  In this study we use the Magnetospheric Multiscale (MMS) mission to
investigate the electron acceleration and thermalization occurring along the
magnetic reconnection separatrices in the magnetotail. We find that initially
cold electron lobe populations are accelerated towards the X line forming beams
with energies up to a few keV's, corresponding to a substantial fraction of the
electron thermal energy inside the exhaust. The accelerated electron
populations are unstable to the formation of electrostatic waves which develop
into nonlinear electrostatic solitary waves. The waves' amplitudes are large
enough to interact efficiently with a large part of the electron population,
including the electron beam. The wave-particle interaction gradually
thermalizes the beam, transforming directed drift energy to thermal energy.
