On the threshold electric field for the ∼1‐m auroral irregularity appearance

We study the electric field threshold for 1‐m auroral electrojet irregularity excitation, using data from the special Finnish EISCAT experiment held on November 17, 1983. We find that at times irregularities can arise when the ionospheric electric field is 5–15 mV/m, i.e., well below the Farley‐Buneman instability threshold. The electron density fluctuations derived here are increased when the E field becomes more than 15 mV/m. As a result the dependence of electron density fluctuation amplitude on electric fields has a kind of a “soft” E field threshold. Possible reasons for such a dependence are discussed. We show that unresolved ionospheric microstructure in the radar scattering volume can cause this “threshold spreading.”