Suprathermal electron acceleration in the near‐Earth flow rebounce region

Abstract

Flux pileup regions (FPRs) are traditionally referred to the strong‐Bz bundles behind dipolarization fronts (DFs) in the Earth's magnetotail and can appear both inside earthward and tailward bursty bulk flows. It has been widely reported that suprathermal electrons (40–200 keV) can be efficiently accelerated inside earthward FPRs, leaving the electron acceleration inside tailward FPRs as an open question. In this study, we focus on the electron acceleration inside a tailward FPR that is formed due to the flow rebounce in the near‐Earth region (XGSM ≈ −12 RE) and compare it quantitatively with the acceleration inside an earthward FPR. By examining the Cluster data in 2008, we sequentially observe an earthward FPR and a tailward FPR in the near‐Earth region, with the earthward one belonging to decaying type and the tailward one belonging to growing type. Inside the earthward FPR, Fermi acceleration and betatron cooling of suprathermal electrons are found, while inside the tailward FPR, Fermi and betatron acceleration occur. Whistler‐mode waves are observed inside the tailward FPR; their generation process may still be at the early stage. We notice that the suprathermal electron fluxes inside the tailward FPR are about twice as large as those inside the earthward FPR, suggesting that the acceleration of suprathermal electrons is more efficient in the flow rebounce region. These acceleration processes have been successfully reproduced using an analytical model; they emphasize the role of flow rebounce in accelerating suprathermal electrons and further reveal how the MHD‐scale flow modulates the kinetic‐scale electron dynamics in the near‐Earth magnetotail.

Statistical Properties of Sub‐Ion Magnetic Holes in the Dipolarized Magnetotail: Formation, Structure, and Dynamics

“…Note that adiabatic heating is expected to be the dominant energization process for high‐energy electrons that are not significantly affected by nonadiabatic wave‐particle interactions around the front (Fu et al, ; Fu et al, ). Although, for this population, the adiabatic heating model agrees well with spacecraft observations (e.g., Liu, Fu, Xu, Wang, et al, ; Liu, Fu, Xu, Cao, et al, ; Liu, Fu, Cao, Xu, Yu, et al, ), such high‐energy electrons likely cannot be trapped in MHs. Thus, we consider here the thermal electron population (dominating in the electron temperature; heating of this population has been shown to scale with | B z |; see Runov et al, ).…”

Rapid Pitch Angle Evolution of Suprathermal Electrons Behind Dipolarization Fronts

“…The linear fitting of energy gain (dashed lines) is consistent with that suggested in adiabatic model (e.g., Fu et al, ; Liu, Fu, Xu, Cao, et al, ; Pan et al, ). For suprathermal electrons, since the PSD decreases in the perpendicular direction (Figures a and b), betatron cooling thus indeed works during the evolution (e.g., Fu et al, ; Fu, Khotyaintsev, et al, ; Liu, Fu, Xu, Wang, et al, and Liu, Fu, Xu, Cao, et al, ). However, for the low‐energy electrons (<30 keV), for example, electrons at PEACE energy ranges, we find that they were accelerated during the evolution (their energy gains are positive).…”

“…Considering that the entropy parameter is conserved during these two stages, the expansion should be quasi‐adiabatic (e.g., Dubyagin et al, ; Wolf et al, ). Expansion of the flux tube certainly leads to the betatron cooling of suprathermal electrons (e.g., Fu et al, ; Fu, Khotyaintsev, et al, ; Liu, Fu, Xu, Wang, et al, and Liu, Fu, Xu, Cao, et al, ), and consequently the drop of PSD in the perpendicular direction (see Figures a and b). To investigate whether the betatron cooling is indeed responsible for the temporal evolution in these two stages, we analyze the energy gain of electrons during the evolution.…”

“…During the earthward propagation of DFs, suprathermal electrons (40–200 keV) are frequently observed (e.g., Asano et al, ; Fu et al, ; Fu, Khotyaintsev, et al, ; Huang et al, ; Vaivads et al, ; Zhou et al, ), particularly inside the magnetic field pileup region (FPR) (Khotyaintsev et al, ) or the dipolarizing‐flux bundles (DFBs) (Liu et al, ). These electrons are either energized by adiabatic processes (e.g., Ashour‐Abdalla et al, ; Birn et al, ; Duan et al, ; Fu et al, ; Gabrielse et al, ; Liu, Fu, Xu, Wang, et al, ; Liu et al, ; Lu et al, ; Pan et al, , ; Wu et al, ) or nonadiabatic processes (e.g., Deng et al, ; Fu et al, ; Hwang et al, ; Khotyaintsev et al, ; Kronberg et al, ; Liu, Fu, Xu, Wang, et al, ; Liu et al, ; Panov et al, ; Yang et al, ; Zhou et al, ). Specifically, the adiabatic processes elevate electron fluxes in the field‐aligned direction through the Fermi acceleration (e.g., Fu et al, ; Liu, Fu, Xu, Wang, et al, ; Wu et al, ) or elevate electron fluxes in the perpendicular direction through the betatron acceleration (e.g., Fu et al, ; Liu, Fu, Xu, Wang, et al, ; Wu et al, ); the nonadiabatic processes scatter electrons from field‐aligned direction to perpendicular direction (during electron energization) or inversely from perpendicular direction to field‐aligned direction (during electron deceleration) (e.g., Hwang et al, ; Khotyaintsev et al, ).…”

Broadband high‐frequency waves detected at dipolarization fronts

“…The electric structures at the DFs [ Zhou et al ., ; Fu et al ., ; Sun et al ., ] as well as the acceleration of electrons [e.g., Fu et al ., , ; Ashour‐Abdalla et al , ; Lu et al ., ; Duan et al ., ] and ions [e.g., Zhou et al ., ; Artemyev et al ., ] inside the FPRs have been widely reported. Usually, the electron acceleration can be attributed to either adiabatic process associated with betatron and Fermi mechanisms [ Fu et al ., , ; Liu et al ., ] or nonadiabatic process associated with wave‐particle interaction.…”