Electron and Ion Acceleration in the Unstable Auroral Ionosphere

Abstract

It is demonstrated that energetic electrons accelerated in the field-aligned electric wavefield of kinetic Alfvén waves in near Earth space destabilize the ionosphere to secondary Alfvén waves via the well known ionospheric feedback instability. From spacecraft observations and simulations it is shown how this instability may drive waves at wavelengths perpendicular to the geomagnetic field of the order of the electron inertial length (λe) and ion gyro-radius (ρi). These secondary waves then lead to further electron acceleration and ion trapping. The trapping process results in coherent acceleration up to energies where the ion gyro-radius is equal to half the wavelength of the wave transverse to the geomagnetic field (λ⊥). For sufficiently large wave amplitudes at these scales the orbit of the ions in the wavefield is disrupted and may become stochastic as has been suggested previously from observations by others.