Unexpected spectral characteristics of VHF radar signals from 150-km region over Jicamarca

The physical mechanisms that cause daytime 150-km echoes at equatorial latitudes are still puzzling. However, there is a general consensus that the radar signals coming from this altitude region should present a narrow spectra (<20 m s(-1)). Moreover 150-km echoes are believed to come from field-aligned irregularities, and to be very aspect sensitive, i.e., they come from regions where the radar line of sight is perpendicular to the magnetic field (B). In this letter, we present evidence of a surprising result: radar signals around 150 km altitudes are also obtained when the radar points few degrees off-perpendicular to B, and more importantly, their spectra are very wide (>1000 m s(-1)). On the other hand, the time and altitudinal characteristics of these off-perpendicular observations are the same as those presented by the so-called 150-km echoes. Moreover, the off-perpendicular echo strengths are several orders of magnitude weaker than perpendicular echoes. Our preliminary interpretation of these results suggests that the spectra of these signals are controlled by ion (electron) dynamics when they are observed off-perpendicular ( \perpendicular) to B.