The Io Plasma Torus (IPT) is a cloud of charged particles in orbit
around Jupiter and centered on the centrifugal equator. It extends
from inside Io's orbit (about 5 Rj) out to around Europa's orbit,
though exact estimates of the outer boundary vary.
The IPT consists primarily of ionized sulfur and oxygen, and
electrons. The main source of these particles is Io, though Europa
may also contribute. The particles orbit Jupiter at a velocity
which lags slightly behind the corrotation velocity.
The parameters relevant to the torus are difficult to quantify as
they vary with radial distance, Jovian latitude, Jovian longitude,
and even with time. The following description (adapted from
[BAGENAL1994]) describes the characteristics of five radially
The inner torus 5-5.4 Rj. The plasma is cold (~1 eV) with few
suprathermal ions or electrons. The composition is dominated by
S+ (45-75% of charge fraction) and O+ (20-45% charge fraction)
with small, quantities of O++ and SO2+.
The precipice region 5.4-5.6 Rj. There are sharp changes in the
slope of the number of ions per shell of magnetic flux (NL^2)
and in the composition. The supra thermal ions comprise >10% of
The ribbon region 5.6-6 Rj. Here are the highest densities
(>3000 cm^-3). The temperature and composition (for >5.75 Rj
~40% O+, ~17% S++, ~10% S+ of charge fraction) are similar to
the rest of the warm torus (>5.7 Rj).
The ledge region 6-7.5 Rj. This is the uniform region of the
warm torus, where there are gradual changes in density,
composition, and temperature.
The ramp region > 7.5 Rj. There is a drop in NL^2, perhaps due
to impoundment of the torus material by the middle
magnetospheric ring current. The current sheet begins to affect
the magnetic field. The ions change in composition towards
higher ionization state and higher oxygen fraction. Suprathermal
ions are diffusing inwards. Europa is a possible source of
plasma in this region.
Results from Voyager 1 investigations of the IPT include
[BRIDGEETAL1979A] (PLS), [BROADFOOTETAL1979] (UVS), and
[WARWICKETAL1979A] and [BIRMINGHAMETAL1981] (PRA electron
studies). [BROWNETAL1983] gives a post Pioneer and Voyager
description of the IPT's characteristics. [BAGENAL1994] models the
plasma conditions between 5 and 10 Rj using updated analyses of
Voyager 1 observations. [SCHNEIDER&TRAUGE1995] discuss IPT
asymmetries. Periodicities in the brightness of the torus are
discussed in [DESSLER&SANDEL1992], and [BROWN1995].