Field-aligned acceleration of ions by the Alfvén waves in the polar regions Original Research Article

Ponderomotive field-aligned acceleration of ions by Alfvén waves is a fundamental process in terrestrial and celestial plasmas. Ponderomotive forcing may transfer wave energy and momentum to plasmas on small scales as well as on large scales. The process is expected to play an important role in energizing ionospheric ions over the polar region, leading to outflow of ions into the outer magnetosphere. We discuss the acceleration of upward moving heavy ions like O+ and how they behave while approaching the resonant altitude, i.e., where the gyrofrequency equals the wave frequency. In the vicinity of the resonant altitude the so-called magnetic moment pumping (MMP) process dominates [Lundin, R., Hultqvist, B. Ionospheric plasma escape by high-amplitude electric fields: magnetic moment “pumping”. J. Geophys. Res. 94, 6665–6680, 1989; Guglielmi, A., Lundin, R. Ponderomotive upward acceleration of ions by ion cyclotron and Alfvén waves over the polar regions. J. Geophys. Res. 106, 13,219–13,236, 2001]. The problem is to remove the singularity, the pole, of the MMP ponderomotive potential at the resonance altitude where the solution, the heavy ion energy goes to infinity. To eliminate this singularity we present three different modifications to remove the pole of the MMP force equation: (a) assuming a finite transverse size of the wave field, (b) introducing an effective collision frequency, and (c) assuming a finite ion-transit time.