Solar Physics at Evergreen

1. Solar Physics at Evergreen Dr. E.J. Zita(zita@evergreen.edu) The Evergreen State College American Physical Society – NW section Spokane-Pullman, 21 May 2004 This work supported by NASA's  Sun-Earth Connection Guest Investigator Program, NRA 00-OSS-01 SEC

2. ABSTRACT 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. Collaborators 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. HAO colleagues have also shared data from computer simulations of magnetohydrodynamics (MHD) in the chromosphere, generated with the Institute for Theoretical Astrophysics (ITA) at the University of Oslo. 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. E.J. Zita (The Evergreen State College, Olympia WA 98505), T.J. Bogdan (HAO, NCAR, Boulder, CO 80307-3000), M. Carlsson (University of Oslo, Institute for Theoretical Astrophysics, Blindern N-0315 Norway), P. Judge (HAO, NCAR, Boulder, CO 80307-3000), and Evergreen alummi N. Heller, M. Johnson (Physics Dept., University of California, Santa Cruz CA 95064), and S. Petty (Center for Solar Physics and Space Weather, Catholic University, Washington DC 20064) We are investigating the role of magnetic waves in heating the solar atmosphere. Comparing data from satellites and simulations shows that acoustic oscillations from the photosphere cannot effectively propagate into the chromosphere, but that magnetic waves may carry energy up toward the hot, thin corona. We find that acoustic waves can transform into magnetic waves, especially near the magnetic "canopy", a region where the sound speed is comparable to the Alfvén speed. Understanding MHD wave transformations and their role in energy transport can help answer outstanding questions about the anomalous heating of the solar atmosphere.

3. … ABSTRACT We are investigating the role of magnetic waves in heating the solar atmosphere. Comparing data from satellites and simulations shows that acoustic oscillations from the photosphere cannot effectively propagate into the chromosphere, but that magnetic waves may carry energy up toward the hot, thin corona. We find that acoustic waves can transform into magnetic waves, especially near the magnetic "canopy", a region where the sound speed is comparable to the Alfvén speed. Understanding MHD wave transformations and their role in energy transport can help answer outstanding questions about the anomalous heating of the solar atmosphere. E.J. Zita (The Evergreen State College, Olympia WA 98505), T.J. Bogdan (HAO, NCAR, Boulder, CO 80307-3000), M. Carlsson (University of Oslo, Institute for Theoretical Astrophysics, Blindern N-0315 Norway), P. Judge (HAO, NCAR, Boulder, CO 80307-3000), and Evergreen alumni N. Heller, M. Johnson (Physics Dept., University of California, Santa Cruz CA 95064), and S. Petty (Center for Solar Physics and Space Weather, Catholic University, Washington DC 20064)

4. Magnetic dynamics may heat the solar atmosphere

5. Magnetic outbursts affect Earth Recent Solar Max: • 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

6. 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.

7. Results: Simulations Magnetic energy fluxes grow; MS and Alfvén out of phase. Pressure-mode energy flux decreases with height.

8. Conclusions: Simulations • Parallel acoustic waves are channeled along field lines • Oblique component of acoustic waves can excite magnetic waves • Strong mode mixing near b=1 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

9. 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

10. h T l 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

11. 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?

12. Observations Schematic Mathematical model x Methods: Theory • Model sheared field region with a force-free magnetic field: • Bx=0, 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.

13. Results: Theory k ||  k || B B B v v Alfvén waves Magnetosonic waves The wave equation describes how forces displace plasma. 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.

14. Critical frequencies: p2 = 0 when 2 = and p0 = 0 when 2 = Conclusions: Theory • Magnetic energy travels along or across magnetic field lines. • Twisting or shearing increases magnetic energy • Shearing  mode transformation • Twisting  tearing  release of magnetic energy. • Waves oscillate along x when kx = real (p0 > 0 and p2 > 0), for frequencies 2 > 22 and 2 > 02 (high frequencies). • Waves damp along x when kx = imaginary: • LF case: (p0 < 0 and p2 > 0) 2 < 02 • MF case: (p0 > 0 and p2 < 0) 02< 2 <22

15. Summary • Something carries energy from the solar surface to heat the solar atmosphere, … • … but photospheric pressure modes weaken with altitude. • p-modes transform into magnetohydrodynamic modes, especially where b~1 or vA ~ cs … • … then Alfvénic, magnetosonic, and hybrid 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.

16. Acknowledgements We thank the High Altitude Observatory (HAO) at the National Center for Atmospheric Research (NCAR) for hosting our summer visits; BC Low for suggesting the form of the sheared field; and computing staff at Evergreen for setting up Linux boxes with IDL in the Computer Applications Lab.

17. References • 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 • 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: waves from localized sources in magnetic flux concentrations”, ApJ 597 • 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” • * papers: http://academic.evergreen.edu/z/zita/research.htm (zita@evergreen.edu) • 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.edu/canfield/ • SOHO = Solar Heliospheric Observatory: http://sohowww.nascom.nasa.gov/ • SUMER = Solar Ultraviolet Measurements of Emitted Radiation: http://www.linmpi.mpg.de/english/projekte/sumer/