Mechanism responsible for the emission.

The low frequency emission is generated by relatively low energy electrons spiraling in the Jovian magnetic field. In the case of the Io-related emission, the electrons are accelerated in the electric field induced by the jovian magnetic field across Io. These electrons spiral down to the planet along the magnetic flux tube passing through Io. The emission is produced by electrons ascending the Io magnetic flux tube after having mirrored near the top of the Jovian ionosphere. The emission occur at a frequency slightly above the local gyrofrequency.

The figure above illustrates the generalized decametric and decimetric emission regions. The solidly outlined area depicts decameter emission for a magnetic field of 10 gauss RJ ; the dashed area, 4 gauss  RJ3 . Adapted from Dolores Krausche's dissertation, (1975)

The mechanism responsible for the emission is believed to the cyclotron maser instability. The emission is beamed into a thin hollow cone with axis parallel to the direction of the magnetic field lines in the region where the emission originates (near the Jovian magnetic poles). The opening angle of the hollow cone seems to be around 70-80 degrees. The emission is detected on the Earth when the walls of the hollow cone intersect the direction of the Earth.