On the character and distribution of lower-frequency radio emissions at Saturn and their relationship to substorm-like events
With the arrival of the Cassini spacecraft at Saturn in July 2004, there have been quasi-continuous observations of Saturn kilometric radiation (SKR) emissions. Exploration of the nightside magnetosphere has revealed evidence of plasmoid-like magnetic structures and other phenomena indicative of the Kronian equivalent of terrestrial substorms. In general, there is a good correlation between the timing of reconnection events and enhancements in the auroral SKR emission. Eight of nine reconnection events studied occur at SKR phases where the SKR power would be expected to be rising with time. Thus, while the recurrence rate of substorm-like events at Saturn is likely much longer than the planetary rotation timescale, the events are favored to occur at a particular phase of the rotation. We show three examples in each of which the SKR spectrum extends to lower frequencies than usual. This can be interpreted as an expansion of the auroral particle acceleration region to higher altitudes along magnetic field lines as a direct consequence of an increase in the magnetosphere-ionosphere current density driven by substorm-like events. We then conduct a survey of such low-frequency extensions during the equatorial orbits of 2005-2006 and place some constraints on visibility of these radio emissions.