Zita's research: stellar magnetohydrodynamics

• Why does the Sun's magnetic field flip every 11 years? (Solar Dynamo at NASA and HAO)
• How does this contribute to the magnetic storms Earth experiences when the Sun's field reverses?
• How do dynamic interactions between material properties of the Sun and its magnetic field change each other and affect the solar dynamo?
• Can we predict short term details and long term patterns of the Sun's magnetic reversals?

Click on links below to get to our papers and presentations.

How to work remotely at HOA - 2007 update

2005: Jada Maxwell studied solar magnetic waves sources for auroral infrasound, and Zita continued solar dynamo work with Mausumi and Eric.
(photo by Mark Urwiller)

Summer 2004: Night Song worked on solar dynamo simulations, with Dr. Mausumi Dikpati and Eric McDonald, and
Chris Dove analyzed simulations of magnetic waves in the Sun's atmosphere, with Dr. Tom Bogdan.

Summer 2002: Noah Heller analyzed data from a satellite observing the Sun, with Dr. Phil Judge

Plasma is hot ionized gas.  Heat a solid, and it melts to liquid.  Heat a liquid, and it vaporizes to gas.  Heat a gas, and it ionizes to plasma.  While plasma is blood to biologists, it is something completely different but just as vital to physicists.  Plasma, a neutral fluid of charged particles, comprises most of the visible universe.   The Sun and every shining star is made of plasma.  Galaxies are made of stars, and the spaces between stars are filled with thin hot gas.  The glowing red aurora in the photo above is from plasma:  charged particles stream from the Sun, become trapped in Earth's magnetic field, and excite gases in our upper atmosphere.  We are interested in how plasmas interact with - and create - magnetic fields. 

Plasma physicists investigate questions such as: 
* Fusion:  How can magnetic trapping help researchers build a sun in the lab, for safer, cleaner energy? 
* Magnetic stars:  Can magnetic dynamics help explain observed fluctuations in hot stars?
* Solar magnetism:  Why does the Sun's magnetic field flip every 11 years?
* Solar weather:  Why is the Sun's atmosphere millions of degrees hotter than the solar surface? How do magnetic waves help heat the Sun's atmosphere?
* Space weather: How does solar radiation change, and how does it affect Earth?

All these investigations involve magnetohydrodynamics (MHD), the study of motions of magnetized fluids, especially plasmas.  Mathematically, magnetohydrodynamics can be described with Maxwell's equations (electromagnetic theory) and conservation laws (for mass, momentum, and energy).  This coupled set of nonlinear differential equations can be solved exactly only in special cases (analytical MHD theory).  In more complex, realistic cases, these equations can be solved approximately by supercomputers (MHD computation).  We aim to compare analytical and computational results with physical data.  Where they agree, we can learn new insights about how magnetic plasmas work.

Zita, E.J. & Dikpati, M., ”Sensitivity of a Babcock-Leighton Flux-Transport Dynamo to Magnetic Diffusivity Profiles,” Solar Physics, submitted August 2007

Zita, E.J., *Song, N., McDonald, E., Dikpati, M., Influence of depth-dependent diffusivity profiles in governing the evolution of weak, large-scale magnetic fields of the Sun, in Large Scale Structures and their Role in Solar Activity, eds: K. Sankarasubramanian, M. Penn and A. Pevtsov, 18-22 October 2004 @ National Solar Observatory/Sacramento Peak Observatory, Sunspot, New Mexico, USA, ASP Conference Series (2005)

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., Waves in the magnetized solar atmosphere II: Waves from localized sources in magnetic flux concentrations. Bogdan et al., ApJ 599, 626-660 (2003)

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., “Waves in magnetic flux concentrations: The critical role of mode mixing and interference,” Astronomische Nachrichten 323, 196-202 (2002)

Zita, E.J., A Turbulent Dynamo for Stars Lacking Convection, in Stellar Dynamos: Nonlinearity and Chaotic Flows, ASP Conference Series, Vol. 178, eds. M. Nuñez & A. Ferriz-Mas, (San Francisco: Astron. Soc. of the Pacific), 203 (1999) (Abstract)

