Although the Earth’s Van Allen radiation belts were discovered over 50 years ago, the dominant processes responsible for relativistic electron acceleration, transport and loss remain poorly understood. Here we show evidence for the action of coherent acceleration due to resonance with ultra-low frequency waves on a planetary scale. Data from the CRRES probe, and from the recently launched multi-satellite NASA Van Allen Probes mission, with supporting modelling, collectively show coherent ultra-low frequency interactions which high
energy resolution data reveals are far more common than either previously thought or observed. The observed modulations and energy-dependent spatial structure indicate a mode of action analogous to a geophysical synchrotron; this new mode of response represents a significant shift in known Van Allen radiation belt dynamics and structure. These periodic collisionless betatron acceleration processes also have applications in understanding the dynamics of, and periodic electromagnetic emissions from, distant plasma-astrophysical systems.