Ringing Magnetic Stars A colloquium at the University College of the Cariboo, Kamloops, BC Many stars - including the sun - have magnetic fields, and many - including the sun - ring with sound waves. Why do stars ring? What, if anything, do magnetic fields have to do with the ringing? What light does this shed on the lives of stars and life on Earth? Dr. E.J. Zita, The Evergreen State College, Olympia, WA

Outline: Which stars ring? How can we tell? What do sound waves look like in stars? Why do stars ring? How do magnetic fields affect ringing? Who is studying these effects?

Which stars ring? The Sun “5-minute oscillations” Peculiar A stars “mHz p-modes” Diagrams courtesy of NASA and SIMBAD (DO Eri), sounds from Stanfordsounds

How can we tell they ring? reveal expanding and contracting gas. reveal changes in shape of slowly rotating stars. Diagrams courtesy of JCD and D.W.Kurtz, 1990 Doppler shifts on the surface of the Sun... Luminosity fluctuations in peculiar A stars...

What do sound waves look like? Short-wavelength p-modes checker the Sun’s surface and interior Long-wavelength p-modes distort the surface of peculiar A stars Movies courtesy of Hathaway and Kawaler (WET)

Why do stars ring? Convection compresses the photosphere and rings the Sun’s surface Peculiar A stars ring similarly, but not due to convection! Diagram courtesy of JCD

Magnetosonic waves in roAp stars The twisted magnetic field B oscillates and creates a perpendicular electric field E... … an ExB drift compresses the plasma driving sound waves v r.

How do magnetic fields affect ringing stars? When sound waves bounce into magnetic sunspots... … energy is lost to Alfven waves: magnetic fields can DESTROY sound waves. We have seen that magnetic fields may DRIVE ringing in peculiar A stars. But in the Sun... Diagram courtesy of JCD

Sunspots and the solar dynamo Differential rotation twists magnetic field lines... … and magnetic buoyancy lifts kinks to the surface Diagrams courtesy of NASA

Helioseismology gives insight into the Sun’s structure and evolution Solar core rotates slower than equatorial surface. Understanding flows helps test dynamo models. Solar dynamo cycle  magnetic activity  increased radiation to Earth. Diagrams courtesy of JCD and HAO

Summary Magnetic fields can absorb ringing in the Sun Magnetic fields may drive ringing in peculiar A stars Helioseismology of p-modes reveals inner details of Sun Ringing p-modes reveal shape of stars surfaces. Diagram courtesy of HAO and WET

Who studies helioseismology & stellar seismology ? Bogdan & Rosenthal, 94+ D.W. Kurtz, E.J. Zita 97, 98, 00, 01 Dziembowski & Goode 96 Gough & Cunha, 2000 Shibahashi et al, JCD= Global Oscillation Network Group