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 .