Abstract
Abstract. The fortunate location of Cluster and the THEMIS P3 probe in the near-Earth plasma sheet (PS) (at X ∼ −7–−9 RE) 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 BZ magnetic field lasted ∼ 13 min and it was comprised of several BZ 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 BZ field, and it was comprised of multiple short (∼ few minutes) electron injections associated with the BZ pulses. This behavior can be explained by the combined action of local betatron acceleration at the BZ 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 BZ 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 ∼ 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 BZ pulses. Keywords. Magnetospheric physics (Magnetotail; plasma sheet) – Space plasma physics (Transport processes)
Highlights
Rapid increase in the northward component of the Earth’s magnetotail magnetic field (BZ), called dipolarization, is one of the key processes in tail dynamics
Spacecraft observations have shown that rapid enhancements in the BZ field represent spatial structures – dipolarization fronts (DFs) – which are typically observed at the leading edge of the earthwardmoving bursty bulk flows (BBFs)
The dipolarization was associated with braking and azimuthal diversion of multiple bursty bulk flows with DFs and it was followed by substorm current wedge (SCW) formation
Summary
Rapid increase in the northward component of the Earth’s magnetotail magnetic field (BZ), called dipolarization, is one of the key processes in tail dynamics. Zhou et al (2011) traced protons coming from the solar wind in the time-dependent magnetic and electric fields obtained from a global MHD simulation of a substorm They explained the observation of dispersed ion injections by acceleration in two sources: (i) around the near-Earth X line (at X ∼ −20 RE), where particles are mostly accelerated nonadiabatically by strong electric fields (both inductive and potential), and (ii) in several localized regions between X = −7 RE and X = −18 RE, where particles were accelerated in nonadiabatic motion under the potential electric field.
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