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

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.