Changing electrical nature of Saturn's rings: Implications for spoke formation

[1] During Cassini's orbit insertion at Saturn, the trajectory took the spacecraft overtop the planet's famed ring system. At this time, the Cassini Radio and Plasma Wave Science (RPWS) instrument obtained unprecedented observations of electron density in the vicinity of the rings. Using this information and a model of photoemission anticipated from the rings, we demonstrate that the ring surface potential undergoes a seasonal change in electrical configuration, being primarily unipolar (of one charge polarity/potential) during the Voyager era and now bipolar (two separate polarities/potentials) during the Cassini era. We calculate the approximate ring/sun opening angle required for the transition from unipolar to bipolar configuration. Using electron density profiles, we explicitly examine the conditions for current balance on the B ring and show which regions are most likely to charge to positive potentials and those that may remain negative as a function of ring opening angle relative to the sun. Finally, we demonstrate that the current body of observations of Saturn ring spokes is consistent with their formation only on negatively-charged surfaces, and suggest future times and locations to look for spoke activity based upon the model.