Solar Physics at Evergreen

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Solar Physics at Evergreen
Dr. E.J. Zita (zita_at_evergreen.edu) The Evergreen
State College Southwest Washington Astronomical
Society 12 Jan. 2005, SPSCC This work was
supported by NASA's  Sun-Earth Connection Guest
Investigator Program, NRA 00-OSS-01 SEC
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We have recently established a solar physics
research program at The Evergreen State College.
Famed for its cloudy skies, the Pacific Northwest
is an ideal location for solar physics research
activities that do not require extensive local
observations. Colleagues from the High Altitude
Observatory (HAO) at the National Center for
Atmospheric Research (NCAR) have shared solar
data from satellite-borne instruments such as
TRACE and SUMER. Colleagues from HAO and the
Institute for Theoretical Astrophysics (ITA) at
the University of Oslo have also shared data from
computer simulations of magnetohydrodynamics
(MHD) in the Sun.
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Evergreen students and faculty have learned to
analyze data from satellites and simulations, at
Boulder and Oslo, and established an
infrastructure for performing these analyses at
Evergreen. Partners in crime E.J. Zita (The
Evergreen State College, Olympia WA
98505) Evergreen alumni Noah Heller, Matt
Johnson (Physics Dept., University of California,
Santa Cruz CA 95064), Sara Petty (Center for
Solar Physics and Space Weather, Catholic
University, Washington DC 20064), Chris Dove
(UW-Seattle), Night Song (Evergreen) HAO-NCAR
colleagues Chromosphere Tom Bogdan and Phil
Judge Convection zone Mausumi Dikpati and
Eric McDonald Mats Carlsson and team (University
of Oslo, Institute for Theoretical Astrophysics,
Blindern N-0315 Norway)
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Interesting regions in the Sun
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Questions about the Sun

• In the convection zone (CZ)
• Why does the Suns magnetic field flip every 11
  years?
• How do the physical properties of the Suns
  plasma affect the evolution of the Suns magnetic
  flux?
• In the chromosphere
• Why is the Suns upper atmosphere millions of
  degrees hotter than its surface?
• How do magnetic waves carry energy from
  photospheric sound waves up into the chromosphere?

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Suns magnetic field flips

• O-effect Differential rotation creates toroidal
  field from poloidal field
• a-effect Helical turbulence twists rising flux
  tubes, which can tear, reconnect, and create
  reversed poloidal field
• Meridional circulation surface flow carries
  reverse poloidal field poleward equatorward
  flow near tachocline is inferred

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Dikpatis code models evolution of solar magnetic
field in CZ
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How does magnetic diffusivity affect solar field
evolution?
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Preliminary conclusions about Suns magnetic
field evolution

• Diffusivitysurface
• If h is too low at the surface, the field becomes
  concentrated there particularly at the poles
• If h is high the field diffuses too much
• Diffusivitytachocline
• If h is low near the base of the convection zone,
  then the field is frozen near the equator and
  tachocline
• Shape
• Linear h(r) can handle the greatest range of
  diffusivity
• Gradients in h(r) cause flux concentration

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Going up
Photospheric acoustic waves drive magnetic waves
in the Suns atmosphere
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Magnetic waves may heat the solar atmosphere
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Magnetic outbursts affect Earth

• Solar Max 2002
• More magnetic sunspots
• Strong, twisted B fields
• Magnetic tearing releases energy and radiation ?
• Cell phone disruption
• Bright, widespread aurorae
• Solar flares, prominences, and coronal mass
  ejections
• Global warming?
• next solar max around 2011

CME movie
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Methods Simulations

• Nordlund et al. 3D MHD code models effects of
  surface acoustic waves near magnetic network
  regions.
• Students wrote programs to analyze supercomputer
  data from ITAP ?HAO.
• Calculated energy fluxes out of each region.

Pressure (p-)mode oscillates in left half of
network region at photosphere. Waves travel up
into chromosphere.
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Results Simulations
Magnetic energy fluxes grow MS and Alfvén out of
phase.
Pressure-mode energy flux decreases with height.
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Conclusions Simulations

• Parallel acoustic waves are channeled along field
  lines
• Oblique component of acoustic waves can excite
  magnetic waves
• Strong mode mixing near b1 regions
• Magnetosonic and Alfvénic waves can carry energy
  to high altitudes

Matt Johnson, Sara Petty-Powell, E.J. Zita, 2001,
Energy Transport by MHD waves above the
photosphere
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Methods Observations
UV oscillates in space (brightest in magnetic
network regions) and in time (milliHertz
frequencies characteristic of photospheric
p-modes).
SOHO telescope includes SUMER, which measures
solar UV light
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Results Observations

• Fourier analyze UV oscillations in each
  wavelength
• Shorter-wavelength UV at higher altitudes, where
  chromosphere is hotter
• P-mode oscillations weaken with height

Noah S. Heller, E.J. Zita, 2002, Chromospheric UV
oscillations depend on altitude and local
magnetic field
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Conclusions Observations

• Magnetic waves carry energy to higher altitudes
  while p-modes weaken.
• Lower frequency oscillations stronger in magnetic
  regions.
• Higher frequency oscillations stronger in
  internetwork regions magnetic shadowing?

