Louis et al.  present exciting new observations of radio emissions on Jupiter from the NASA Juno spacecraft – the first direct detection of Jovian decametric radio emissions induced by the interaction between Jupiter and the moon Ganymede. These observations were made as Juno crossed a polar region of the Giant Planet where the magnetic field lines are connected to Ganymede.
The radio emissions were produced by electrons at relativistic energy (a few thousand electron volts) in a region where the electron’s oscillation frequency (“plasma frequency”) is much lower than its gyration frequency (“cyclotron frequency”). Such electrons can amplify radio waves very close to the electron cyclotron frequency very rapidly, via a physical process called electron cyclotron maser instability (CMI). They can as well produce aurora in the far-ultraviolet – which was also observed by the camera on Juno.
Juno was traveling at a speed of approximately 50 kilometers per second, and it spent at least about 5 seconds crossing the source region of the emission, which was therefore at least about 250 kilometers in size.
The observed decametric radiation on Jupiter is clearly the “shorter cousin” (in wavelength) of the auroral kilometric radiation on both Earth and Saturn: the CMI being responsible for their production on the three planets.
Citation: Louis, C. K., Louarn, P., Allegrini, F., Kurth, W. S., & Szalay, J. R. . Ganymede‐induced decametric radio emission: In situ observations and measurements by Juno. Geophysical Research Letters, 47, e2020GL090021. https://doi.org/10.1029/2020GL090021
―Andrew Yau, Editor, Geophysical Research Letters
25 January 2021: This article was originally published on 16 December 2020. It has since been updated to clarify that the radio emissions are not from Ganymede per se but are related to Ganymede’s interaction with Jupiter.