Investigating the origins of the Jovian decimetric emission's variability

The long-term variability of the Jovian radiation observed at 13-cm wavelength over 30 years is investigated using a physical model of the radiation belts. The model enables us to quantify the effect of the geometric parameter DE on the synchrotron emission. When a full Jupiter rotation is taken into account in the computations, the variability of the total radio flux with DE is only similar or equal to 1% of that measured. Such a dependence is not detectable. In contrast, radio fluctuations observed at a central meridian longitude (CML) and caused by DE changes can be very important, up to 10% of that measured. This strong dependence of the total radio flux on the geometric factor can result in difficulties for examining the origins of the Jovian synchrotron emission's fluctuations during periods where times of observation are not covering large CML range values. The numerical computations reveal that the variability either of the populations injected near the outer boundary of our simulations (Io's orbit) or the inward radial transport can, independently or in combined action, conduct variability in the decimetric emission. The theoretical fluctuations required in our computations for reproducing the radio measurements do not confirm any direct correlation between the long-term changes in the radio data and the radial transport driven by solar wind conditions or solar radio fluxes. Nevertheless, a linear relationship between the total radio flux and solar wind ram pressure for the period of 1970 to 2002 is established. The maximum correlation is reached when the P-sw data are shifted by similar or equal to 2.7 years prior to the radio observations. Using this time-lag in the computations, the simulation results show that ( Psw) g can be associated with the variations of the particle injections and the radial transport. The correlation coefficient, associated with the fit between radio data and simulated radio flux densities, is 0.59 for the period of 1971 - 2002, 0.93 for the period of 1971 - 1972, and 0.83 for the periods of 1975 - 1989 and 1992 - 1995. The index g is set to 0.35 when only particle injections are fluctuating with time, to 0.50 when temporal variations are driving the radial transport, and to 0.15 - 0.20 when the particles injected in the inner magnetosphere and their transport are simultaneously following the variations of the solar wind ram pressure. The model suggests that the solar wind ram pressure fluctuations can be related to variations of the Jovian decimetric emission on timescales of months, particularly to the enhancements in total radio flux observed in 1987, 1988, 1990 and 1994. Our results thus support the idea that the increase in radio emission in July 1994 was due to the impact of Comet Shoemaker-Levy 9 into Jupiter, in addition to the response of the Jovian magnetosphere to steep changes in solar wind conditions. The divergence between observations and simulations indicates that mechanisms other than those discussed in the modeling must govern the radiation belts dynamics for the periods of 1972 - 1975 and 1996 - 2002.