence fromother SOHO

experiments (LASCO and/or EIT) suggests thatthese shocks may

be associatedwith CME's(CoronalMassEjections).About18

hoursafter the 1997 February 9 shock passage,the PMobserved

anunusualdensityrarefaction.(SeeFigure7.)Thedensityre-

mained below 1 cm ^{- 3}for about 5 hours, attaining a minimum value

of ~ 0.2 cm^{- 3}.These arethe lowestdensities observed by the PM

in 14 months of operation.The very low solar wind ram pressure

atthis time would be expected to cause the earth'smagnetopause

and bow shock to expand to almost twice their nominal locations.

The CELIAS Proton Monitor solar wind data are provided within

a few minutes (typically <= 30 min) of data acquisition to the solar-

terrestrialspacephysicscommunity and to the generalpublicvia

the world wide web (http://umtof.umd.edu/pm).This site is inregu-

lar use by the NOAA SEL and the USAF for space weather predic-

tions.Forexample,theProton Monitormadeandreportedthe

firstin situmeasurementsof theinterplanetarymanifestation of

the CME eventwhose solar launch wasobserved by LASCO on

January 6, 1997.

Solar Wind Elemental AbundancesThe CELIASMass Time-of-

Flight (CELIAS/MTOF) sensor is a high resolution solar wind mass

spectrometer (M/ DM>= 100)whichmeasurestheelementaland

isotopic composition of the solar wind over a wide range of solar

wind bulk speeds.TheMTOFsensor hasmetalldesign goals,

and no degradation in instrumentperformance hasbeen observed

since launch.MTOF has already madeobservations of previously

undetected species in the solar wind (Figure 8).Therare elements

phosphorus, chlorine, potassium, titanium, chromium, manganese

Figure7.SOHO-CELIAS/PMmeasurements(10min

averages) for the 3-day period 1997 February 8-10. A high

speed stream arrives on February 8, a shock on February

9, and the density rarefaction on February 10.

and nickel have been measured in the solar wind for thefirst time.