Io's Interaction with the Plasma Torus: Galileo Magnetometer Report

Abstract

an 898 km) and reached 610 n T within a background field of -1000 n T at 17:44:50 UT. In this region, the Galileo magnetometer data at 0.22-second resolution reveal a complex interaction KK96 model gives a field magnitude of between lo and the flowing plasma of the lo torus. The highly structured magnetic field about 1800 nT. The field signature bedepression across the downstream wake, although consistent with a magnetized lo, is came markedly different in the region immodified by sources of currents within the plasma that introduce ambiguity into the mediately downstream of 10 relative to the interpretation of the signature. Highly monochromatic ion cyclotron waves appear to be flow direction of torus plasma, a region correlated with the local neutral particle density. The power peaks in the range of that we refer to as the geometric wake of molecular ion gyrofrequencies, suggesting that molecules from lo can remain undisso-10. The field magnitude leveled off, and its ciated over a region of more than 15 lo radii around lo.fluctuations became small. Near the outbound crossing of the geometric wake, the field again decreased, and its magnitude began to fluctuate much as it did during O n 7 December 1995, the Galileo orbiter between 10 and the plasma that flows by it. the inbound interval. The field returned passed within 0.5 R,, (10 radii) above the Before the encounter, the magnetic field to within 3% of model values at -17:56 surface of Jupiter's moon 10. The magne-B was predominantly in the 0 direction UT. Estimates of expected perturbations tometer (1 ) recorded magnetic field vec-with small B, and B+ components, and the in a magnetohydrodynamic simulation (2, tors with 0.224 time resolution and stored magnitude was increasing, as expected for 6-8) of a conducting 10 are too small to them on the spacecraft tape recorder for approach to Jupiter (Fig. 1). The Khurana-account for the observations, whereas a transmission in mid-June 1996. The mag-96 Jupiter magnetic field model (KK96) magnetized 10 model overestimates the netic signature based on 1-min averages (5) represented the measurements quite field decrease in the geometric wake and

1. Transport of Mass, Momentum and Energy in Planetary Magnetodisc Regions

“…Figure 31 shows Galileo trajectories for five crossings of the Io wake region. On the first orbit, on December 7, 1995 (flyby I0), Galileo flew 900 km downstream of Io at closest approach, revealing strong magnetic fluctuations with periods ∼ 2-3 s, corresponding to the gyrofrequencies of SO + 2 and SO + (Kivelson et al 1996). The waves were observed inbound and outbound (sinusoidal signatures), extending over distances of ∼ 20 R I (Io radii) and ∼ 7 R I , respectively.…”

Effects of magnetohydrodynamic waves in Jovian decametric emission

“…The frequency ∼0.5 Hz of heaviest SO + 2 /SO + ions dominates in the magnetometer data of Galileo (Kivelson et al 1996;Russell et al 2003). In this context, the 2s-periodicity in the autocorrelation function of the S-burst emission record seems very interesting (Fig.…”

“…The ion cyclotron waves with frequencies near the equatorial gyrofrequencies of SO + 2 and SO + have been discovered with the Galileo space probe near the Io wake, in the Io plasma torus around Jupiter (Kivelson et al 1996;Russell et al 2003). It is very important that the amplitude of SO + 2 /SO + cyclotron waves (250 nT) is comparable to the transverse Io's disturbance of the Jovian magnetic field (300 nT; Kivelson et al 1996).…”

“…Near the planet, the local cyclotron frequency of ions (mainly protons) exceeds the frequency of ULF wave a lot, which is Alfvénic in the region of DAM sources. Such an Alfvén wave could modulate the acceleration of electrons (hence the DAM IRA, Kharkiv) showing the existence of a dominating wave with 1.5−2 s period, which corresponds to the SO + 2 /SO + ion cyclotron waves found in the Io plasma torus (Kivelson et al 1996;Russell et al 2003). Fig.…”

“…It is very important that the amplitude of SO + 2 /SO + cyclotron waves (250 nT) is comparable to the transverse Io's disturbance of the Jovian magnetic field (300 nT; Kivelson et al 1996). As the Io's disturbances (Alfvén wings) are considered a stimulus of Io-related DAM, L-emission (Bagenal & Leblanc 1988) as well as S-bursts (Zarka 1998;Ergun et al 2006), the ion cyclotron waves could have an influence on the Jovian radio emission, too.…”