First results from the Freja HF snapshot receiver

Abstract

The Freja plasma wave instrument has measured electric field waveforms up to 4 MHz in the auroral ionosphere near 1700 km altitude. The HF snapshot receiver responds to natural signals during every passage through the auroral ionosphere and we have currently identified two kinds of signals: broadband whistler mode emissions with a cut‐off at the plasma frequency, narrow band Langmuir wave emissions at the plasma frequency, and mixtures of both wave emissions. The Langmuir wave emissions are frequently narrow band (Δf/f ⪝ 10 −2 ) and exhibit a variety of modulational features. These Langmuir waves exist up to amplitudes of roughly 1 volt/m. At larger amplitudes (a few V/m and ϵ o E²/2nkT ≅10 −2 ) the wave spectra broaden and the waveforms appear to be composed of individual wave packets, each with 5–10 wave periods. The narrow band Langmuir waves appear to be very common and are observed on nearly every auroral zone pass in which precipitating electrons are observed.