Low-latitude field-aligned irregularities observed in the E region with the Piura VHF radar:: First results

We have used the Piura VHF radar (5 degrees 12'S, 80 degrees 38'W, similar to 7.5 degrees N geomagnetic latitude, just outside the magnetic equator) in northern Peru to gather echoes from 3-m E region field-aligned irregularities. We present statistical results of these echoes: percentage of occurrence and histograms of radial velocities and spectral widths, obtained with a 15-day data set gathered in 1996. These E region echoes are confined to 95- to 120-km altitude and present spectral characteristics similar to equatorial electrojet (EEJ) echoes generated by a gradient drift, instability, i.e., type 2. However, they appear mainly at nighttime and early morning (1800-0800 LT) and therefore do not present a temporal similarity to EEJ echoes. Moreover, we observe the existence of two well-defined types of echoes: (1) lower E region echoes (95-105 km) and upper E region echoes (105-120 km). Both regions show different spectral and temporal characteristics. It is difficult to determine at this point the physical mechanisms responsible for the formation of the irregularities without measurements of E fields and density gradients, which we plan to do in a future experiment.