SCIENCE OBJECTIVES:

Determine the structure and dynamics of the magnetic fields at the sources of the solar wind:
a. How does the magnetic field in the solar wind source regions connect to the photosphere and the
heliosphere?
b. How do the observed structures in the corona evolve into the solar wind?
c. Is the source of the solar wind steady or intermittent?

Trace the flow of the energy that heats the solar corona and accelerates the solar wind:
a. How is energy from the lower solar atmosphere transferred to and dissipated in the corona?
b. What coronal processes shape the non-equilibrium velocity distributions observed throughout the
heliosphere?
c. How do the processes in the corona affect the properties of the solar wind in the heliosphere?

Determine what mechanisms accelerate and transport energetic particles:
a. What are the roles of shocks, reconnection, waves, and turbulence in the acceleration of energetic particles?
b. What are the seed populations and physical conditions necessary for energetic particle acceleration?
c. How are energetic particles transported radially and across latitudes from the corona to the heliosphere?

Explore dusty plasma phenomena and their influence on the solar wind and energetic particle formation:
a. What is the dust environment of the inner heliosphere?
b. What is the origin and composition of dust in the inner heliosphere?
c. What is the nature of dust–plasma interactions and how does dust modify the spacecraft environment close to the Sun?
d. What are the physical and chemical properties of dust-generated

Solar wind observations collected by the Ulysses spacecraft during two separate polar orbits of the Sun, six years apart, at nearly opposite times in the solar cycle. Near solar minimum (left) activity is focused at low altitudes, high-speed solar wind prevails, and magnetic fields are dipolar. Near solar maximum (right), the solar winds are slower and more chaotic, with fluctuating magnetic fields. (Courtesy of Southwest Research Institute and the Ulysses/SWOOPS team)

(Click image to enlarge)

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.

Structure in the solar wind: Structures observed out to 30 RS by SOHO/LASCO in polar solar corona holes have densities at least four times the background. No corresponding density structures are identified in the solar wind measured by Ulysses/SWOOPS at 2 AU above the poles. Solar Probe will directly sample structure, from the wind into the corona, as it passes to a perihelion of 4 RS in its orbit (yellow line shows track of solar flyby).

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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.

Streamers are related to the slow solar wind. Images for SOHO/UVCS in O5+ (red), neutral hydrogen (blue), and SOHO/LASCO electron scattering (green) of a streamer. Contours of solar wind outflow speeds are plotted at 0, 50, and 100 km/s, increasing outward. Blobs of out-flowing material seen in LASCO movies accelerate along the sides of streamers in the locations show with arrows.

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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?