Nonlinear features of electrostatic ion cyclotron instability driven by counterstreaming ion beams in equatorial outer plasmasphere

Abstract

Abstract. A commonly observed feature of the field-aligned flow of ions on closed geomagnetic field lines is that the pitch angle distributions are considerably broadened. Such pitch angle distributions facilitate in trapping ions and thus contribute to the process of plasmaspheric refilling. Since counterstreaming ion beams are a common feature of the early stage refilling as shown by models, we have investigated the role of ion-beam-driven instabilities in broadening the pitch angle distribution of the ion beams. The study is performed using a 2.5-dimensional particle-in-cell code. It is shown that for sufficiently fast ion beams as expected in the outer region of the plasmasphere, the ion cyclotron modes, which appear at frequencies less than the ion-cyclotron frequency •'•i in the rest frame of the nondrifting electrons, are the dominant unstable wave modes. In the early stage of the instability a nearly purely growing mode dominates, which efficiently mediates in transferring a part of the parallel drift energy into the perpendicular energy, significantly broadening the ion pitch angle distribution. The transfer abruptly saturates when d•(/q_p•f)-->0, where .In( ) is the Bessel function of an appropriate order n, kñ is the perpendicular wave number of the dominant mode, and p•f is the maximum value of the Larmor radius of the beam ions in anomalous cyclotron resonance with the wave. This behavior of the dominant mode in conjunction with the conservation of ion energy allows a method for including such wave-particle interaction effects in mesoscale models for the plasmaspheric refilling. The perpendicular acceleration of ions by this instability reduces the parallel drift only by a relatively small fraction, leaving the counterstreaming intact. However, the perpendicular acceleration produces a pitch angle distribution which is inducive for the trapping of the counterstreaming ions in the plasmaspheric flux tubes.

Numerical simulation of waves driven by plasma currents generated by low‐frequency Alfvén waves in a multi‐ion plasma

“…We use a standard 2.5‐D electrostatic particle‐in‐cell code [ Birdsal and Langdon , 1985], which we have described in previous studies [ Singh and Leung , 1999; Singh , 2000]. Specifically, we simulate two‐dimensional plasma of dimension L x × L y in the x‐y plane with periodic boundary conditions on both particles and fields.…”

Inhomogeneous magnetic-field-aligned ion flow measured in a Q machine

“…14 The separate observations in the ionosphere of counterstreaming beams and multiple wave harmonics separated by half the ion-cyclotron frequency is interesting in light of the theory of beam-driven ion cyclotron waves that predicts such a frequency spectrum. [15][16][17][18] Counterstreaming ion beams with inhomogeneous density profiles perpendicular to the magnetic field are observed by the Fast Auroral Snapshot ͑FAST͒ satellite. 19 Instruments aboard this satellite are capable of making high-resolution measurements of ion distributions.…”

“…Singh and Leung use a 2.5-dimensional particle-in-cell code to study the early and late nonlinear stages of the purely growing mode and of the ion-cyclotron mode at half the ion gyrofrequency. 18 They investigate the effect of ion-temperature anisotropy on the nonlinear evolution of the instability, concentrating on the zero-frequency mode that results from the coupling of the fundamental Doppler-shifted ion-cyclotron modes associated with the two counterstreaming beams.…”

End-to-end study of the transfer of energy from magnetosheath ion precipitation to the cusp

“…The electrostatic PIC model is that described previously by Singh and Leung (1999). Periodic boundary conditions on both particles and fields are used.…”