  By analysing a database of 26 years of observations of Jupiter with the
Nancay Decameter Array, we unambiguously identify the radio emissions caused by
the Ganymede-Jupiter interaction. We study the energetics of these emissions
via the distributions of their intensities, duration, and power, and compare
them to the energetics of the Io-Jupiter radio emissions. This allows us to
demonstrate that the average emitted radio power is proportional to the
Poynting flux from the rotating Jupiter's magnetosphere intercepted by the
obstacle. We then generalize this result to the radio-magnetic scaling law that
appears to apply to all plasma interactions between a magnetized flow and an
obstacle, magnetized or not. Extrapolating this scaling law to the parameter
range corresponding to hot Jupiters, we predict large radio powers emitted by
these objects, that should result in detectable radio flux with new-generation
radiotelescopes. Comparing the distributions of the durations of
Ganymede-Jupiter and Io-Jupiter emission events also suggests that while the
latter results from quasi-permanent Alfven wave excitation by Io, the former
likely results from sporadic reconnection between magnetic fields Ganymede and
Jupiter, controlled by Jupiter's magnetic field geometry and modulated by its
rotation.
