Particle-in-cell simulations of particle energization in the auroral region

Electromagnetic particle-in-cell (PIC) simulations are performed to study the particle energization mechanisms in the auroral region. First, three-dimensional electromagnetic PIC simulations are conducted to study the properties of current shear-driven (CSD) instabilities, which are driven by the free energy stored in the inhomogeneous transverse magnetic field and associated with the transverse gradient of the field-aligned current. The simulation results demonstrate that the electromagnetic fluctuations generated by CSD instabilities have characteristics similar to the broadband ELF (BBELF) fluctuations observed in the topside auroral region and at higher altitudes. It is also shown that CSD instabilities have the capacity to accelerate ions transversely. Comparison of the PIC simulations with the satellite observations and three-dimensional, two-fluid MHD simulations supports CSD instabilities as potential candidates for the generation of BBELF fluctuations and the correlated transverse acceleration of ions (TAI). Second, two and three-dimensional electromagnetic PIC simulations are performed to study the particle energization by oblique inertial Alfven waves in the auroral region. The results demonstrate the parallel acceleration of electrons and TAI by the nonlinear evolution of an oblique inertial Alfven wave. The wave-particle interactions between the Alfven wave and the electrons accelerate electrons along the geo-magnetic field to form a field-aligned electron beam, which may explain both the time-dispersive electron signature and the suprathermal electron burst observed by spacecraft. The steepening and subsequent breaking evolution of the Alfven wave generates a combination of ion cyclotron and ion acoustic waves which, in turn, heat the ions transversely. The parallel-accelerated electron beam itself may be unstable to ion cyclotron and ion acoustic modes, but it is generally too weak to cause significant TAI. Furthermore, the three-dimensional simulations results confirm that the sheared field-aligned current sheet, embedded in a transversely finite Alfven wave is subject to CSD instabilities. It is very possible that the ion cyclotron and ion acoustic waves generated by the wave breaking process, as well as the low-frequency electromagnetic fluctuations generated by CSD instabilities, are present in observed BBELF fluctuations and contribute to the associated TAI.