An interpretation of the broadband VLF waves near the Io torus as observed by Ulysses

Abstract

The requirements for the Ulysses trajectory to attain high ecliptic latitudes using a Jovian gravitational assist resulted in a fortuitous passage through the Io torus region. Specifically, the spacecraft spend may hours at latitudes just above the torus. During this time the low-frequency cutoff of an ordinary mode (O mode) emission allowed a determination of the local electron plasma frequency (i.e., electron density) along the northern flank of the torus. Also, near a Jovian System III longitude of 100°, the spacecraft flew past a set of active field lines that have been previously identified to be associated with the hectometric generation region. During the passage, Ulysses observed a newly discovered O mode component and a whistler mode emission similar to that observed by Voyager 1 13 years previously. All of the broadband VLF emissions imply the presence of a particular population of electrons. We suggest that broadband VLF emissions can be used as a ``particle detector'' to qualitatively measure the electron plasma conditions in the torus region and identify active regions.