Variations of Jupiterʹs synchrotron radiation: a link with solar activity?

The jovian synchrotron radiation has been regularly monitored with the Nançay Radiotelescope (NRT) between April 1994 and June 1999 at 21.3, 18.0 and 9.1 cm wavelength in horizontal and vertical linear polarization. Our five years data set clearly reveals the presence of medium-term (months) and long-term (years) natural variations in addition to the variations caused by the impacts of comet Shoemaker-Levy 9 in July 1994. The spectral index of the nonthermal flux density varies between −0.7 and 0.2 and is strongly correlated with the 9.1 cm flux. The long-term variation of the radio flux measured at the three wavelengths grossly follows the solar activity: we perform a correlation study of several parameters (temperature, density, dynamic pressure, solar energy flux, etc.) with the jovian flux density. The best correlation with the proton temperature, flow speed, dynamic pressure, ion thermal pressure and energy flux occurs for a time lag of ∼245 days. However the dynamic pressure, ion thermal pressure and energy flux curves also suggest a possible correlation with a time lag between 600 and 700 days. Clearly, other factors than the solar wind exert a control on the radiation belts in the close environment of Jupiter (like whistler-mode waves and ionospheric induced electric fields) that contribute to medium-term variations of the radio emission.