Ponderomotive upward acceleration of ions by ion cyclotron and Alfvén waves over the polar regions

Abstract

A ponderomotive theory that describes the acceleration of magnetospheric ions along the geomagnetic field lines is developed and compared to Viking and Freja satellite observations showing that low‐frequency electric field fluctuations correlate with the upward escape of ions. From the general expressions of the averaged field‐aligned wave forces, we develop a theory for the nonresonance acceleration of the polar wind and resonant acceleration of a small admixture of heavy ions under the action of Alfvén and ion cyclotron waves. Using a hydrodynamic polar wind model, we show analytically and numerically that the critical point of supersonic transition shifts toward the Earth under the ponderomotive action of the waves. Furthermore, the ion velocity, as well as the acceleration of the flow, increases as the wave amplitude increases. It is shown that the critical velocity always equals the local sound velocity, independent of whether Alfvén waves are present or not. The asymptotic behavior of the flow (far above the critical point) is also modified substantially under the action of Alfvén waves. In particular, the mere existence of waves in the flow causes an increasing upward acceleration with increasing distance from the Earth. We emphasize that the subsonic polar breeze is not subject to a qualitative modification under the action of Alfvén waves, as opposed to the supersonic polar wind. In addition, we present a ponderomotive mechanism for the generation of field‐aligned small‐scale plasma density depletions which have been discovered by polar‐orbiting satellites. The acceleration of heavy ions has been examined from the viewpoint of single‐particle motion. The emphasis is on a phenomenon termed ponderomotive resonance. It is found that the energy gain of heavy ions increases as the first power of the wave amplitude provided the ion intersects the resonance region from the bottom upward, despite the fact that the ponderomotive force (“magnetic moment pumping”) is proportional to the second power of the wave amplitude. This leads to fast acceleration of heavy ions provided that high‐amplitude waves propagate along geomagnetic field lines.