Electric field and electron density thresholds for coherent auroral echo onset

Simultaneous observations of the Polar Geophysical Institute 83‐MHz auroral radar and the European Incoherent Scatter facility were used to study the coherent echo threshold phenomenon. It is shown that, for electric fields not far from the Farley‐Buneman instability threshold of ∼20 mV/m, the onset of echo occurrence was due to an electron density increase above some threshold value. This value was estimated to be ∼2.5 × 1011 m−3. For smaller electric fields of 5‐10 mV/m, echoes were not observed even for electron densities double the above threshold value. The importance of both factors, electric field and electron density, and some other effects are discussed in light of recent studies of coherent echo intensities and their dependence on plasma conditions at electrojet altitudes.