[1] In this paper, we attempt to clarify the relationship between Jovian hectometric (HOM) and non-Io-related decametric (non-Io-DAM) radio structure. For that purpose, we extend the analysis by including more data and investigating statistical properties of the Jovian DAM and HOM radio emissions based on Cassini and Voyager observations, especially below 16 MHz. We have investigated these emissions observed by the Cassini, Voyager 1, and Voyager 2 spacecraft for specific Jovigraphic latitudes in the range of À3.7°-7.3°a nd local times in the range of 0.76-21.4 hours. We show a statistical comparison of Cassini, Voyager 1, and Voyager 2 data for occurrence probability in Central Meridian Longitude (CML) versus Io phase and in CML versus Frequency. The main results are as follows: (1) the detailed frequency structures of non-Io-related components can be seen for different spacecraft's local time and Jovigraphic latitude, (2) the high frequency of HOM extends up to near 10 MHz, and (3) a new DAM component, named the non-Io-D, appears from 40°to 60°CML in the frequency range of 7-11 MHz. On the basis of additional information of different behaviors of non-Io-B and non-Io-A structures in longitude depending on pre-and post-encounter of Cassini data, we improved the DAM angular beaming model that shows the cone half-angle of the emitting cone decreases as a function of frequency. We conclude that the changing beaming angle is not affected by Jovigraphic latitude of the spacecraft, but rather due to different local time of the source regions.

[1] Observations of the low frequency part of Jupiter decameter wavelength (DAM) emissions were made using the Cassini radio and plasma wave science (RPWS) instrument. We have analyzed non-Io-DAM occurrence dependence from 4 MHz to 16 MHz based on the System III central meridian longitude (CML) of the Cassini spacecraft and calculated the occurrence probability for each frequency. As a result of this analysis, the two peaks of non-Io-B and non-Io-A occurrence probability showed a dramatic change in longitude between 9 MHz and 16 MHz. At 16 MHz two peaks of probability occurred at 160°and 240°CML. As the frequency decreases to 9 MHz, the two peaks converged to become one peak near 205°CML at 9 MHz. This peak gradually disappeared below 9 MHz. Based on Jupiter's magnetic VIP4 model, an angular beaming model was made to explain these observational results by taking into account the decreasing cone half-angle of the emitting cone from 16 MHz down to 9 MHz. We found the active magnetic flux tubes of non-Io-B and nonIo-A sources are localized at about 180°± 10°of System III longitude projected on Jupiter's surface.

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The result of a measurement of the period of rotation of Jupiter's inner magnetosphere with unprecendented precision is presented. The measurement was made from the University of Florida database of 35 apparitions of Jovian decametric observations at frequencies of 18, 20, and 22 MHz between 1957 and 1994. The mean of our 24 independent measurements was 9h55m29s.685, and the standard deviation of the mean was 0s.0034. The System III (1965) Jovian rotation period value that is currently accepted by the International Astronomical Union is greater than our value by 7.4 times our standard deviation; it appears to be in need of revision. We set an upper limit of 27 milliseconds per year on a possible drift of the rotation period as measured by our method.

New observations of Jupiter's decametric radio emissions have been made with the Long Wavelength Array Station 1 (LWA1), which is capable of making high-quality observations as low as 11 MHz. Full Stokes parameters were determined for bandwidths of 16 MHz. Here we present the first LWA1 results for the study of six Io-related events at temporal resolutions as fine as 0.25 ms. LWA1 data show excellent spectral detail in Jovian DAM such as simultaneous left-hand circular (LHC) and right-hand circular (RHC) polarized Io-related arcs and source envelopes, modulation lane features, S-burst structures, narrow band N events, and interactions between S bursts and N events. The sensitivity of the LWA1 combined with the low-radio-frequency interference environment allow us to trace the start of the LHC Io-C source region to much earlier CML III than typically found in the literature. We find that the Io-C starts as early as CML III = 230• at frequencies near 11 MHz. This early start of the Io-C emission may be valuable for refining models of the emission mechanism. We also detect modulation lane structures that appear continuous across LHC and RHC emissions, suggesting that both polarizations may originate from the same hemisphere of Jupiter. We present a study of rare S bursts detected during an Io-D event and show that drift rates are consistent with those from other Io-related sources. Finally, S-N burst events are seen in high spectral and temporal resolution and our data strongly support the cospatial origins of these events.