Do magnetic waves heat the solar atmosphere? APS poster, Portland, May 2003

Astronomy at Evergreen, a talk given at South Puget Sound Community College, 2003

Magnetic waves in sheared field regions, Banff poster, E.J. Zita, 2002

2002: Magnetic wave transformations in sheared field regions:  analytical calculations, presented at the ITP Program on Solar Magnetism and Related Astrophysics at UCSB in January 2002.  (Work in progress - last updated 4.Feb.02) 

2001:  Sun-Earth connections, a talk given at Evergreen

2001: Magnetic effects in peculiar, pulsating stars, in Magnetic fields across the HR diagram, in Magnetic Fields Across the Hertzsprung-Russell Diagram, ASP Conference Proceedings Vol. 248, G. Mathys, S. K. Solanki, and D. T. Wickramasinghe (eds.), (San Francisco: Astronomical Society of the Pacific), p.345

2001:  Ringing magnetic stars, a talk given at Kamloops, BC

2000, Magnetic oscillations in radiative stars, in The Impact of Large Scale Surveys on Pulsating Star Research,
IAU Colloquium No. 176, ASP Conference Series, Vol. 203, L. Szabados & D. Kurtz (eds.), (San Francisco: Astronomical Society of the Pacific), pp. 386-387

summer 1998 abstract for Chapman conference on magnetic helicity: Turbulent stellar dynamo for stars without convection

1997, A magnetic model for acoustic modes in roAp stars, in A Half Century of Stellar Pulsation Interpretations, ASP Conference Series, Vol. 135, P.A. Bradley & J.A. Guzik (eds.), (San Francisco: Astronomical Society of the Pacific), pp. 414-422 (pdf format)

1992, MHD computation of feedback of resistive-shell instabilities in the Reversed Field Pinch, Zita, E.J., Prager, S.C., Ho, Y.L., Schnack, D.D., Nuclear Fusion 32, 1941

1992, Turbulent transport in the Madison Symmetric Torus reversed-field pinch, Rempel, T. D.; Almagri, A. F.; Assadi, S.; et al, Phys. Fluids B 4, 7, 2136-2141

• Derive frequencies for plasma oscillations and synchrotron radiation (ref: Chen Ch.4)
• Derive the frozen-in-field condition for conducting plasmas
• Derive the Alfven speed
• Magnetic (resistive) diffusion timescale
• Derive the magnetic reconnection timescale (ref: Tandberg-Hanssen and Emslie Ch.7)
• Scale height
• Dispersion relation for solar p-modes

Workshops for Astronomy and Sailpower

Jada Maxwell (TESC), Jiong Qiu and Richard Canfield (MSU), Analyzing Flare Ribbons to Determine Magnetic Reconnection Flux and its Relationship to Flux Rope Formation, American Physical Society NW, Pocatello ID, May 2007

Jada Maxwell, Can Magnetic Waves in the Auroral Region Transform into Acoustic Waves? American Physical Society NW, PLU, Tacoma WA, May 2006

Night Song, E.J. Zita, Mausumi Dikpati, Eric McDonald, Influence of depth-dependent diffusivity profiles in governing the evolution of weak, large-scale magnetic fields of the Sun, HAO-NCAR, Boulder, July 2004

Chris Dove, E.J. Zita, Tom Bogdan, Dispersion diagrams of waves in the simulated solar chromosphere, (paper) 2004

Chris Dove, Tom Bogdan, E.J. Zita, Dispersion diagrams of chromospheric MHD waves in a 2D simulation, HAO-NCAR, Boulder, July 2004 (presentation)

Research paper and Banff poster, Chromospheric UV oscillations, by Noah S. Heller and E.J.Zita, 2002

An observational analysis of the solar chromospheric intensity, 2003, master's thesis by Niklas Karlsen, student of Mats Carlsson at the University of Oslo

Solar MHD: research paper and webpage, by Matt Johnson and Sara Petty-Powell, 2001

