Contrasting dynamics of electrons and protons in the near-Earth plasma sheet during dipolarization

The fortunate location of Cluster and the THEMIS P3 probe in the near-Earth plasma sheet (PS) (at X similar to -7-9- R-E) allowed for the multipoint analysis of properties and spectra of electron and proton injections. The injections were observed during dipolarization and substorm current wedge formation associated with braking of multiple bursty bulk flows (BBFs). In the course of dipolarization, a gradual growth of the B-Z magnetic field lasted similar to 13 min and it was comprised of several B-Z pulses or dipolarization fronts (DFs) with duration <= 1 min. Multipoint observations have shown that the beginning of the increase in suprathermal (> 50 keV) electron fluxes - the injection boundary - was observed in the PS simultaneously with the dipolarization onset and it propagated dawnward along with the onset-related DF. The subsequent dynamics of the energetic electron flux was similar to the dynamics of the magnetic field during the dipolarization. Namely, a gradual linear growth of the electron flux occurred simultaneously with the gradual growth of the B-Z field, and it was comprised of multiple short (similar to few minutes) electron injections associated with the B-Z pulses. This behavior can be explained by the combined action of local betatron acceleration at the B-Z pulses and subsequent gradient drifts of electrons in the flux pile up region through the numerous braking and diverting DFs. The nonadiabatic features occasionally observed in the electron spectra during the injections can be due to the electron interactions with high-frequency electromagnetic or electrostatic fluctuations transiently observed in the course of dipolarization. On the contrary, proton injections were detected only in the vicinity of the strongest B-Z pulses. The front thickness of these pulses was less than a gyroradius of thermal protons that ensured the nonadiabatic acceleration of protons. Indeed, during the injections in the energy spectra of protons the pronounced bulge was clearly observed in a finite energy range similar to 70-90 keV. This feature can be explained by the nonadiabatic resonant acceleration of protons by the bursts of the dawn-dusk electric field associated with the B-Z pulses.