That interpretation was based on the conclusion that the observed fluctuations were predominantly right elliptically polarized in the solar wind rest frame.
Instabilities excited by protons appear unlikely to account for the observations.
We conclude that a resonant instability excited by relativistic electrons streaming along the magnetic field away from the Jupiter is a likely source of free energy consistent with the observations. Evidence for the existence of such a population of electrons has been found in both the Low Energy Charged Particle experiments and cosmic ray experiments on Voyager 2. This new interpretation is reminiscent of observations reported by Smith et al.