Fig. 4: Electron energy and velocity distributions within the Callisto flux tube.

a Electron energy distribution. Colored curves represent the distribution measured with a 1 s resolution during the flux tube crossing. The gray curves highlight the electron distributions measured during 30 s prior to the crossing, outside of the Callisto flux tube. b Electron distribution function in the velocity space \(({{{\rm{v}}}}_{{{\rm{||}}}},{{{\rm{v}}}}_{{{\rm{ \perp }}}})\) at the beginning of the flux tube crossing. The size of the loss cone is indicated by the red line. The gray circle indicates the resonance circle that maximizes the growth rate, enabling wave amplification. Note that at \({{{\rm{v}}}}_{{{\rm{||}}}}\) = 0 m.s^-1, the iso-energy contours decrease, which is due to spacecraft shadowing⁸⁸.