IDL tutorials, by Matt Johnson, 2002

Oscillators, by Jonathan Gibson, 2000-2001

Observing the Sun at 20.1 MHz:  Radio Jove Receiver and Antenna, by Sara Petty-Powell, 2000

Introduction to solar physics, by Caylin Mendelowitz and Claire Rosen, 2000

A proposal for the Evergreen State College Observatory, by Mike Covello, 2000

Observing with the Manastash Ridge Observatory, by Sara Petty-Powell and Andrea Nowicki, 1999

Christopher Wolfe's visualizations of field dynamics in roAp stars, 1998

Learning about magnetosonic waves in roAp stars, by Tomoko Adachi with Karen O'Meara, 1998

Undergraduate posters at APS NW in Vancouver, BC, May 1999

Undergraduate posters at APS NW in Portland, OR, May 2003 (plus those in photo)

Posters at APS NW in Pullman, WA, May 2004 (Joey won first prize!)

Posters at APS NW in Victoria, BC, May 2005; Jada Maxwell on Auroral infrasound
 

Acknowledgements

News, March 2001: Dr. E.J. Zita of The Evergreen State College and Dr. Thomas Bogdan of the High Altitude Observatory (HAO) at the National Center for Atmospheric Research in Boulder have been awarded a 3-year research grant by NASA.  This $225,5000 grant will support their research into the role of magnetic fields in heating the atmosphere of the Sun.  Zita and Bogdan began their work together in Spring 2000, when Zita's expiring National Science Foundation (NSF) grant funded one quarter of unpaid leave from Evergreen.  Their continuing analytic work on the effect of magnetic shear on plasma dynamics will be combined with numerical calculations on heating processes in the Sun, and compared with extensive datasets from solar observatories and satellites.  The NASA grant provides for Evergreen students to contribute to new computations and data analysis, on campus and in visits to HAO, in collaboration with colleagues from Boulder, Oslo, and London.  This new work builds on Zita's past efforts to expand opportunities for Evergreen students interested in astronomy and astrophysics, including access to the UW's Manastash Ridge Observatory and construction of a modest on-campus observatory for the College's Meade LX-200 telescope, funded by the NSF.  Zita continues to welcome new students to her research team, many of whom start by taking her spring Astronomy and Cosmologies program.

Summer 2001:  Matt Johnson and Sara Petty-Powell spent July in Boulder working with Zita and Bogdan, and August at Evergreen continuing their work.  They analyzed data from Rosenthal and Nordlund's 3D MHD code, calculated energy flux near a vertically oscillating sunspot, and found that energy oscillates between magnetic compression and magnetic bending as MHD waves rise from the photosphere.  As photospheric sound waves rise, they transform into shocks and Alfvenic and magnetosonic waves, which may contribute to heating.  Zita got the sheared-field wave equations into a tractable form by transforming everying to the sheared frame. We all presented our work at the Institute for Theoretical Physics in Santa Barbara in Jan 2001. 

Summer 2002: Noah Heller and Zita worked with Phil Judge at HOA and Evergreen. We analyzed chromospheric UV oscillation data from SUMER and found that sound waves (2-5 mHz) do lose power as they rise above the photosphere, as inferred from our simulation analyses last year. Lower frequency oscillations (0-2 mHz) are stronger in magnetic (network) regions of the chromosphere, and higher frequency oscillations (5-10 mHz) are stronger in internetwork regions. Zita found solutions to a special case of the wave equations in a sheared magnetic field. We presented these results at the SHINE meeting in Banff in Aug.2002.

2001:  Our one-page plan, internal report, and first-year report to NASA. 2002 NASA report. 2003 NASA report

Astronomers can tell stars are ringing when their brightness varies in characteristic ways.  We know why the Sun rings:  convection cells make the photosphere oscillate.  We know why delta-Scuti stars ring:  opaque regions of the star block the outflow of radiation, exciting convection.  But we don't know why rapidly oscillating peculiar A (roAp) stars ring.   The usual explanations (convection or opacity mechanisms) don't seem to work in roAp stars.  Astronomers are pretty sure that roAp sound waves are somehow connected with the stars' strong magnetic fields, because they have the same spatial symmetry, and because Ap stars don't ring unless they have a strong magnetic field.  But how, exactly, do the stars' magnetic fields affect their sound waves?

