Standing hydromagnetic waves in the Io plasma torus: Voyager 1 observations

Abstract

Geomagnetic pulsations are one of the dominant features of the dynamics of the Earth's solar wind‐magnetosphere‐ionosphere coupling system. Whether such ultralow‐frequency waves are also excited within the Jovian magnetosphere has been the subject of a close inspection of Voyager 1 magnetic field observations during its close encounter with Jupiter. These observations clearly indicate the existence and an increase of ultralow‐frequency wave activity and indicate that the activity becomes more regular as soon as Voyager 1 entered the Io plasma torus at around 0700 spacecraft event time on March 5, 1979. In particular, periodic transverse and compressional magnetic field fluctuations with periods of about 1200 s and 800 s, respectively, are observed with the different periods pointing toward a decoupling between these two different types of oscillations. The coincidence between the increase in wave activity and the entry into the Io plasma torus is in support of treating the torus as a low Alfvén velocity region and thus as a hydromagnetic waveguide. A first theoretical treatment of hydromagnetic wave propagation within the torus suggests that decoupling of toroidal and poloidal type oscillations can occur under the condition of axisymmetry of the wave field. Numerical calculations of the fundamental mode toroidal and first harmonic poloidal eigenperiods for a model Jovian magnetosphere give values quite in accord with the observed periods. We thus conclude that nearly axisymmetric, decoupled toroidal and poloidal mode eigenoscillations of the Io plasma torus are observed, indicating a large‐scale source mechanism for the magnetic field fluctuations detected. The total electromagnetic energy of the observed torus oscillations is of the order of 1013 J and is thus significant with respect to the energetics of the Io plasma torus.