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SCIENCE OBJECTIVES: Determine the
structure and dynamics of the magnetic fields at the sources of
the solar wind: Trace the flow
of the energy that heats the solar corona and accelerates the solar
wind: Determine what
mechanisms accelerate and transport energetic particles: Explore dusty
plasma phenomena and their influence on the solar wind and energetic
particle formation:
SCIENCE BACKGROUND Present observation, theory, and modeling provide the following general picture of the corona and solar wind. At times of lower solar activity, the solar wind is bimodal, consisting of a dominant quasi-steady high-speed wind that originates in open-eld polar coronal holes and a variable, low-speed wind that originates around the equatorial streamer belt. With increasing activity, this orderly bimodal configuration of the corona and the solar wind breaks down, as the polar holes shrink and streamers appear at higher and higher heliographic latitudes. At these times, the bimodal wind structure is replaced by a complex mixture of fast flows from smaller coronal holes and transients, embedded in a slow-to-moderate speed wind from all latitudes. The energy that heats the corona and drives the wind derives from photospheric motions and is channeled, stored, and dissipated by the magnetic fields that emerge from the photosphere and structure the coronal plasma. Several fundamental plasma physical processes—waves and instabilities, magnetic reconnection, turbulence—operating on a vast range of spatial and temporal scales are believed to play a role in coronal heating and solar wind acceleration. At solar minimum the solar wind is dominated by a highspeed flow from polar coronal holes, with a slower, variable flow emanating from the equatorial streamer belt. Since the discovery of the solar wind by Mariner 2, NASA missions have taken two complementary paths toward understanding the origin of the solar wind. The first path measures the properties of the wind in situ. Sun-Solar System Connection (SSSC) missions on this path are Voyager, which has entered the heliosheath, the region beyond the termination shock; Ulysses, which is studying the high-latitude heliosphere at 2 AU; and Wind and ACE, which are measuring the solar wind near the Earth at 1 AU. The second path uses remote sensing instruments to analyze the solar corona. SEC missions on this path are SOHO, with a diverse complement of imaging and spectrographic instruments; and TRACE, with very high spatial and temporal resolution imaging of corona structure. These missions were joined in 2004 by MESSENGER, which measures the solar wind from Mercury orbit at 0.4 AU; and STEREO, which will combine in situ measurements and remote sensing from positions ahead of and behind the Earth in its orbit. Even so, the innermost heliosphere remains one of the last unexplored regions of the solar system. The closest approach ever made to the Sun, 0.31 AU (67 solar radii RS), by the Helios spacecraft in the 1970s, is more than twice the outer limit of any remote sensor. The smoothing of solar wind structures, caused by solar rotation and variations in propagation speeds, make detailed connections between the in situ and remote sensing measurements impossible. Past missions have revealed many aspects of solar wind acceleration, but compelling science questions remain unanswered. Coronal holes persist at the solar poles through much of solar cycle. During sunspot minimum, the polar holes are present and the solar wind is well organized, with the fast wind from the polar holes filling much of the heliosphere. The solar magnetic fields restructure themselves during the 11-year sunspot cycle. During sunspot maximum, the polar holes are absent; the solar wind is mixed, with fast and slow speed wind seen at all latitudes. The restructuring of the solar magnetic field, modifying the corona and wind, gives important information about the general sources of the fast and slow winds, but the physical processes that accelerate the different wind speeds are not understood.
Observations from SOHO's Ultraviolet Coronagraph Spectrometer (UVCS) imply that the wind acceleration operates over the range of 2 to 10 RS in coronal holes. The wind flow appears anti correlated with structures in the coronal holes, as shown in the image below. Low-density regions have higher temperatures, non thermal, probably wave motions, and are possibly the source of the fast solar wind. How do the structures of the solar corona, so clearly correlated with the flow of the solar wind, evolve to the smoother wind observed at a distance? The generation of the slow-speed wind within the streamers is a perplexing problem. SOHO/UVCS observations detected the wind outflow along the outer edges and from the tops of streamers.
At the same locations, discrete blobs of material are seen moving outward in images from SOHO/ LASCO images, but masses are too small to match the solar wind flux. Why does the wind from the streamers not accelerate to the faster speeds seen from coronal holes? Is the boundary between the closed and open magnetic fields at the edges of streamers unstable to wave motions or discrete disconnections that drive the blobs and remove energy from the wind flow?
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