In this paper we present the first results of the mapping of the spatial distribution of auroral echoes by a 398‐MHz phased array radar recently installed at Homer, Alaska. The results are shown in the form of two‐dimensional spatial maps of auroral echoes. These maps are classified into types that are extensions of the ‘diffuse’ and ‘discrete’ echo types introduced by Leadabrand et al. (1959) and Presnell et al. (1959) for narrow beam radars. The auroral echo signatures typically seen during the afternoon and evening sectors are then described and discussed in relation to radar auroral echo characteristics found by previous investigations and other auroral processes. We show that interpretation of the occurrence of auroral echoes in terms of control by an electric field and by electron density in the radar scattering volume produces a consistent morphology of the radar aurora in relation to the other observed auroral processes. Finally, we include a dynamic auroral echo signature that may be related to the growth phase of an auroral substorm.

This paper presents the results of a study to determine the relationships between radar aurora, visual aurora, and auroral electrojets in the evening sector. The data, taken on March 21, 1973 (UT), consisted of (1) auroral echo maps obtained with a 398-MHz phased array radar located at Homer, Alaska, (2) magnetograms and all-sky-camera photographs taken along the Alaskan meridian network of stations, and (3) Chatanika incoherent scatter radar measurements. The study showed that diffuse auroral echoes obtained at 398 MHz can be used to map the spatial distribution of the eastward electrojet. Discrete visual arcs were almost invariably found to occur poleward of (but often adjacent to) the eastward electrojet (as defined by the diffuse radar aurora) and within the westward electrojet region (as defined by ground magnetometer data). When discrete radar echoes were observed, they were associated only with bright visual arcs. Furthermore, the association was usually confined to only the most equatorward visual arcs located closest to the poleward boundary of the eastward electrojet. This confinement was interpreted as being produced by a decrease with latitude in electric field strength, associated with the Harang discontinuity region.

During routine UHF auroral radar investigations an unusual daytime auroral effect has been discovered. It apparently occurs most frequently when: (1) the reflecting region is sunlit; (2) the atmosphere is undergoing its greatest change (early morning and late afternoon). There is a minimum of echo occurrence at noon when atmospheric conditions are stable. Daytime aurora is distributed over a larger region of space than the more commonly observed night‐time aurora. The night‐time and daytime echoes are labeled discrete and diffuse, respectively. They can be differentiated in several ways. Discrete echoes are identified by their relatively short duration, their occurrence only at night, and their orientation in the E‐layer along a plane at right angles to radar beam; hence, the echo does not shift in range with change in elevation angle of the radar antenna. Diffuse echoes last longer, occur only during the day, and are apparently oriented in the E‐layer along a plane almost parallel to the surface of the earth; hence, the echo does shift in range when the radar‐antenna elevation angle is changed. The primary effects of increasing the observation frequency are decreasing echo amplitudes and decreasing maximum off‐perpendicular angle. The observed aspect sensitivity and the wavelength dependence are interpreted in terms of the scattering approach of Booker. Using the experimental UHF results, a model of the underdense ionosphere has been developed consisting of irregularities which have dimensions of 0.1 meter across and 3.5 meters along the magnetic field lines. The echo results are compared with auroral zone effects, and described together with measurements of the frequency spectra (Doppler shift and spread) of an aurorally reflected continuous‐wave signal.

In this paper we address the question of whether auroral echoes are dependent on the ionospheric electric field. This question naturally arises from the concept of a plasma instability as the source mechanism producing the electron density irregularities responsible for the auroral echoes. Using the data from a backscatter radar located at Homer, Alaska, and the Chatanika incoherent scatter radar in a joint experiment, we show that there is a distinct dependence of the occurrence of diffuse auroral echoes on a threshold electric field strength. The threshold value for the 398‐MHz radar aurora is shown to be approximately 30 mV/m. When the electric field strength is greater than 30 mV/m, the auroral echo strength is shown to be always positively correlated with the electric field strength, regardless of the mean electron density. On the other hand, the auroral echo strength is not always positively correlated with the electron density. The results are shown to be consistent with the quasi‐linear plasma instability theory of Sudan et al. (1973) and Greenwald (1974).

In recent years the diffuse radar aurora has been shown to be closely associated with the auroral electrojets. Greenwald et al. (1973, 1975) showed that the range‐integrated amplitude of the diffuse radar aurora is linearly proportional to the horizontal component of the magnetic perturbations found beneath the echoing region. Tsunoda et al. (1974) showed that the evening diffuse radar aurora was characterized by a distinct poleward boundary that was usually aligned along a magnetic L shell. They hypothesized that such a boundary could be produced by a latitudinal gradient in either the ionospheric electric field or the conductivity. In either case, field‐aligned currents might be expected to flow in such a region. An initial study was therefore undertaken to examine the spatial relationship of the radar aurora to the field‐aligned currents observed by virtue of the transverse magnetic disturbances detected by the Triad satellite. By utilizing a 398‐MHz phased array radar located at Homer, Alaska, event‐by‐event comparisons were made between the evening radar aurora and the field‐aligned currents in the 1700–2100 MLT sector. The results of this study include the following findings: (1) the downward field‐aligned currents in the evening sector are closely associated with the eastward electrojet; (2) the upward field‐aligned currents in the same time sector are associated with the visual aurora; (3) there is a poleward‐directed electric field across the oppositely directed field‐aligned currents, implying at least partial closure through the ionosphere via a Pedersen current; and (4) the downward field‐aligned currents are associated with the diffuse particle precipitation responsible for the production of the diffuse radar aurora (and hence the eastward electrojet). However, the downward field‐aligned currents must be carried by precipitating protons and/or upward‐moving low‐energy electrons.