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
b. How do the observed structures in the corona evolve into the
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
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
c. How do the processes in the corona affect the properties of the
solar wind in the heliosphere?
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?
plasma phenomena and their influence on the solar wind and energetic
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 dustplasma interactions and how does
dust modify the spacecraft environment close to the Sun?
d. What are the physical and chemical properties of dust-generated
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)
image to enlarge)
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 processeswaves
and instabilities, magnetic reconnection, turbulenceoperating
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
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
which is studying the high-latitude heliosphere at 2 AU; and Wind
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;
with very high spatial and temporal resolution imaging of corona
These missions were joined
in 2004 by MESSENGER,
which measures the solar wind from Mercury orbit at 0.4 AU; and
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
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.
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).
image to enlarge)
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.
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.
image to enlarge)
At the same locations,
discrete blobs of material are seen moving outward in images from
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?