Abstrakt
Measurements by the Cassini spacecraft during a close flyby of Saturn's moon Rhea on March 2, 2010, show the presence of intense plasma waves in the magnetic flux tube connected to the surface of the moon. Three types of waves were observed, (1) bursty electrostatic waves near the electron plasma frequency, (2) intense whistler-mode emissions below one half of the electron cyclotron frequency, and (3) broadband electrostatic waves at frequencies well below the ion plasma frequency.
The waves near the electron plasma frequency are believed to be driven by a low energy (similar to 35 eV) electron beam accelerated away from Rhea. Their bursty structure is believed to be due to a nonlinear process similar to the three-wave interaction that occurs for Langmuir waves in the solar wind.
The whistler-mode emissions are propagating toward Rhea and are shown to be generated by the loss-cone anisotropy (at parallel cyclotron resonance energies around 230 eV) caused by absorption of electrons at the surface of the moon. Scattering by these whistler-mode waves may be able to explain previously reported depletions of energetic electrons in the vicinity of the moon.
The low-frequency waves may play a role in nonlinear three-wave interactions with the bursty electrostatic waves.