Radio reflections from aurora

This paper outlines observations and experiences associated with a continuing investigation of radio echoes from aurora at Saskatoon, Canada. For the three frequencies investigated—3000, 106, and 56 Mc/s—echoes have been observed with the last two. Echoes occur when the auroral forms exhibit some ray structure and, then, only from parts of the aurora at elevations less than 15° above the horizon. Using range-amplitude and range-azimuth displays of echoes in combination with the corresponding photographs of the aurora, it is shown that the echoes originate from levels close to the lower edge of the aurora. Height distributions of the centers from which the echoes come agree closely with height distributions found from parallactic photographs of aurora in other regions. Using aerial arrays for the 106- and 56-Mc/s frequencies that are mounted on a rotatable tower so that each beam scans simultaneously the same part of the sky, the 106-Mc/s echoes are found to occur most frequently within the auroral zone and the 56-Mc/s echoes some distance to the south. Relative frequencies and diurnal variations of the two groups of echoes, as well as their heights and geographical distributions, indicate that echoes originate from a threshold process. It is suggested that the echoes arise by critical reflection from centers of high electron density (1.4× 08 and 4×107 electrons per cm3, respectively) that are formed close to but above the levels where the electron collision frequency exceeds the gyro-frequency of the electrons.