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The Magnetic Dynamo Theory &

Stellar Magnetic Activity

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MDT: When and Who?

First model proposed in 1961 by Horace Babcock

(American)

-Babcock ModelBabcock: Son of an astronomer

-astronomical instruments

-adaptive optics-specialized in study of magnetic fields and

spectroscopy

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MDT: What is it?

Explains many components of the solar cycle

Generally successful but still many unresolved issues

Consider magnetic field lines are frozen into gas:

Poloidal field field w/ toroidal componentWhy?: differential rotation of the Sun and the turbulent

convection zone

-this zone has the effect of twisting the lines magnetic ropes(intense magnetic fields)

-direction of magnetic field along ropes determines polarity ofsunspots

-every lead spot will have same polarity in its hemisphere andthe other lead spots in the other hemisphere will have the

opposite polarity

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MDT: What is it? Cont.

The initial small twisting occurs at higher

latitudes during the sunspot minimum

Differential rotation continues to drag the fieldlines along and convective turbulence ties

them in a knots

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MDT: What is it? Cont.

These knots form at intermediate latitudes leadto more sunspots here and producing thesunspot maximum

Number of sunspots near equator is small dueto cancellation of polarity of leading spots andthis cancellation returns to sun to poloidal

field but with its original polarity reverse (11years)

Solar cycle: 22 years

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Evidence of Magnetic Activity in

Other StarsObservations indicate that other cool main-sequence stars possess activity cyclessuggesting that the basic ideas behind solar

cycle are correctHot Coronae and angular momentum lost to

stellar winds agree with onset of surfaceconvection in low mass-stars (majorcomponent of MDT)

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Flare Stars

Main-sequence class M stars with some rapid

fluctuations in brightness

Possible explanation to short observed changesin luminosity of these stars:

-flares size of those on sun occurring on these stars

Larger flares may be generated on other stars

-4/24/04, GJ 3685A

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Starspots

Observed on other stars from through their

effects on luminosity

Two classes of stars show significant long-termvariations b/c large fractions of surface

covered by starspots

-RS Canum and By Draconis stars

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Starspots Cont.

Magnetic fields detected directly on cool main-

sequence stars by measuring Zeeman-

broadened spectral lines

Field strengths of several tenths of a tesla

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Thank You

Any Questions?

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SOURCES

Carroll, Bradley W., and Dale A. Ostlie.An

Introduction to Modern Astrophysics. San

Francisco: Pearson Addison-Wesley, 2007.

Print.