Instabilities in the equatorial electrojet

From recent radar data, we draw two new conclusions. First, the so-called magnetic ‘threshold effect’ has little physical significance at radar frequencies of 50 MHz or less and results simply from the way in which past data have been presented. At these frequencies, type II irregularities give strong echoes for electron drift velocities well below the two-stream instability threshold. At 150 MHz, however, only type I irregularities appear to be important, and hence there is a true threshold. Second, our data indicate that the electrojet is highly ‘turbulent’ over distances of hundreds of meters. We believe that it is the local drift velocity within the large-scale irregularities that controls the excitation of instabilities on a smaller scale, and that this local velocity may differ substantially from the mean electron velocity. With this hypothesis, it is possible to account at least qualitatively for the hitherto puzzling observations that (1) type I echoes appear nearly simultaneously over a range of elevation angles, (2) type II and type I echoes are observed with a vertically directed radar, and (3) the characteristic Doppler velocity of the type I echoes is apparently always the ion-acoustic velocity, whereas theory predicts that the irregularity velocity should increase with electron drift velocity when and if the latter exceeds the two-stream threshold.