No 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.

True random number generator (TRNG) is a critical component in hardware security that is increasingly important in the era of mobile computing and internet of things. Here we demonstrate a TRNG using intrinsic variation of memristors as a natural source of entropy that is otherwise undesirable in most applications. The random bits were produced by cyclically switching a pair of tantalum oxide based memristors and comparing their resistance values in the off state, taking advantage of the more pronounced resistance variation compared with that in the on state. Using an alternating read scheme in the designed TRNG circuit, the unbiasedness of the random numbers was significantly improved, and the bitstream passed standard randomness tests. The Pt/TaO /Ta memristors fabricated in this work have fast programming/erasing speeds of ∼30 ns, suggesting a high random number throughput. The approach proposed here thus holds great promise for physically-implemented random number generation.

Ultralow frequency (ULF) waves have long been known to resonate with magnetospheric charged particles through their drift and bounce motions. Most research interest has focused on the resonance with drift motion, which can accelerate charged particles at very high energies. The role of the bounce motion, especially for particles with lower energies, has attracted less attention so far. Here we start from the general theory of wave-particle interactions to predict the characteristic, observable signatures of drift-bounce resonance. Such signatures can be described in the particle pitch angle spectrum as a series of inclined stripes, with the inclination angle depending on the latitude of the observing spacecraft. Each stripe is also twisted at two conjugated pitch angles, suggesting significant phase shifts across resonant pitch angles. These predicted signatures are found consistent with observations from the THEMIS (Time History of Events and Macroscale Interactions during Substorms) spacecraft, and therefore provide an identification of drift-bounce resonance together with a validated picture over the importance of particle's bounce motion in the ULF wave-particle interactions.