Ringing Magnetic StarsA 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

http://192.211.16.13/individuals/zita/home.htm

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 Stanford

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 vr.

B(-) vr

E

B(+)

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?

http://helios.tuc.noao.edu/

http://wet.iitap.iastate.edu/

Bogdan & Rosenthal, 94+D.W. Kurtz, 1985 +E.J. Zita 97, 98, 00, 01Dziembowski & Goode 96Gough & Cunha, 2000Shibahashi et al, 1985 +JCD=http://www.obs.aau.dk/helio_outreach/

http://www.astro.ubc.ca/MOST/

Global Oscillation Network Group