VHF and UHF radar observations of the aurora at College, Alaska

Abstract

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.

On the diurnal variation of the E-region coherent HF echo occurrence

“…Presnell et al, 1959;Leadabrand et al, 1965;Unwin, 1966b;McNamara, 1972;Hanuise et al, 1991;Kustov et al, 1996;Makarevitch et al, 2002;Meyer et al, 2004;Carter and Makarevich, 2009). These studies have shown that the diurnal variations of echo occurrence at different radio frequencies/ wavelengths were quite similar.…”

“…The exact definitions have varied from study to study and have been mainly dependent on temporal and spatial resolutions of a particular radar system (e.g. Presnell et al, 1959;Leadabrand et al, 1965;Greenwald et al, 1975;Kustov et al, 1996).…”

Radar auroral echo characteristics as seen by a 398-MHz phased array radar operated at Homer, Alaska

“…The results are then presented in the form of spatial maps of auroral echoes, the various characteristic signatures, and their relationships to conventional auroral echo types [Presnell et al, 1959]. The results are then presented in the form of spatial maps of auroral echoes, the various characteristic signatures, and their relationships to conventional auroral echo types [Presnell et al, 1959].…”

“…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 results are shown in the form of two-dimensional spatial maps of auroral echoes.…”

“…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. The results are then presented in the form of spatial maps of auroral echoes, the various characteristic signatures, and their relationships to conventional auroral echo types [Presnell et al, 1959]. Despite the many years of intense investigation, the usefulness of these studies in characterizing auroral phenomena has been limited for a number of reasons.…”