In a recent study, Allison et al. found that depletions in plasma allowed electrons to accelerate to ultra-relativistic energies. The study used data from the Van Allen Probes as well as machine learning techniques to make these findings, which were published in Space Advances.
“This study shows that electrons in the Earth’s radiation belt can be promptly accelerated locally to ultra-relativistic energies, if the conditions of the plasma environment — plasma waves and temporarily low plasma density — are right. The particles can be regarded as surfing on plasma waves. In regions of extremely low plasma density they can just take a lot of energy from plasma waves. Similar mechanisms may be at work in the magnetospheres of the outer planets such as Jupiter or Saturn and in other astrophysical objects,” — Yuri Shprits