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