Cassini observations of narrowband radio emissions in Saturn's magnetosphere

Abstract

The Radio and Plasma Wave Science instrument (RPWS) on board the Cassini spacecraft has detected numerous narrowband radio emissions in Saturn's magnetosphere from September 2005 to May 2007. Typically, the narrowband radio emissions occur after an intensification of Saturn kilometric radiation (SKR) and last for several days. These emissions often occur near 5 kHz, with a modulation period of about 10.8 h, similar to the planet's rotation period. Other apparently associated bands also occur at higher frequencies, typically from 20 to 30 kHz. Both 5 kHz and 20 to 30 kHz emissions tend to decrease in intensity with increasing time and slowly drift upward in frequency after the onset. Our study shows that the narrowband radio emissions tend to be observed on the nightside at a radial distance range from 5 to 50 Saturn radii and tend to be observed more frequently at higher latitudes. The rotational modulation of narrowband radio emissions in Saturn's magnetosphere acts like a flashing light rather than a rotating beacon, similar to the source of SKR. These emissions are believed to be generated by mode conversion from anisotropydriven electrostatic instabilities near the upper hybrid frequency. Comparisons with Cassini neutral particle imaging data show that transient hot plasma clouds, corotating with the planet inside the orbit of Titan, might be the source for narrowband radio emissions. Comparisons with Cassini Magnetometer data reveal that the narrowband radio emissions tend to occur 1 or 2 days after the compression of the magnetotail. Copyright © 2010 by the American Geophysical Union.