A theory of scattering by nonisotropic irregularities with application to radar reflections from the aurora

A model of the distribution of ionization in an aurora is developed that can explain the aspect sensitivity of radar echoes obtained at VHF during auroral activity. Even when the radar is in the midst of a zone of auroral activity, echoes are obtained only from low angles of elevation in roughly the northern quadrant. However, both the horizontal and vertical angles over which echoes are obtained increase Significantly as frequency is decreased from 100 to 25 Mc/s. These observations can be explained in terms of columns of ionization parallel to the earth´s magnetic field. The size of the columns must, however, be much smaller than suggested by Chapman (1952) or by Booker, Gartlein, and Nichols (1955). The column size required is of the order of 40 m in length and 1 m in diameter, smoothed so as to avoid discontinuities at the surfaces. It is quite doubtful whether the formation of such short columns of ionization can be associated directly with the formation of visual rays, even though the same axis of symmetry is involved in Both cases. The size of irregularities required to explain the observations is in fact of the order of magnitude likely to be involved in atmospheric turbulence, and it is quite likely that the earth´s magnetic field could create substantial nonisotropy in the associated irregularities of electron density. Thus the strength of auroral echoes and their aspect sensitivity could be entirely explained as backscatter arising from nonisotropic atmospheric turbulence in an E region having a maximum electron density about a hundred times the normal value. Simple turbulence cannot, however, explain the remarkable fading phenomena associated with auroral reflections (Bowles, 1954).