Fermi and betatron acceleration of suprathermal electrons behind dipolarization fronts

Abstract

[1] Two dipolarization front (DF) structures observed by Cluster in the Earth midtail region (X GSM ≈ −15 R E ), showing respectively the feature of Fermi and betatron acceleration of suprathermal electrons, are studied in detail in this paper. Our results show that Fermi acceleration dominates inside a decaying flux pileup region (FPR), while betatron acceleration dominates inside a growing FPR. Both decaying and growing FPRs are associated with the DF and can be distinguished by examining whether the peak of the bursty bulk flow (BBF) is co-located with the DF (decaying) or is behind the DF (growing). Fermi acceleration is routinely caused by the shrinking length of flux tubes, while betatron acceleration is caused by a local compression of the magnetic field. With a simple model, we reproduce the processes of Fermi and betatron acceleration for the higher-energy (>40 keV) electrons. For the lower-energy (<20 keV) electrons, Fermi and betatron acceleration are not the dominant processes. Our observations reveal that betatron acceleration can be prominent in the midtail region even though the magnetic field lines are significantly stretched there.

Energetic electron acceleration by unsteady magnetic reconnection

“…Figure 2f shows the pitch angle distribution of the energetic electrons. During the rising speed portion of the plasma jets the electrons tend to be perpendicular to the magnetic field, whereas during the falling speed portion of the jets the electrons tend to be parallel to the field, as reported recently 24,26 . This characteristic is clearer in Fig.…”

“…1c). In this situation, the local magnetic field is enhanced and betatron acceleration occurs 24 . During the jet speed falling, the trailing part of the tube is moving away from the leading part of the tube; the volume of flux tubes expands, so that the localized betatron cooling occurs.…”

“…Each jet consists of a rising speed portion (orange) and a falling speed portion (blue). In the rising speed portion of the jets, the flux tubes contract and localized betatron acceleration of electrons occurs 24 ; in the falling speed portion, the flux tubes expand and localized betatron cooling of electrons is expected.…”

Whistler mode waves at magnetotail dipolarization fronts

“…We see that v x increases in the FPR up to a maximum of 450 km/s. The velocity peak is located behind the DF, so in this case we are observing a growing flux pileup region [Fu et al, 2011]. The plasma flow velocity is not high enough to cause significant Doppler shift of the measured frequency (maximum frequency shift is 5%).…”

“…DFs are identified by a sharp increase of B z GSM and are associated with electron and ion acceleration [Asano et al, 2010;Zhou et al, 2010;Fu et al, 2011] as well as various wave activities, e.g., electron holes, whistler, lower hybrid, and electron cyclotron waves [Le Contel et al, 2009;Sergeev et al, 2009;Zhou et al, 2009;Deng et al, 2010;Khotyaintsev et al, 2011;Hwang et al, 2011].…”