Broadband ELF plasma emission during auroral energization: 1. Slow ion acoustic waves

Abstract

High‐resolution measurements by the Freja spacecraft of broadband extremely low frequency (BB‐ELF) emission from dc up to the lower hybrid frequency (a few kHz) are reported from regions of transverse ion acceleration (TAI) and broad‐energy suprathermal electron bursts (STEB) occuring in the topside ionospheric auroral regions. A gradual transition of the broadband emission occurs near the local O+ cyclotron frequency (ƒO+ ≈ 25 Hz) from predominantly electromagnetic below this frequency to mostly electrostatic above this frequency. The emission below 200 Hz often reach amplitudes up to several hundred mV/m and density perturbations (δn/n) of tens of %. An improved analysis technique is presented, based on the quantity |δE/(δn/n)| versus frequency and applied to the Freja plasma wave measurements. The method can be used to infer the dispersion relation for the measured emission as well as give estimates of the thermal plasma temperatures. The BB‐ELF emission is found to consist partly of plasma waves with an ion Boltzmann response, which is interpreted as originating from the so‐called slow ion acoustic wave mode (SIA). This emission is associated with large bulk ion (O+) temperatures of up to 30 eV and low electron temperatures (1–2 eV) and therefore occurs during conditions when Te/Ti ≪ 1. The BB‐ELF emissions also contain other wave mode components, which are not equally easy to identify, even though it is reasonably certain that ion acoustic/cyclotron waves are measured. The ion Boltzmann component is characterized by a dominantly perpendicular polarization with respect to the Earth's magnetic field direction and a small magnetic component with amplitudes around 0.1–1 nT. The ion Boltzmann component dominates the lower‐frequency part (30–400 Hz) of the BB‐ELF emissions. The BB‐ELF emission have often an enhanced spectral power when certain waveform signatures, interpreted as solitary kinetic Alfvén waves (SKAW), or when large‐amplitude electric fields, possibly related to black aurora, are encountered in regions often associated with large‐scale auroral density depletions. A scenario where the SKAW provides the original free energy and via the BB‐ELF emission causes intense transverse ion heating (TAI) is suggested.