Source characteristics and radiation mechanism of Jovian anomalous continuum

We investigate the characteristics of the Jovian anomalous continuum (JAC) in interplanetary space and in the magnetosheath, using Ulysses observations. Some new source characteristics of JAC were obtained in addition to those found by previous authors [e.g., Kaiser, 1998]. JAC tends to occur when the solar wind dynamic pressure decreases after a rapid increase. We confirm and show more concretely that the commencement of JAC has significant dependence on local time, i.e., its appearance is most likely when the System III longitude of the subsolar point is near 270°. The periodic appearance of JAC is not due to its intensity modulation but to its repeated individual excitation. JAC exhibited an abrupt amplitude change across the bow shock, possibly suggesting a mode conversion from quasi-electrostatic to electromagnetic waves at the bow shock. We also evaluate possible sources of JAC and hypothesize that its origin is Langmuir waves excited at the magnetopause by energetic particles such as quasiperiodic bursts ejected from the polar magnetosphere. The relation of magnetospheric disturbances to the generation of JAC is also discussed.