Newsgroups: sci.physics.plasma
From: keller@noao.edu (Christoph Keller)
Organization: National Optical Astronomy Observatories, Tucson AZ
Subject: Re: Can Gravity be Induced?

These are some follow-up comments on Frank Crary's comments on my recent
mail. I think that in general we agree quite well, however, there are
some remarks by Frank that need some further discussion:

In article <39th5k$7dh@mojo.eng.umd.edu> fcrary@benji.Colorado.EDU (Frank Crary) writes:

on neutrinos:

> There is also a more direct explanation, although it isn't
> quite as popular (probably because it would be very
> difficult to prove.) A basic fact of heat conduction is
> ...(text removed)
> constant temperature all year long. In the case of the
> sun, it takes hundreds of years (a few thousand, possibly:
> I don't remember the figure exactly) for heat to
> be conducted from the core to the solar surface.

It is about 3e+7 years. A rather reasonable estimate
can be obtained by dividing the gravitational potential
energy by the luminosity. The potential energy comes in
via the virial theorem. It is called the Kelvin-Helmholtz
time scale.

> the average fusion rate. So it is entirely possible
> that fusion in the sun's core varies with time, on
> a scale of decades. If we happened to be looking
> during a time of minimal fusion, we would see
> far fewer neutrinos than we would expect, based on
> the observed, average energy output. Nor would
> this imply periodic variations in solar energy
> output: The process of heat conduction would
> average out these decade-long variations reached
> the observable regions of the sun.

This explanation is very unlikely by now thanks to accurate
helioseismological observations. Global oscillations do not have the
time lag of temperature fluctuations. Furthermore a low-temperature
core model would need a helium abundance that is lower than the
primordial helium abundance (at least the more popular non-standard
solar models).

> As well as the non-thermal heating processes, it is also possible
> that the corona is simply not as hot as it looks. This is
> actually a good exercise for students of plasma physics.
> "Temperature" is really the average kinetic energy of the
> particles. But often, the average doesn't mean much.
> Fluid mechanics, as well as the solar observations of the
> corona, assume that the particles are in a maxwellian
> distribution: A single population well-described by the
> average. But many space plasmas are not so easily
> described. Often the particles are in a much more complex
> distribution. Now observations of the corona's temperature
> are based on a particular sort of weighted average
> (weighted by collision cross-sections and the emissions
> they excite) while heat transfer equations are based
> on a different, weighted average (based on the
> kinetic energy transferred by collisions). If the
> particles were in a simple, maxwellian distribution,
> these two averages would be the same. But if the
> particles aren't in such an idealized state, then the
> tow sorts of averages might be very different:
> "Temperature", for the purposes of observed, emitted
> light might be quite different from "temperature"
> for the purposes of heat conduction.

I am aware of coronal heating theories that use non-Maxwellian
distributions (e.g. Scudder 1992, ApJ 398, 319). However, there is
currently no viable theory on how you get the non-Maxwellian distribution
at the base of the corona and certain observed quantities are not
reproduced. Also note that thermal X-rays from the corona are
observed, not just emission lines.

>>Rotation cannot produce magnetic fields.

> While I agree with many of your earlier remarks, this is not
> correct. The essence of a dynamo is the creation of
> a magnetic field drawing the required energy from
> rotational motion. Even in the absence of an initial
> magnetic field, a dynamo is unstable and would produce
> a strong magnetic field from even the most trivial
> disturbance. Rotation in a plasma can, under the
> right conditions, produce a magnetic field. Rotation
> alone, however, is certainly not enough.

I agree that in a very general term you may create magnetic fields
from certain plasmas. We were talking about the solar case, and I am
not aware of any theory that may create magnetic fields from rotation
and does that in a period that is short with respect to the age of the sun.
(please note: I am aware of theories on the generation of seed fields.)
The usual dynamo equations derived in magnetohydrodynamics do not
contain a term that creates magnetic fields from a field-free state.
I would be interested to hear how you get a term that creates fields.

------------------------------------------------------------------------------
Christoph Keller Internet: ckeller@noao.edu
National Optical Astronomy Observatories/National Solar Observatory
950 N. Cherry Avenue, P.O.Box 26732, Tucson, AZ 85726-6732, USA