Solar Physics at Evergreen Dr. E.J. Zita 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

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.

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)

Interesting regions in the Sun:

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

Sun’s magnetic field flips Ω-effect: Differential rotation creates toroidal field from poloidal field  -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

Dikpati’s code models evolution of solar magnetic field in CZ tachocline photosphere

How does magnetic diffusivity affect solar field evolution? 10^12 10^ r/R 1.0

Preliminary conclusions about Sun’s magnetic field evolution Diffusivity surface : If  is too low at the surface, the field becomes concentrated there – particularly at the poles If  is high the field diffuses too much Diffusivity tachocline : If  is low near the base of the convection zone, then the field is frozen near the equator and tachocline Shape: Linear  r) can handle the greatest range of diffusivity Gradients in  r) cause flux concentration

Going up… Photospheric acoustic waves drive magnetic waves in the Sun’s atmosphere

Magnetic waves may heat the solar atmosphere

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

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.

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

Conclusions: Simulations Parallel acoustic waves are channeled along field lines Oblique component of acoustic waves can excite magnetic waves Strong mode mixing near  =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

Methods: Observations SOHO telescope includes SUMER, which measures solar UV light UV oscillates in space (brightest in magnetic network regions) and in time (milliHertz frequencies characteristic of photospheric p-modes).

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 T h

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?

Methods: Theory ObservationsSchematicMathematical model x Model sheared field region with a force-free magnetic field: B x =0, B y = B 0 sech(ax), B z = B 0 tanh(ax) Write the wave equation in sheared coordinates. Solve the wave equation for plasma displacements. Find wave characteristics in the sheared field region.

The wave equation describes how forces displace plasma.  = frequency,  = displacement, c s = sound speed, v A = Alfvén speed B = total magnetic field, B 0 = mean field, b 1 = magnetic oscillation Alfvén waves Magnetosonic waves B v B v k || B k ||  Waves transform as they move through a sheared magnetic field region. Results: Theory

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. Critical frequencies: p 2 = 0 when and p 0 = 0 when Waves oscillate along x when k x = real (p 0 > 0 and p 2 > 0), for frequencies  2 >  2 2 and  2 >  0 2 (high frequencies). Waves damp along x when k x = imaginary: LF case: (p 0 0)  2 <  0 2 MF case: (p 0 > 0 and p 2 < 0)  0 2 <  2 <  2 2

Summary Flows in the convection zone change and twist the Sun’s 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.

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.

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: NCAR= National Center for Atmospheric Research: Montana St. Univ., SOHO = Solar Heliospheric Observatory: SUMER = Solar Ultraviolet Measurements of Emitted Radiation: Papers online: