Turbulent Plasma Jet Fronts and Related Ion Acceleration

We investigate a series of earthward bursty bulk flows (BBFs) observed by the Magnetospheric Multiscale (MMS) spacecraft in the Earths magnetotail (XGSM ~ -23.88 RE, YGSM ~ 6.72 RE, ZGSM ~ 4.06 RE). 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 (~1 RE) dipolarizations, and another with a large scale (~10 RE) dipolarization. Although the two structures have different scales, both of these two structures are associated with flux increases of the supra-thermal ions (~200 keV). We investigate the ion acceleration mechanism, its dependence on the mass/charge-state and discuss the adiabaticity of the energisation process. We show that the ions are accelerated beyond the maximum energy reachable for an ion that undergoes direct acceleration by cross-tail electric field. We show that the energy changes associated with the small scale structures exhibit deviations from the conservation of the magnetic moment, which, however, remains valid in the case of the large structure.