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.

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Auroral echoes have been detected using a radar at 106.1 Mc located at 43 ø geomagnetic latitude. The geometry of reflection for ionization aligned with the earth's magnetic field lines is such that, for a geomagnetic latitude of 43 ø, reflection can occur as high as 300 km. The results of these observations are presented, with an interpretation of the height of reflections and a discussion of the advisability of making low-latitude auroral echo investigations. Introduction--Considerable work has been done in the past on the investigation of the characteristics of auroral ionization by the radar technique. Much of this work has been at frequencies between 30 and 100 Mc [Curtie, Forsyth, and Vawter, 1953; Harang and Landmark, 1954(a); Booker, Garilein, and Nichols, 1955], and almost all of it has been at geomagnetic latitudes above 56 ø. Previous HF investigations of auroral echoes at the low geomagnetic latitude of 43 ø .have been conducted at only a very few locations to date [Leadabrand and Peterson]. This paper is a description of observations made at 106.1 Mc at a geomagnetic latitude of 43 ø .

By means of a relatively sensitive 400-Mc radar located at College, Alaska, the variation of Doppler shift of auroral echoes has been determined as a function of the following parameters: (1) azimuth angle of the radar ray; (2) off-perpendicular intersection angle of the radar ray and the earth's magnetic field; (3) elevation angle of the radar ray; (4) range of the echoes; (5) altitude of the reflection centers; (6) time of day; (7) number of occurrences; (8) strength of the echoes. These data have been further delineated in terms of the type of echo seen (discrete or diffuse) and whether the data were taken before or after magnetic midnight. An estimate of the spectrum spread of auroral echoes has also been determined by pulse and by CW techniques. A consistent trend in these data has been found which would indicate an east-west motion of the ionospheric irregularities. There is no appreciable variation in the direction of motion with time of day or with respect to magnetic midnight. The mean velocity of the east-west motion appears to be 500 meters per second. These conclusions agree with those of Kim and Curtie [1958] but disagree with those of Lyon and Kavadas [1958], Nichols [1957], and Bullough et al. [1957].