This study shows that the presence of an obstacle embedded in the magnetic field of a larger scale that has a trapped plasma population results in the generation of waves. However, the presence of a satellite, or a satellite with a magnetic field is not sufficient to generate waves. Galileo observations of waves near Io (6 R J ) and Callisto (26.3 R J ), did not show strong whistler waves. In the vicinity of Io high plasma density results in a very high ratio of plasma to gyrofrequency which creates unfavorable conditions for the local acceleration of electrons by waves. In the magnetosphere of Saturn, strong waves were also observed in the vicinity of Rhea10 and Enceladus20. It was also argued that waves may be responsible for depleting electrons in the vicinity of Rhea10. In a similar way, waves can produce local acceleration if the plasma environment is preferential for energy diffusion. The intensity of the waves may depend on a number of parameters such as the amplitude of the magnetic field, ratio of plasma to gyro-frequency, external magnetic field and the presence of neutral particles. Note that enhanced wave activity may be also produced by objects that do not have an internal magnetic field10,20. However, the internal magnetic field will increase the size of the obstacle for the particles streaming near the object, resulting in the generation of stronger waves. Finding exact scaling laws and predicting maximum amplitudes that may result from non-linear saturation should be a subject of future research. Follow-up studies should verify that depleted pitch angle distributions contain sufficient free energy for the excitation of waves.

Since whistler mode waves propagate in a cone of angles about the direction of the background field, and field and density gradients refract the waves, it is likely that some waves are able to escape from field lines threading Ganymede and Jupiter. By analogy with the Earth, whistler mode waves are also reflected at the lower hybrid resonance frequency and thus waves could return to the equatorial region on a different field line after reflection at higher Jovian latitudes, provided Landau wave damping is not severe. It is therefore possible that the moons provide a source of waves that is not just confined to the magnetosphere of Ganymede but could extend outside the region. Future ray tracing studies should be focused on the propagation and reflection of waves.

If the waves escape the moons’ magnetosphere, the moons may have a pronounced effect on the radiation environment of Jupiter’s magnetosphere which may in turn affect the particle environment through wave–particle interactions. Even though the increase in wave power cannot be seen outside of ~2 moon radii, waves may be efficiently accelerating particles that are later transported radially. The increase in wave intensity that we observe may be already moderated by the wave–particle energy exchange. In other words, by the time waves are observed, they may have already given a significant portion of energy to the charged particles. These waves may allow the transport of energy from low energy and low pitch angles that are lost to Ganymede’s loss cone, and generate waves that will accelerate high energy particles.

Comparative studies of the radiation and wave environments are important for understanding the dynamics of the radiation belts, of the outer planets and Earth, and understanding of fundamental physical processes in the radiation belts such as acceleration and scattering. Comparative studies may also help us to understand the critical parameters and scaling laws that determine the behavior of plasma populations in the solar system and beyond.

This study is particularly relevant to the ongoing Juno and upcoming JUICE missions which will allow us to build a more detailed picture of these waves and their interactions with particle populations of the Jovian magnetosphere. The Juno mission reached Jupiter’s magnetosphere on July 4, 2016 and will focus on the exploration of Jupiter’s deep atmosphere and magnetosphere, while the later scheduled European Space Agency JUICE mission will have a particular emphasis on Ganymede, and a few flybys of Europa. The planned NASA Europa Clipper mission may fly by Europa >45 times. This study may additionally help with NASA’s next planned discovery mission, “Psyche”, that will explore the origin of planetary cores by studying the heaviest known M-type asteroid 16 Psyche that may potentially have a magnetic field. This study may be also relevant to laboratory studies where objects embedded in the streaming plasma may produce strong waves as well as acceleration or loss of charged particles.

Statistical observations of waves presented in this study indicated that the processes of wave generation observed for the Jovian moons embedded in the magnetosphere of Jupiter are universal and should occur for other astrophysical objects, e.g., that are stellar magnetic fields embedded in the interplanetary medium, magnetospheres of the exoplanets and magnetospheres of the moons of exoplanets. The increase in chorus wave power in the magnetospheres of exoplanets may provide free energy for the acceleration of electrons to ultra-relativistic energies. The intense synchrotron radiation from such electrons may aid in the detection of the magnetospheres of such objects.