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

In article <3aidqn$fl0@mojo.eng.umd.edu>,
Stephen Goodfellow <llrowla@cms.cc.wayne.edu> wrote:
>Does the Webber-Davis model comment on the plasma sheath situated at
>Jupiter's ecliptic?

No, the Webber-Davis model doesn't really apply. It says that,
given the gravity, temperature and magnetic fields at the
surface of Jupiter, there shouldn't be any "planetary wind"
flowing away from the planet. That's good, because we've
observed no such wind. The plasma torus around Jupiter
is supplied (primarily) by particles escaping from Io, the
inner most, major moon. This sort of external source is
completely beyond the scope of the Webber-Davis model, which
(after all) is supposed to model the solar wind even
if it can also be applied to planetary winds.

>...Do you happen to know if the flux tubes between Io and
>Jupiter's magnetic poles are mentioned or the sulfur 'lane' that Io orbits
>Jupiter? I mention these, because it seems to me that the accumulation of
>these may add considerable drag to the Jovian system.

The torus does produce some drag, but it can be estimated and
is negligible. The source of the drag is rather interesting:
Plasma confined in Jupiter's magnetic fields "co-rotates"
with the planet. Basically, if the plasma were moving
at a different rate, electric fields would be generated.
Since Jupiter has a conductive ionosphere, these fields
would drive a current flow, closing through the ionosphere,
and that current flow would produce a JxB force, speeding
the plasma up to co-rotation and trying to slow the
ionosphere. Of course, slowing a planet's ionosphere is
not all that easy: It is collisionally tied to the lower
atmosphere and viscosity keeps it moving at more or
less the same speed as the planet. Since most of the
plasma in the torus is already co-rotating, it
doesn't cause any torque on Jupiter. But there are two
exceptions: New particles ionized at Io are moving 56 km/s
slower than co-rotation. They are accelerated and
produce a torque on Jupiter. Also, as plasma diffuses
outwards from the torus, it must be accelerated: The
co-rotation velocity increases with distance (for a
constant angular velocity.) Near Io, the acceleration
of new particles is observed to cause a 1 or 2% departure
from co-rotation. What's really "slipping" is the
Jovian ionosphere: On the one hand, viscosity is
forcing it to rotate with the rest of the atmosphere.
On the other hand, the currents from the plasma torus
are trying to slow it down. The balance along the
field lines connected to Io is _almost_ perfect
co-rotation. Something similar also happens with the
material diffusing outwards, but here the problem is
worse: The energy required to accelerate the plasma
increases with distance and the magnetic fields
(and therefore the coupling to the ionosphere) get
weaker. At some point (around 20 or 50 Jovian radii out),
the process brakes down and the plasma no longer
co-rotates at all. All this definitely heats
the ionosphere, but the torque doesn't have much of
an effect on the planet: Estimates put it as
much less than the torque from other things, like
the tidal forces coupling Jupiter to its major
satellites.
(Sorry if I've gone on at length... The Io-Jupiter
system is what I mainly work on.)

>Also, I apologize for not making myself clear; I was considering proximity
>(Moons/Jupiter-Sun/Planets) rather than actual mass. Those moons are
>relatively close in relation to their size.

That's certainly true, although Pluto/Charon is a much more
extreme case. Io, for example, is only 6 Jovian radii away
from the planet, while the Earth's moon is about 60 Earth
radii away and the closest planet to the Sun is perhaps
100 solar radii out. But then, the processes which produced
these bodies are thought to be very different (the Earth's
moon, for example, is thought to have been produced by
a very large impact, i.e. between the then-forming Earth
and a Mars-sized impactor.)

Frank Crary
CU Boulder