In December 1995 the Galileo spacecraft arrived at Jupiter and has remained in orbit around the gas giant until September 2003. At the end of the mission (i.e. near the en dof the spacecraft fuel) Galileo was plunged into Jupiter in order to comply with the planetary protection requirements, such that it cannot contaminate the Galielean moons with an impact.

The PWS experiment aboard Galileo was responsible for measuring low-frequency planetary radio emissions (5.62 Hz - 5.62 MHz). This kind of radiation is produced by an interplay of magnetic field and moving plasma inside Jupiter's magnetosphere.

Galileo's PWS recordings were compared to radio observations performed by the WIND satellite in Earth's orbit. Galileo and WIND are more than 4.2 AU apart and simultaneous radio observations between both spacecraft seem to be a very rare phenomenon. Nevertheless, on the basis of a magnetic field model for Jupiter the source location inside Jupiter's magnetosphere can be derived from simultaneously observed radio events. Moreover, parameters for the special emission geometry (hollow cone) of Jupiter's radio emission can be calculated.

 

The sketch shown above illustrates the geometry between Galileo, WIND and the source region required for a simultaneous observation. Both spacecraft have to be positioned inside the cone's mantle of the emitting hollow cone structure in order to be able to record emission from the same source region.

One of the surprises from the magnetic field data was the discovery of an internal magnetic field at Ganymede. Next to that, there are now strong indications that there are liquid layers (oceans) underneath the ice surface of Europa and Ganymede.

Further information on Galileo is found at NASA.