Source Location of Narrowband Radio Emission Detected at Saturn

Details Source Location of Narrowband Radio Emission Detected at Saturn Source Location of Narrowband Radio Emission Detected at Saturn

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.p43a1017y&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #P43A-1017

Physics

6275 Saturn, 6984 Waves In Plasma (7867)

Scientific paper

Since Cassini's arrival at Saturn in 2004, the Radio and Plasma Wave Science (RPWS) instrument has detected a series of narrowband radio emissions in Saturn's magnetosphere. It is believed that these narrowband emissions are first excited as electrostatic waves by electrons when the matching condition fuh = (n+1/2)fce is met, where fuh is the upper hybrid frequency and fce is the electron cyclotron frequency. These upper hybrid waves can mode convert to electromagnetic L-O waves in the presence of density gradients. We determined the polarization of the narrowband emissions and found it to be consistent with the prediction of linear mode conversion theory. To model the source region of narrowband emissions, we numerically calculated the locations where the matching condition is met in a meridian plane, using a scale height electron density model [Persoon et al., 2006] and a dipole magnetic field model. For the 5 kHz narrowband emission, the matching condition fuh = 3/2 fce is met at a radial distance of 5.6 RS and latitude of 32.6 degrees. For the 20 kHz emission, fuh = 3/2fce is met at 3.3 RS and 15.7 degrees latitude. In several selected events, Cassini went through the source region of narrowband emissions, as indicated by a strong electrostatic band preceding the radio emission. The coordinates of Cassini when the source of narrowband emissions are encountered closely match that predicted by the model. For these encounters with the narrowband emission source, we investigate the electron distribution functions obtained from the electron spectrometer (ELS) onboard Cassini, and perform an initial wave growth analysis for the electrostatic waves. For the electromagnetic radio component of narrowband emission, we employ a polarization reversal technique to determine the source locations. While the Cassini spacecraft is rotating, the apparent polarization of the radio emission measured by RPWS switches its rotation sense when the source goes through the antenna plane. By finding the intersection of the plane which contains the source with a ring of possible source locations predicted by our model, we can determine the longitudinal coordinate of the source.

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