2. The Outer Solar Atmosphere and the Solar Wind

SOHO studies the outer atmosphere and solar wind with two groups of instruments:EIT, CDS, and SUMER study the transition

region and inner corona at moderate to high spatial resolution and UVCS, LASCO, and SWANprovide data on the outer corona

including the solar wind acceleration region.MDI supportsthese data sets by providing a record of the surface magnetic field

and its evolution.

The primary goalof the coronalinstrumentsis to investigate the physicalmechanismsthatheatthe corona and accelerate the

solar wind.While several space-based instruments have focused on these problems, SOHO's is the first comprehensive instru-

ment system with sufficienttime resolution to follow the dynamics, spatial resolution to image the many distinct features, and

spectral resolution to determine accurately the temperature, density, and velocity.Previous satellite instruments have suggested

that much of the energy that heats the corona is input on small spatial and temporal scales and that there are larger scale flows of

energy toward and away from the surface.SOHO, with its unique combination of spatial, spectral, and temporal resolution, has

finally revealed in detail the richnessof the structure and dynamicsof the quiet solar transition region and corona.

Although the first year of SOHO operation has not definitively answered the global questions of coronal heating and solar wind

acceleration, the unprecedented detail it provides hasled to significant new insights and understanding of the solar atmosphere.

For example, while it is too early to be certain, it appears that the location of the accelerating region, and the rate of acceleration,

for the low-latitude, slow solar wind have been discovered, and unprecedented spectroscopy of the extended corona is revealing

evidence forMHD waveenergy deposition aswellaslargedifferencesfromphotospheric abundances.In whatfollows,we

presentsome of the new resultsand insights derived fromthe SOHO data.

Dynamics and Heating

Transition Region Dynamics.Although highly dynamic events can show large upward mass flows, one of the great paradoxes

of transition-region physicsisthathigh spectralresolution observationsshow thatmost locationsin the quietsolar transition

region show redshifts--the plasma flow is predominately downward.Before SOHO, accurate transition-region flow velocities

could only be measured up to temperatures of a little more than 10 ⁵K.Now SUMER has extended those observations through

the upper transition region to the low corona.These studies show that over most of the quiet solar surface the downflow velocity

peaksatatemperaturenear10⁵K,butthatpersistent

downflowsremain up to the base of the corona.While

explosive events produce some upflows, they do not ap-

peartoprovideenoughmas st oaccountforthe

downflows.Only additional,detailed,coordinatedob-

servationsamong thetransition-regionandcoronalin-

struments on SOHO will solve this puzzle.

Active Regions.Even though SOHO is operating through

the currentsolar minimum, ithasobserved many active

regionsfromboth the old and new solarcycles.These

observations have enhanced our understanding of active

region structure and dynamics. (See, for example, Figure

5.) Combined observations from EIT, CDS, and SUMER

clearly show that active regions consist of collections of

loops with a variety of properties in close physical prox-

imity to each other.Many of the loops contain only tran-

sition-region plasma.CDS has found that active regions

often contain very bright, compact areas seen at 2x10⁵K

and 4x10⁵K (O V and Ne VI lines),sometimesextend-

ing in temperature up to 6x10⁵K (Mg VII line),butnot

Figure 5. SOHO-CDS image of a system of loops on the E limb of

the Sun in O V 629 Å (formed at ~ 0.25 MK). Along any one loop,

and from loop to loop, there are large differences in line-of-sight

velocities. SOHO has given us the first opportunity to image such

featuresin several temperatureregimessimultaneously, while

accumulating line profile information in every image pixel.

reaching 9x10⁵K (Mg IX).

The lifetimesofthe bright

sourcesareatleast12 hours,with one observed to last

for more than five days.These intense compact sources

arelocated very close to sunspots, overlying atleastthe

sunspotpenumbra,and sometimesalso partofthe um-

bra.

Proposal forSOHO SolarMaximum Science Program-

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