Jovian hectometric emission: Attenuation band due Io torus plasma
The RPWS experiment recorded Jovian radio emissions during the Cassini flyby of Jupiter. The capability of the experiment allowed frequency coverage from a few Hertz to 16 MHz with a large dynamic range of about 80 dB. Jovian radio components (DAM, HOM and KOM) were regularly observed several weeks before and after the closest approach to the planet on 30th Dec., 2000, at a distance of 137 Jovian radii. The figure (upper-panel) shows the Jovian radio emission versus time (UT, horizontal axis) and frequency (kHz , vertical axis). In the spectrum above 100 kHz one can see the hectometric (100 kHz to about 3 MHz) and the decametric (5 MHz to 16 MHz) radio emissions. These emissions are principally modulated by the planetary rotation which is on the order of 10 hours. One full Jovian rotation recorded on 28th Dec. 2000 is shown in the figure in the time interval between 04:00 and 14:00 UT. Detailed spectral features have been studied using a combination of the High Frequency Receiver (HFR), which is tuneable from 125 kHz to 16 MHz, with a bandwidth of 25 kHz, and the Wide Band Receiver (WBR), which can analyze the given bandwidth with a spectral resolution of about 100 Hz.
The sensitivity of the RPWS experiment permitted the attenuation bands to be detected at a far distance from Jupiter, at least several weeks before the closest approach. The figure (upper-panel) shows a dynamic spectrum where typical attenuation bands are fully and partially seen. The outline and the trace of the attenuation band on the dynamic spectrum appear as an incomplete sinusoid as displayed in the lower-panel. During a full Jovian rotation one observes a succession of two incomplete sinusoids associated with the northern and the southern hemispheres. These sinusoids cross each other at about 06:15 UT and 16:45 UT as seen in the figure. This intersection corresponds to the time when the spacecraft is in the Jovian magnetic equatorial plane, i.e. the spacecraft magnitude latitude is almost equal to zero. The discovery during the Voyager mission of an attenuation band in the HOM emissions was interpreted as a refraction effect which occurs inside the Io torus.
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