Radar studies of the equatorial electrojet at three frequencies

Abstract

Radar measurements at 16.25, 49.92, and 146.25 MHz of electrojet irregularities at Jicamarca indicate that (1) the plasma-wave velocities observed for the type I (two-stream) irregularities are approximately proportional with ratios 0.9:1.0:1.1; (2) these velocities can vary throughout the day by as much as 50%; (3) the mean velocities of the type II irregularities are equal at 16 and 50 MHz; (4) the strength of the type II irregularities decreases much more rapidly with wavelength than the strength of the type I, and as a result the type II signal predominates at 16 MHz, but only type I is observed at 146 MHz; and (5) the dependence of the scattering cross section on electrojet strength is much more pronounced at 146 MHz than at 16 MHz, probably because of the different character of the signals. On the basis of these and other observations, we conclude that (1) the Doppler velocity of the type I echoes corresponds to the acoustic velocity rather than the electron streaming velocity, a result contrary to the prediction of linear theory; (2) the acoustic velocity varies with wavelength in at least approximate agreement with theory; and (3) the type II irregularities observed by the radar are probably not excited directly, but rather result from nonlinear interactions among larger irregularities.