Energetic Electron Acceleration at Reconnection Jet Fronts in the Magnetotail

International audienceMagnetic reconnection plays a key role for particle acceleration in many astrophysical plasmas. Important examples are reconnection during planetary substorms and solar flares. A number of recent numerical simulations as well as of in situ observations in the magnetotail indicate that strong acceleration can occur at reconnection jet fronts, separating jetting from ambient plasma, and in the jet braking region, where jets eventually stop/dissipate. Yet the details of the acceleration mechanisms are not completely understood. Here we present ESA/ Cluster observations of energetic electron (>30 keV) acceleration at the fronts of earthward-directed reconnection jets in the magnetotail. We find that jet fronts are highly structured on the spatial scale of a few ion gyroradii suggesting a filamentation of the reconnection jet possibly due to interchange instability. Strong electron energization is associated to such filamented jet fronts. Electrons are typically energized in the direction perpendicular to the magnetic field indicating betatron acceleration. Waves in the whistler frequency range are associated to the strongest perpendicular flux increases. Yet the observed electron distributions and waves indicate that more complicated acceleration mechanisms can also occur at localized structures within the fronts