A 50-MHZ RADIO DOPPLER EXPERIMENT FOR MIDLATITUDE-E REGION COHERENT BACKSCATTER STUDIES - SYSTEM DESCRIPTION AND 1ST RESULTS

Contrary to equatorial and auroral electrojet plasmas, studies on scattering of VHF waves from midlatitude E region field-aligned irregularities are limited, and the physics of the phenomena is not well understood. In this paper we provide first an update of the subject and then present a new experiment, denoted as Sporadic E Scatter Experiment (SESCAT), designed for high-time resolution coherent backscatter measurements of 3-m E region irregularities from a middle latitude location. The technique, which is used successfully in auroral latitudes for years, is capable of measuring the Doppler spectrum of the echoing signal at a fixed range with great accuracy. The experiment is a state of the art bistatic CW Doppler radar operating at 50.52 MHz with the transmitting and receiving arrays beaming northward to a region perpendicular to the Earth's magnetic field at the E region peak. The viewing area, determined by the narrow beam intersecting patterns and the field-aligned character of the irregularities, is fixed at about 15 x 40 km2 and located over the southern Aegean at 30.8-degrees invariant magnetic latitude (L = 1.35). First results show that 50-MHz backscatter events, with snrs as large as 25 dB and lifetimes of several minutes to more than an hour, do exist at midlatitudes in accordance with previous experiments which related these echoes to sporadic E layers. The observed events, except for an early afternoon case, occurred during dark hours in the period between a few hours before and after local midnight. The backscatter can be continuous but in some events a quasi-periodic behavior was found with periods between 2 and 10 min. Associated to the backscatter are symmetric narrow Doppler spectra with mean velocities in the +/- 80 m s-1 range and mean widths about 30 to 60 m s-1. In terms of spectrum width, the midlatitude E region echoes do not compare to the low-velocity type 2 echoes observed in the equatorial and auroral electrojets. Finally, very strong meteor-induced backscatter with abrupt onsets and short lifetimes up to 3 min were also detected. These echoes have relatively large velocities at onset and in one case an ion acoustic velocity component was seen briefly at 320 m s-1, suggesting that the Farley-Buneman instability may also be functioning at times in the midlatitude E region.