Comparative study of the 'radio-planets'

Four planets -the Earth, Jupiter, Saturn and Uranus -are known to be sources of low frequency "auroral" emissions, due to energetic electrons precipitating in regions of high latitude.These emissions might differ from planet to planet either because the emission mechanism is different, or because the source structure and energetic electron populations are different.Hence it is very interesting to compare the known characteristics of the four planetary "auroral" emissions.In the first part, we emphasize on the difficulties of this comparative study, due mainly to the limited amount of information we have so far.In the second part, we summarize the main properties of the planets on which the radio emission is dependent, i.e. the magnetic field and the cold and hot plasma populations in the source, around it and more generally in the planet's inner magnetosphere.Then we present a comparative study of the four planetary emissions, using the ground observations of Jupiter, the data obtained by the Planetary Radio Astronomy experiment on the Voyager for Jupiter, Saturn and Uranus, and for the Earth the observations of several Earth orbiting satellites.We consider successively the high frequency limit of the emissions, the total energy radiated and the source brightness temperature, the source location, emission diagram and source structure, the wave polarization, the fine structures in the emission dynamic spectra and finally the modulation by the planetary rotation.It is shown that there are many similarities between the four planets' emissions which seems to justify the search for a common emission mechanism.On another hand one must not ignore the differences between the emissions, which reflect probably differences of physical conditions in the source and of the origin of the energetic electrons.The more striking of these differences are still unexplained.