In situ evidence of ion acceleration between consecutive reconnection jet fronts

Processes driven by unsteady reconnection can efficiently accelerateparticles in many astrophysical plasmas. An example are the reconnection jetfronts in an outflow region. We present evidence of suprathermal ionacceleration between two consecutive reconnection jet fronts observed by theMagnetospheric Multiscale mission in the terrestrial magnetotail. An earthwardpropagating jet is approached by a second faster jet. Between the jets, thethermal ions are mostly perpendicular to magnetic field, are trapped and aregradually accelerated in the parallel direction up to 150 keV. Observationssuggest that ions are predominantly accelerated by a Fermi-like mechanism inthe contracting magnetic bottle formed between the two jet fronts. The ionacceleration mechanism is presumably efficient in other environments where jetfronts produced by variable rates of reconnection are common and where theinteraction of multiple jet fronts can also develop a turbulent environment,e.g. in stellar and solar eruptions.