  We investigate a series of Earthward bursty bulk flows (BBFs) observed by the
Magnetospheric Multiscale (MMS) spacecraft in Earth's magnetotail at (-24, 7,
4) RE in Geocentric Solar Magnetospheric (GSM) coordinates. At the leading
edges of the BBFs, we observe complex magnetic field structures. In particular,
we focus on one which presents a chain of small scale (~0.5 RE)
dipolarizations, and another with a large scale (~3.5 RE) dipolarization.
Although the two structures have different scales, both of these structures are
associated with flux increases of supra-thermal ions with energies > 100 keV.
We investigate the ion acceleration mechanism and its dependence on the mass
and charge state. We show that the ions with gyroradii smaller than the scale
of the structure are accelerated by the ion bulk flow. We show that whereas in
the small scale structure, ions with gyroradii comparable with the scale of the
structure undergo resonance acceleration, and the acceleration in the larger
scale structure is more likely due to a spatially limited electric field.
