SOHO, STEREO and Hinode Observations of Polar Coronal Holes at Solar Minimum

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SOHO, STEREO and Hinode Observations of Polar
Coronal Holes at Solar Minimum

• Barbara Thompson, NASA GSFC
• David Alexander, Rice University
• Jonathan Cirtain, NASA MSFC
• Craig DeForest, SWRI
• Sarah Gibson, UCAR
• Donald Hassler, SWRI
• Michael Kaiser, NASA GSFC
• Antonia Savcheva, SAO

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What do solar polar observations accomplish?

• Solar polar observations can lead to significant
  advances in our understanding of the Sun-Earth
  system.
• As yet, the polar regions of the Sun remain a
  largely untapped resource in our attempts to
  understand the Sun and its influence on the near
  Earth environment.

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What do solar polar observations accomplish?

• 3D Solar Wind and Heliospheric Structure 360o
  view of streamer belt, global structure of
  corona, fast/slow wind interactions
• Coronal Hole Structure and Phenomena polar
  coronal holes, unipolar regions, jets, polar
  plumes
• Solar Activity formation and evolution of
  solar structures, solar irradiance
• Suns Influence on Earth angular momentum
  loss, cosmic-ray modulation
• Helioseismology solar dynamo, polar fields,
  meridional circulation, field reversal

FUTURE PLANS
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3D Heliospheric Structure

• The 3D structure of the corona and inner
  heliosphere is tremendously complex, even at
  solar minimum.
• The interactions of fast/slow solar wind, the
  expansion into the heliosphere, and rotational
  effects are most directly studied with a solar
  minimum corona.
• Additionally, the Sun is not as likely to change
  for an entire solar rotation during solar
  minimum, and CMEs are not as likely to disturb
  the structure.

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• mccomas et al 2003

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Whole Heliosphere Interval
Originating from Carrington Rotation 2068 March
20 - April 16, 2008

• What is WHI?
• Internationally coordinated observing and
  modeling effort

• What are WHIs science goals?
• Characterize the 3-D solar minimum heliosphere
• Trace the effects of solar structure and
  activity through the solar wind to the Earth and
  other planetary systems

• How does WHI work?
• Will involve ground- and space-based
  observations
• From the solar interior, through geospace, to
  the heliopause
• Over 200 international scientists already
  participating

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How to get involved? http//ihy2007.org/WHI
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STEREO 3D Solar wind flows

• Movies quickly demonstrate that the flow of solar
  wind structures can be traced out to 1 AU.
  Tomographic techniques give us new insight into
  the azimuthal topology.

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Coronal Hole Structure and Phenomena Polar Plumes

• A major highlight of the SOHO mission was the
  ability to view polar plumes, using multiple
  imagers and spectrometers, out to 30 solar radii.
  
• The structure of these high-contrast features
  allowed us to trace the magnetic field emanating
  from poles, and variability in the emission has
  served as a diagnostic of wave activity.

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SOHO plume maps out to 30 solar radii allow us to
trace the super-radial expansion and thus the
fast solar wind. Tracking variations in plume
structure have revealed quasi-periodic
oscillations, traveling 75 - 150 km/sec, with
periods of 10-15 minutes (indicative of slow-mode
MHD waves).
Deforest et al., 1997, 2000
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Coronal Hole Structure and Phenomena Polar Jets

• Polar jets are small-scale eruptive events that
  provide insight into magnetic evolution and
  reconnection, Alfvénic properties, and mass
  loading of the solar wind.
• They are relatively small phenomena that rapidly
  evolve in 3 dimensions. Prior to STEREO and
  Hinode observations, the resolution in time and
  space were, for the most part, insufficient to
  explore the underlying physics.

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Hinodes high-resolution spectral observations
have allowed new insights into polar jets.
Observations of nearly 10 jets/hour bring us
closer to understanding the role of small-scale
structure and magnetic reconnection in solar
coronal energetics. Jet expansion speeds ( 35
km/sec) indicate the rate of pressure-driven
expansion and serve as a diagnostic of the
pressure of the fast solar wind. Alfvénic
fluctuations have been observed at both at slow-
and fast-mode speeds, enabling us to further
sample MHD parameters on the pole, and providing
insights into astrophysical jets.
Hinode XRT observations (Cirtain et al, 2007
Savcheva et al, 2007)
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A twisted mess Combining Hinodes observations
with STEREOs 3D imaging capability, we obtain a
more clear view of a jets motion, including
rapid rotation about the magnetic axis.
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What can direct polar observations bring us?

• There are several major limitations to our
  observations of the poles thus far
• We only see a small fraction of the pole at a
  given time.
• We view the poles from a high inclination angle
  - nearly perpendicular. Our view of the
  footpoints and the magnetic field is severely
  restricted.
• Complementary in situ observations are extremely
  valuable but have been scarce.

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Solar Activity as viewed from the poles will
enable the following

• Long-term evolution of the Suns magnetic field
  in particular, meridional flows field reversal
• A greater understanding of the poloidal component
  of solar field, believed responsible for much of
  the interplanetary magnetic field
• Formation, evolution and demise of solar
  structures (e.g. active regions, helmet
  streamers)
• Global dynamics and rotation of the Sun
• True solar radiance establishing a 3D
  determination of the energetic input to the
  Earths atmosphere and ionosphere

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CMEs and Space Weather

• New observations from out of the ecliptic plane
  provided by Solar Orbiter will yield unique
  information about the coronal effects of CMEs and
  solar flares in addition to providing continuous
  coverage of Earth-directed disturbances. A
  mission to the poles will enable unprecedented
  exploration of the
• initiation and evolution of Earth-directed CMEs
• global extent of CMEs and propagation of CME
  disturbance around Sun
• acceleration and transport of solar energetic
  particles

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Helioseismology the origin of the solar cycle
and the Suns magnetic field

• Continuous coverage of the polar regions will
  provide otherwise unattainable information about
  the large-scale solar dynamo. Observations above
  the poles will enable unprecedented exploration
  of the
• internal structure of the Sun
• pure small-scale fields with no AR
  contamination
• polar magnetic field and field dynamics
• polar convective patterns
• interaction of convection, rotation and magnetic
  field
• solar rotation at the poles
• polar vortex and circumpolar jet streams
• reversal of the large-scale magnetic field

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Conclusions

• SOHO, STEREO and Hinode data possess three chief
  improvements over previous polar observations
  extended fields of view, improved resolution
  (temporal, spatial, spectral), and multiple
  vantage points.
• They have provided several new and exciting
  insights into the poles of the Sun. The 3D
  structure of the heliosphere, and the flow of
  both fast and slow solar wind, are become more
  and more clear.
• Coronal hole phenomena, such as jets and polar
  plumes, also providing insight into the solar
  structure in which they are embedded.
• The new frontier observations from above the
  poles will enable a rich variety of solar and
  Sun-Earth system studies unavailable from the
  ecliptic plane. It will significantly advance
  our knowledge of key solar phenomena, our
  understanding of the Sun as a star and our
  forecasting abilities for the Suns interaction
  with the Earth.