Newsgroups: sci.physics.plasma
From: fcrary@benji.Colorado.EDU (Frank Crary)
Organization: University of Colorado, Boulder
Subject: Re: Can Gravity be Induced?

In article <3astqo$9ja@mojo.eng.umd.edu>,
Stephen Goodfellow <llrowla@cms.cc.wayne.edu> wrote:
>...On the subject of planitary
>rotation, I recall reading an article in a Scientifc American from the
>late 50's, early 60's (Sorry, I'd have to go to the library to give you
>the exact article reference,) on the rotation rate of our planet; that
>for a few days during very high solar activity, the Earth's rotation
>accelerated a noticable amount. Do you know anything about this?

I can't see any way the solar wind could affect the Earth's
rotation on such short time scales. The observed solar
wind simply lacks the energy and momentum for this.
I think this may have been a problem with the old studies.
It is fairly common to measure a planet's internal rate
of rotation by looking at the periodic variations in
magnetic field. The period of Jupiter's internal rotation,
for example (i.e. the "System III" period) was accurately
measured in the early 1960s by this sort of process: The
rotation causes the magnetic fields to wobble once per
"day", and this can be seen in the radio emissions produced
by Jupiter's magnetosphere. I suspect the late 50's
work you are remembering tried to monitor the Earth's
rotation in some similar way. The problem is that
the internal rotation of the Earth is not the only
thing that can perturb the planet's magnetic fields.
A slight contribution comes from the "ring current"
flowing through the Earth's magnetosphere. High solar
activity does affect the ring current very significantly.
So the magnetic fields around the Earth are slightly
changed by solar activity. If the observations of
the Earth's rotation were based on magnetic field data,
this might create the illusion that the Earth was
being slowed down or sped up during periods of high
solar activity. Of course, the people who did that
research wouldn't have known this: In the late 1950s
and early 60s, we barely knew that the Earth had
a magnetosphere at all; the ring current wasn't
discovered until about a decade later and the
affect of the solar wind on the ring current is
even more recent.

Frank Crary
CU Boulder