Analysis and interpretation of aspect-dependent ionospheric radar scatter

Abstract

Aspect-dependent radar scattering from ionospheric phenomena such as the aurora is known to be dependent on the three-dimensional wave-number spectrum of the random electron-density fluctuations as well as the mean-square deviation of the fluctuations. In characterizing the wavenumber spectrum, it is convenient to speak of the longitudinal and transverse scale sizes or coherence lengths. L and T. which describe, respectively, the average distances parallel and perpendicular to the geomagnetic field over which collective scattering effects take place. An analysis of the scattering process is presented in which the relationships between ihese ionospheric quantities and those quantities which are actually observed in an experimental situation are determined. With the use of these relationships, it is shown that the values of the experimental parameters impose fundamental limitations on the extent to which the quantities of interest can actually be measured in any given experimental situation. In particular, it is shown that the vertical antenna aperture D sets the limit on the largest value of L that can actually be determined. If D/L < 1. it is shown that the apparent measured value of L will always be very close to D, regardless of the true value of L; some experimental results aremore » considered in the light of this effect. It is shown that an analogous situation can occur if the transmitted pulse width is too long, but, in practice, this limitation is not as restrictive. The correct normalization of measured values of received signal strengths necessary to determine the scattered power per unit volume is also complicated by the aspect sensitivity of the scattering is also shown that this problem can be considerably alleviated wiih the proper choice of experimenial parameters. From these considerations, an ideal antenna configuration for making such measurements is suggested. An extension of the analysis to bistatic scattering is also considered. The measurement of the variance of the electron-density fluctuations is discussed.« less