In the outer
torus, the main external force acting on the charged particles
along the magnetic field is produced by the centrifugal force
due to plasma corotation. As is well known, since the electrons are much
lighter
than ions, they feel a much smaller centrifugal force and an
ambipolar electric field must exist to preserve local charge
quasi-neutrality. This field confines the electrons in the same
region as ions, i.e. near the point along any given magnetic
field line where the 
-aligned component of the
centrifugal force vanishes ([Gledhill 1967]); this defines the so-called
centrifugal equator (which is slightly shifted from the magnetic
equator since the planet's magnetic and spin axis do not exactly
coincide).
In a first approximation, the particles are thus confined
near the equator by forces deriving from potentials: the
electrostatic force for electrons, the electrostatic force plus
the centrifugal one for ions. Since the main source of these
particles is also near equator, the situation has some
similarity with the problem considered by Scudder (1992a, 1992b)
to interpret temperature inversions in stellar coronae:
basically, since the more energetic particles overcome more easily
the confining potential, their proportion is larger outside the
potential well, so that the mean kinetic energy of particles increases
with
latitude as the density falls. This does not happen with a
Maxwellian distribution because, in this case, the attractive
potential filtrates all particles of the distribution in the same
way (it produces a translation in 
which just multiplies 
by a
constant factor).
To illustrate this velocity filtration effect, we
consider a very simplified model. Since the particle free paths
are much larger than the characteristic scale lengths (see
Section 3.2), we
treat the latitudinal variation over a few scale lengths as a
collisionless problem. In the latitude range considered here
and with a roughly dipolar magnetic field, the field-aligned
component of the centrifugal force on ions of mass 
may be
approximated by
z 
being the distance along magnetic field lines (counted
from the centrifugal equator), which is roughly proportional to the
centrifugal latitude, and 
being the planet's spin angular
frequency (see for example Siscoe (1977)). We neglect
the field-aligned component of the gravitational force, which
is smaller by a factor of 
at the
Jovicentric distance 
, 
being the Jovian mass
and G the
gravitational constant. In the latitude range and with the
parameters considered here, the magnetic field variations are
very small over the latitudinal characteristic length of about 1
, hence we
will also neglect them, i.e., we neglect the magnetic mirror
force.
Since, as we said, the centrifugal force on electrons
is much smaller than the corresponding force 
on ions, the
charge neutrality condition requires that the electrons be subjected
to
an electric field confining them near z = 0, as are the ions.
The corresponding electric potential may thus be taken as
, with 
and 
for 
.
