Parallel electric field structures associated with the low-frequency oscillations in the auroral plasma

Abstract

The nonlinear evolution of low-frequency electrostatic oscillations in a magnetized plasma consisting of protons, electrons and oxygen ion beams has been studied. The fluid equations have been used for the oxygen beam, whereas the Boltzmann distributions are used for the protons and electrons. The coupled system of equations are reduced to a single nonlinear differential equation in the rest frame of the propagating wave for any direction of propagation with respect to the ambient magnetic field. This nonlinear differential equation is solved numerically for the parameters charateristic of the auroral acceleration region. Depending on the wave Mach number, proton and oxygen ion concentrations, and driving electric field, the numerical solutions show a range of periodic solutions varying from sinusoidal to sawtooth and highly spiky waveforms. The effects of the plasma parameters, in particular the oxygen ion concentration and the proton temperature on the evolution of the nonlinear waves are examined. The results from the model are compared with satellite observations.

Nonlinear low-frequency structures in the auroral plasma in the presence of an oxygen beam including charge separation

“…This effect is more significantly in the case of the nonlinear structures ͑i.e., for larger E 0 ͒. For E 0 = 0.8, the period of the spiky electric field of Reddy et al 43 was found to be 4.0 ci compared to 3.5 ci in our case for the same parameters. The value of this period ͑3.5 ci ͒ is consistent with the measurements for E ʈ observed by the FAST satellite in Fig.…”

“…In such situations the term on the left hand side of ͑12͒ can be neglected. This limit would then correspond to the case of quasineutrality as discussed by Reddy et al 43 Consequently our findings in the limit of a strongly magnetized plasma ͑low R values͒ represents an extension of their work. In Fig.…”

“…We have extended the work of Reddy et al 43 by including the Poisson equation ͑their studies were restricted to the quasineutrality condition͒. We have shown that the structures vary from sinusoidal to sawtooth to highly spiky, depending on parameter selections.…”

“…28 Their analysis was based on the quasineutral approximation and could lead to a single nonlinear differential equation in the rest frame of the propagating wave. In this paper, we extend the work of Reddy et al 43 by including the Poisson equation, thereby allowing for the charge separation effect and numerically solve the resulting set of coupled nonlinear equations. In this paper, we assume phase velocity to be greater than the ion beam thermal speed, hence, the assumption of a cold ion beam.…”

“…A relationship between pulse widths and periods of the ESWs was found to be in compliance of Geotail observations. 41 Motivated by the FAST observations of waves near the oxygen cyclotron frequencies in the auroral region, Reddy et al 42,43 studied nonlinear low-frequency electrostatic waves in a magnetized plasma consisting of protons, electrons, and oxygen ion beams. They showed that a range of periodic solutions varying from sinusoidal to sawtooth and highly spiky wave forms could be obtained depending on the plasma parameters considered.…”