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Methods Theory

• Model sheared field region with a force-free
  magnetic field
• Bx0, By B0 sech(ax), Bz B0
  tanh(ax)
• Write the wave equation in sheared coordinates.
• Solve the wave equation for plasma
  displacements.
• Find wave characteristics in the sheared field
  region.

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The wave equation describes how forces displace
plasma.
Results Theory
w frequency, ? displacement, cs sound
speed, vA Alfvén speed B total magnetic
field, B0 mean field, b1 magnetic oscillation






Waves transform as they move through a sheared
magnetic field region.

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Conclusions Theory
Waves oscillate along x when kx real (p0 gt 0
and p2 gt 0), for frequencies ?2 gt ?22 and ?2 gt
?02 (high frequencies). Waves damp along x when
kx imaginary LF case (p0 lt 0 and p2 gt 0)
?2 lt ?02 MF case (p0 gt 0 and p2 lt 0)
?02lt ?2 lt?22

• Magnetic energy travels along or across magnetic
  field lines.
• Twisting or shearing increases magnetic energy
• Shearing ? mode transformation
• Twisting ? tearing ? release of magnetic energy.

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Summary

• Flows in the convection zone change and twist
  the Suns magnetic field
• Something carries energy from the solar surface
  to heat the solar atmosphere,
• but photospheric acoustic modes weaken with
  altitude.
• Acoustic waves become magnetohydrodynamic
  waves, especially where speeds are comparable
• MHD waves carry energy from the photosphere up
  into the chromosphere.
• Magnetic waves may heat the chromosphere by
  tearing, reconnection, and Joule heating
• Magnetic dynamics are important on the Sun and
  affect weather and communications on Earth.

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Acknowledgements
We thank the High Altitude Observatory (HAO) at
the National Center for Atmospheric Research
(NCAR) for hosting our summer visits computing
staff at Evergreen for setting up Linux boxes
with IDL in the Computer Applications Lab and
Physics homeroom and NASA and NSF for funding
this research.
The National Center for Atmospheric Research is
sponsored by the National Science Foundation.
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References

• Song, N., Zita, E.J., McDonald, E., Dikpati, M.,
  Influence of depth-dependent magnetic
  diffusivity on poloidal field evolution in the
  Sun, 2005, Proceedings of the Astronomical
  Society of the Pacific
• Bogdan, T.J., Carlsson, M, Hansteen, V.,
  McMurray, A, Rosenthal, C.S., Johnson, M.,
  Petty-Powell, S., Zita, E.J., Stein, R.F.,
  McIntosh, S.W., Nordlund, Å. 2003, Waves in the
  magnetized solar atmosphere II, ApJ 597
• Bogdan, T.J., Rosenthal, C.S., Carlsson, M,
  Hansteen, V., McMurray, A, Zita, E.J., Johnson,
  M. Petty-Powell, S., McIntosh, S.W., Nordlund,
  Å., Stein, R.F., and Dorch, S.B.F. 2002, Waves
  in magnetic flux concentrations The critical
  role of mode mixing and interference, Astron.
  Nachr. 323, 196
• Canfield, R.C., Hudson, H.S., McKenzie, D.E.
  1999, Sigmoidal morphology and eruptive solar
  activity, Geophysical Research Letters, 26, 627
• Noah Heller, E.J. Zita, 2002, Chromospheric UV
  oscillations frequency spectra in network and
  internetwork regions
• Matt Johnson, Sara Petty-Powell, E.J. Zita,
  2001, Energy Transport by MHD waves above the
  photosphere
• B.C. Low, 1988, Astrophysical Journal 330, 992
• Zita, E.J. 2002, Magnetic waves in sheared
  field regions
• HAO High Altitude Observatory
  http//www.hao.ucar.edu
• NCAR National Center for Atmospheric Research
  http//www.ncar.ucar.edu/ncar/
• Montana St. Univ., http//solar.physics.montana.ed
  u/canfield/
• SOHO Solar Heliospheric Observatory
  http//sohowww.nascom.nasa.gov/
• SUMER Solar Ultraviolet Measurements of Emitted
  Radiation http//www.linmpi.mpg.de/english/projek
  te/sumer/
• Papers online http//academic.evergreen.edu/z/zi
  ta/research.htm (zita_at_evergreen.edu)