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Since October 1983, a new coherent backscatter radar has been in operation at Goose Bay, Labrador, for the purpose of studying small-scale electron density structure in the high-latitude ionosphere. This radar operates over a frequency band that extends from 8 to 20 MHz, and it uses an electronically phased array of 16 log-periodic antennas for both transmission and reception. The radar transmits a seven-pulse pattern that enables one to determine 17-lag complex autocorrelation functions of the backscattered signals as a function of range and azimuth. In this paper we present a complete description of the radar including explanations of the operation of the phasing matrix, the techniques of data acquisition and analysis as implemented in the radar microcomputer, and the possible on-line and automatic operating modes that may be instituted. We also present examples of some of the initial results that we have obtained with the radar during the afternoon and late evening hours. These examples include images of the two-dimensional distribution of small-scale structure and of their associated mean Doppler motion. We also present examples of F region Doppler spectra derived from the complex autocorrelation functions. These Doppler spectra show interesting differences from those of high-latitude E region irregularities. the members of the JHU/APL Electronics Fabrication Group. One of the coauthors (C. H.) is also supported in part by an NSF-CNRS scientific exchange stipend. A portion of the travel funds was provided by NATO grant 529/83 for colloborative research. observations of unstable ionization enhancements in the auroral F region, Geophys. Res. Lett.,

A review of the experimental and theoretical background to the Condor equatorial electrojet campaign is followed by the presentation and discussion of VHF radar interferometer and HF radar backscatter data taken concurrently with two rocket in situ experiments reported in companion papers (Pfaff et al., this issue (a, b). Both experiments were conducted in strongly driven periods with the on‐line radar interferometer displaying signatures of what has been interpreted in earlier radar work (Kudeki et al., 1982) as kilometer scale gradient drift waves. Low‐frequency density fluctuations detected by in situ rocket sensors confirm the earlier interpretation. VHF radar/rocket data comparisons also indicate the existence of a turbulent layer in the upper portion of the daytime electrojet at about 108 km altitude driven purely by the two‐stream instability. Nonlinear mode coupling of linearly growing two‐stream waves to linearly damped 3‐m vertical modes could account for the radar echoes scattered from this layer, which showed no indication of large‐scale gradient drift waves. Nonlinear mode coupling may therefore compete with the wave‐induced anomalous diffusion mechanism proposed recently by Sudan (1983) for the saturation of directly excited two‐stream waves. Nighttime radar data show a bifurcated layer with the two parts having comparable echo strength but oppositely directed zonal drift velocities. The lower layer shows narrow backscatter spectra; the upper layer is characterized by kilometer scale waves and vertically propagating type 1 waves. The charateristics of the topside large‐scale waves show clear consistency with the predictions of nonlocal gradient drift instability theories. The observed sheared flow could be due to neutral winds or a reversal of the vertical polarization fields. Lack of concurrent in situ polarization field or density profile measurements, unfortunately, prevents us from determining the actual cause unambiguously.

In this decade, a new technique for the study of ionospheric electrodynamics has been implemented in an evolving generation of high‐latitude HF radars. Coherent backscatter from electron density irregularities at F region altitudes is utilized to observe convective plasma motion. The electronic beam forming and scanning capabilities of the radars afford an excellent combination of spatial (∼50 km) and temporal (∼1 min) resolution of the large‐scale (∼106 km²) convection pattern. In this paper, we outline the methods developed to synthesize the HF radar data into two‐dimensional maps of convection velocity. Although any single radar can directly measure only the line‐of‐sight, or radial, component of the plasma motion, the convection pattern is sometimes so uniform and stable that scanning in azimuth serves to determine the transverse component as well. Under more variable conditions, data from a second radar are necessary to unambiguously resolve velocity vectors. In either case, a limited region of vector solution can be expanded into contiguous areas of single‐radar radial velocity data by noting that the convection must everywhere be divergence‐free, i.e., ▽ · v = 0. It is thus often possible to map velocity vectors without extensive second‐radar coverage. We present several examples of two‐dimensional velocity maps. These show instances of L shell‐aligned flow in the dusk sector, the reversal of convection near magnetic midnight, and counterstreaming in the dayside cleft. We include a study of merged coherent and incoherent radar data that illustrates the applicability of these methods to other ionospheric radar systems.

Simultaneous data were obtained bet-Various kinds of ground-based facilities also proween the SAFARI HF coherent radars and the EISCAT vide an access to the ionospheric electric field. incoherent scatter experiment in December 1953. Incoherent scatter radars measure most parameters

Common volume observations of E region high‐latitude irregularities at decameter wavelengths have been obtained with the JHU/APL HF radar located at Goose Bay, Labrador, and the SHERPA HF radar located at Schefferville, Quebec. In this paper, we analyze an event with characteristics similar to those of a distinctive type of event described by Villain et al. [1987]. The experimental configuration, which combines the azimuthal‐scanning capability of the Goose Bay radar with the frequency‐scanning operation of the Schefferville radar, has provided unambiguous evidence of the existence of two irregularity layers at different altitudes within the E region. The layers, which exhibit different characteristics, can be related to the action of the gradient drift and ion acoustic instability mechanisms. It is shown that the ion acoustic modes have phase velocities in the range of 400 to 550 m/s and are produced in regions of subcritical perpendicular electron Hall drift. We infer that the observed irregularities are produced through a combination of perpendicular and field‐aligned relative electron‐ion drifts. Features previously observed but not satisfactorily explained by perpendicular drift excitation alone can be understood in terms of field‐aligned drift excitation. We conclude that the role of electron‐ion field‐aligned drift may be much more important than previously realized.

The theory of auroral coherent echoes developed for VHF scattering by Uspensky et al. (1988, 1989) is applied to the interpretation of intensity and Doppler velocity slant range profiles of HF radar aurora. The theoretical model includes the effects of irregularity aspect sensitivity, ionospheric refraction of the radar beam, and the reception of signals from different heights. The predicted profiles of HF radar aurora are compared with Schefferville HF radar observations in the frequency interval of 9–18 MHz. Satisfactory agreement is found between theory and experiment for the intensity profiles. However, there are significant discrepancies for the Doppler velocity profiles. We discuss this lack of agreement in light of other recent observations.

During the Condor campaign a number of instruments were set up in Peru to support the rocket experiments. In this series of papers we report on the results of the experiments designed to study the equatorial F region. In this overview paper we summarize the main results as well as report upon the macroscopic developments of spread F as evidenced by data from backscatter rada•s, from scintillation observations, and from digital ionosonde meaurements. In this latter regard, we argue here that at least two factors other than the classical gravitational Rayleigh-Taylor plasma instability process must operate to yield the longest-scale horizontal organization of spread F structures. The horizontal scale typical of plume separation distances can be explained by invoking the effect of a shear in the plasma flow, although detailed comparison with theory seems to require shear frequencies a bit higher than observations indicate. On the other hand, the largest-scale organization or modulation of the scattering layer cannot be explained by the shear theory and must be due to local time variations in the ionospheric drift or to gravity wave induced vertical motions. Using simultaneous rocket and radar data, we were also able to confirm the oft quoted hypothesis that rapid overhead height variations in the scattering region over