Zita has an idea about how the magnetic fields of roAp stars could cause their sound waves.  The twisted magnetic fields have an equilibrium shape, or minimum energy configuration.  At rest, the magnetic field would maintain its shape, rather like a slinky toy distorted into a spherical shape.  If you stretch and release a slinky toy, it oscillates near its minimum energy state.  Similarly, the magnetic fields in roAp stars can twist and untwist near their equilibrium shape.  These  magnetic oscillations induce new electromagnetic fields, and the fields interact to drive magnetoacoustic modes, or magnetic sound waves, in the edge of the star.  The oscillating surface of the star makes it brighten and dim in a periodic way.

Does this idea work?  It looks good, but we'd like to be sure.  Initial calculations of Zita's magnetoacoustic modes are consistent with the sound waves observed in roAp stars.  They oscillate with periods of several  minutes, or frequencies of a few milliseconds.  There is more than enough magnetic energy available to power the motion of the stellar plasma.  And the magnetic wavelengths are consistent with the observed size of the sound waves.  Zita's  model can be tested by comparing observations with her predictions.  How do the sound waves change in bigger stars, or more magnetic stars?  The model predicts that bigger (or more strongly magnetic) stars will pulse slightly more slowly.  The first Canadian space telescope, the MOST, will be able to test these predictions in 2002.

Students generated predictions from Zita's model so it can be compared to observations of individual stars.  Tomoko Adachi and Karen O'Meara computed hydrostatic equilibria of stars (using Steve Kawaler's easy-to-run ZAMSprogram in the CAL), then calculated the frequency of magnetosonic modes in the stars using Excel.   Karen also used Mathematica to help Zita solve wave equations for how magnetosonic frequencies change with complicated magnetic fields.  Christopher used Mathematica and some of his own C++ programs to visualize what the twisted magnetic fields look like inside the stars, and how they twist and untwist when they oscillate.  Lovely!  Claire Rosen, Michael Martin, and Jenny Pegg have begun a little work in this area too.

Meanwhile, other students learned to make research-quiality observations of stars.  For example, in summer 1999, Andrea Nowicki and Sara Petty-Powell  operated UW's 0.8 m telescope at Manastash Ridge Observatory (MRO), to take digital images of stars, and to analyze the data using IRAF, a standard package of professional astronomy software.  Our colleague, Dr. Paula Szkody has since set up MRO to make remote observations for distant users like Evergreen students.

Our calculations and observations continue.  One product of Zita's model of magnetic oscillations in roAp stars is a new dynamo model that might explain how some hot stars can sustain their magnetic fields without convection.  We would like to see whether this turbulent dynamo can explain new observations of magnetic fields in B stars.  This may require running 3D magnetohydrodynamics calculations on a supercomputer.  Any student interested in carrying out any part of these  investigations is encouraged to contact Zita with a research proposal.

Fusion is the "good" kind of nuclear energy, generated by an artificial sun in the laboratory.  Fusion has the potential to provide much of the world's electricity without burning fossil fuels.  And unlike fission, fusion does not create long-lived, dangerous radioactive sludge.  Fusion research is still experimental, though we hope to see commercial fusion reactors in our lifetime.  Starting with her undergraduate thesis, Zita worked for a decade in fusion research on tokamaks and other devices designed to confine hot plasma in a "magnetic bottle."  See the excellent web pages for General Atomics and University of Wisconsin - Madison, two premier fusion research facilities.  There has been a surge of  funding for fusion research in recent years, facilitating a rennaisance in this field.  Grad schools are actively recruiting students interested in fusion.  Contact Zita if you want to learn more.

Madison Symmetric Torus  Reversed Field Pinch from  http://sprott.physics.wisc.edu/mst.htm

Berkeley fusion link

Colin Rosenthal and Ake Nordlund on MHD waves and p modes