From: stefan@helios.physics.utoronto.ca (Stefan Mochnacki)
Subject: Re: magnetic breaking
Organization: University of Toronto Physics/Astronomy/CITA
Date: Thu, 1 Oct 1992 05:55:43 GMT

cain@geomag.gly.fsu.edu (Joe Cain) writes:


> Can anyone clarify the present ideas on magnetic breaking as the
> mechanism whereby the Sun lost it angular momentum during Solar system
> formation? Specifically I am unclear as to whether this was theorized
> to have taken place before or after nuclear fusion started in the Sun.

The standard model is due to Weber & Davis, 1967 Ap.J. (I don't
have the exact reference - I'm at home )
The problem was that the Weber-Davis model predicted less angular
momentum in the Solar Wind than old observations of comet-tails
and old in-situ measurements indicated. Either way, the present
angular momentum of the solar wind is not high enough to have 
braked the sun AFTER it arrived on the Main Sequence.

BUT: a classic paper by Skumanich (1972, Ap.J. Lett. I think),
showed that both the chromospheric activity AND the rotation rate
of solar-type dwarfs are proportional to the inverse square root
of their age. Assuming solid body rotation and the W-D model,
you very easily can show that the Skumanich Law for rotation implies
that the magnetic field contributing to magnetic braking is PROPORTIONAL
to the rotation rate, and this is nicely consistent with the dependence
of activity on rotation.

This means that if the early Sun was rotating rapidly, its magnetic field
and hence its rate of magnetic breaking were much higher at that time than
now.

Then, in 1983, a paper by Pizzo and others in the Ap.J., using newer
in situ measurements of the tangential component of the solar wind's velocity,
showed that the W-D model does correctly predict the PRESENT angular
momentum content of the solar wind.


Thus, assuming the Skumanich Law and solid body rotation, one can pretty easily
if crudely show that the Sun could have started its main sequence life
rotating very fast ( 100 km/sec or more equatorial speed), and it could
have breaked to the present 2 km/sec. Of course, if the interior is
rotating rapidly, this may not be right, but we DO observe rapidly-
rotating cool dwarfs in young clusters such as the Pleiades. It is therefore
POSSIBLE that the Sun was rotating rapidly at the time of solar system
formation, BUT the "memory" of its early rotation has been lost: it could
have been rotating SLOWLY all along and we cannot tell the difference
from studying the Sun, its wind, and the stars, alone. The only evidence
remaining would be in meteorites and other relics of the very early Solar
System.

> Also, has there been any new work done on why the nebula would have
> been sufficiently ionized to have a good handle on the Sun's field?

I think the nebula would not have had much to do with it: the solar
wind would escape above and below the accretion disk. The wind then, as
now, would have been fully ionized all the way out.

> Hartmann (1993, Moons and Planets) points to a paper by Consolmagno
> and Jokopii (1978) suggesting 26Al or 40K had enough emissions to have
> caused the ionoization, whereas earlier work apparently had used the
> heating from infall. 

That would help to explain conditions in the nebula, which probably
was in the form of an accretion disk of some moderate thickness. If the
Sun at that time was anything like a T Tauri star, intense magnetic
activity would drive a pretty energetic wind, and unless it was very dense
I would assume that it would be sufficiently ionized to carry the magnetic
field with it.


					- Stefan Mochnacki

-- 
Stefan W. Mochnacki          INTERNET - stefan@centaur.astro.utoronto.ca
Astronomy, U. of Toronto     UUCP - {uunet,pyramid}!utai!helios.physics!stefan
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