Determination of the fluctuation level of ionospheric irregularities from radar backscatter measurements

The expression for the radar cross section for backscatter from elongated, field-aligned ionospheric irregularities is used to find a general formula for the backscattered power received by any radar for a given rms electron density fluctuation level. The formula involves an integral of the three-dimensional irregularity power spectrum over a volume defined by the beam width of the antenna and the pulse length. The three cases considered are where: (1) the irregularities fill the beam but the beam width measured parallel to the earth's magnetic field is so large that a wide range of aspect angles falls within the beam, (2) the irregularities fill the beam and the beam width is so narrow that the aspect angle is approximately constant, and (3) the irregularities fill only a small portion of the beam, over which the aspect angle is constant. The three cases are typified by the Goose Bay HF radar, the Jicamarca radar, and STARE-type radars, respectively. An empirical formula for the power spectrum is used to obtain explicit formulas for the three types of radar. The Goose Bay radar works at a low enough frequency so that ionospheric refraction can be used to achieve good aspect sensitivity. The effect of ionospheric refraction and focusing is therefore included in the formulas for backscatter. The results of numerical computations are presented in order to compare the sensitivity of the Goose Bay radar with other types of radar and to estimate the levels of fluctuation needed to produce backscatter. Curves of signal-to-noise ratio versus range are plotted for the radar.