FIRST RESULTS FROM THE FREJA HF SNAPSHOT RECEIVER

The Freja plasma wave instrument has measured electric field waveforms up to 4 MHz in the auroral ionosphere near 1700 km altitude. The HF s napshot receiver responds to natural signals during every passage thro ugh the auroral ionosphere and we have currently identified two kinds of signals: broadband whistler mode emissions with a cut-off at the pl asma frequency, narrow band Langmuir wave emissions at the plasma freq uency, and mixtures of both wave emissions. The Langmuir wave emission s are frequently narrow band (Delta f/f less than or similar to 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 f ew V/m and epsilon(o)E(2)/2nkT congruent to 10(-2)) the wave spectra b roaden and the waveforms appear to be composed of individual wave pack ets, each with 5-10 wave periods. The narrow band Langmuir waves appea r to be very common and are observed on nearly every auroral zone pass in which precipitating electrons are observed.