  Processes driven by unsteady reconnection can efficiently accelerate
particles in many astrophysical plasmas. An example are the reconnection jet
fronts in an outflow region. We present evidence of suprathermal ion
acceleration between two consecutive reconnection jet fronts observed by the
Magnetospheric Multiscale mission in the terrestrial magnetotail. An earthward
propagating jet is approached by a second faster jet. Between the jets, the
thermal ions are mostly perpendicular to magnetic field, are trapped and are
gradually accelerated in the parallel direction up to 150 keV. Observations
suggest that ions are predominantly accelerated by a Fermi-like mechanism in
the contracting magnetic bottle formed between the two jet fronts. The ion
acceleration mechanism is presumably efficient in other environments where jet
fronts produced by variable rates of reconnection are common and where the
interaction of multiple jet fronts can also develop a turbulent environment,
e.g. in stellar and solar eruptions.
