Target Information
Target Overview
      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.
      Target Parameters
      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
      defined regions:
        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 density.
        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.
      Target References
      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].