Ion Cyclotron (IC) Oscillations Excited by Nonlinear Waves Propagating in Collision-free Auroral Ionosphere

Abstract

We study ion cyclotron (IC) oscillations activated by a stochastic, strong space-charge electric wavefield E of nonlinear waves propagating auroral ionosphere. E is in a plane perpendicular to the ambient magnetic field B. The word “strong” means that (1) the conventional linear plasma wave model connected to a perturbed electric field is not suitable to be employed; (2) the E × B drift is comparable to (even higher than) the thermal speed of particles, and drive them away from the initial thermal equilibrium. A physical model is set up for a dense cluster of electron soliton trains with which a magnetic flux tube is teeming. Then, the collision-free Boltzmann equation is solved under the condition that E is temporally constant. With a nonzero initial guiding-center (GC) velocity, ions are found to follow a double-circle trajectory in velocity space with an IC oscillation frequency ω which shifts from the magnetic gyrofrequency Ω = eB/mi (where eand mi are the charge and mass of the singly ionized ions, respectively). Furthermore, the “constant” condition is relaxed by using a simple stochastic E which has 10-step random strengths in 10 different time intervals. The accommodation of ω (as well as other parameters) is illustrated in response to the E switches. At last, the work is generalized by using two random-number generators for the strength and time, respectively. In this case, ω can be shifted to several Ω values. This result is in good agreement with what FAST satellite measured in auroral field-aligned current regions.