SYSTEM-III AND SYSTEM-IV MODULATION OF THE IO PHASE EFFECT IN THE IO PLASMA TORUS

We report two new findings regarding Io control of the EUV brightness of the plasma torus around Jupiter. These new findings relate to the w ell-known periodic brightenings of the torus that are tied to the orbi tal phase of Io (the Io phase effect), the most prominent of the perio dic torus brightness variations seen in the EUV. We first show that th e amplitude of the Io phase effect depends on the system III longitude of the ansa, in addition to the Io phase angle. Because the ribbon ha s a system III dependent motion, the brightness of the EUV ribbon, whi ch is embedded in the torus, is a function of its closest approach to the orbit of Io. We model the observed average Io phase effect with tw o sources of torus power: one, a longitudinally symmetric power source that keeps the Io plasma torus at a temperature of similar to 8 x 10( 4) K and the other a power source located around Io. The amplitude of the Io phase effect is dependent on the distance between the ansa and Io's orbit (not Io itself). We then show that this dependence arises f rom a geometrical effect and self-absorption of the 685-Angstrom line by the torus. The interplay of system III and system IV and its effect on the Io phase effect is investigated for connection between cumulat ive density variations through the Io plasma torus leading to a modifi cation of the heat source near Io. This modification could create the brightness difference in the aligned and antialigned phase of systems III and IV.