Depleted magnetic flux tubes as probes of the Io torus plasma Original Research Article

On the initial pass by Io the Galileo spacecraft detected thin tubes of magnetic flux that had stronger fields than their surroundings indicating that they were depleted in their energy content. These tubes have not been seen on every return to the Io torus, only on the passes with the longest stretches of data. They are also not observed much inside the orbit of Io. The statistics of their occurrence gives us further clues as to their origin and constraints on the properties of the Io torus plasmas. The tubes tend to occur in groups separated by hours of elapsed time suggesting a breakup from a larger structure and an episodic event type origin. The tubes also imply that the temperature of the torus is cooler than if all the plasma were due to heavy ion mass loading. The magnetic field excess inside the tubes also decreases with magnetic latitude following the expected latitude dependence of the beta of the plasma.