Statistical Study of Betatron and Fermi Electron Acceleration at Dipolarization Fronts

Journal article, 2026

Magnetic reconnection jets in Earth's magnetotail are regions of electron heating and acceleration. They are often associated with dipolarization fronts (DFs), which are characterized by sharp increases in the northward magnetic field component . Electrons can be accelerated at DFs mainly through the betatron and Fermi mechanisms. We perform a statistical study of betatron and Fermi electron rate of energy change at Earthward-propagating DFs observed by the Magnetospheric Multiscale (MMS) spacecraft. Our results show that betatron acceleration is dominant ahead of DFs, where suprathermal electron fluxes perpendicular to the magnetic field are detected. Up to ion inertial lengths inside the outflow, Fermi acceleration acts on the low-density electron population, potentially driving them to high energies (similar to 100 ). Spatially in the magnetotail, betatron acceleration is statistically larger near the most probable X-line location, while Fermi-dominated events are observed closer to the Earth.