It is observed that an ionospheric volume in the F layer subjected to high power HF illumination becomes an effective scattering medium for radio signals in the VHF/UHF frequency range. The experimental results are representative of a field‐aligned scattering geometry for which the first such observations of VHF/UHF scattering from a heated ionospheric volume are presented. Two distinct scattering modes are observed, center‐line and plasma‐line scattering. Center‐line scattering is observed at the transmitted radar frequency f; plasma‐line scattering is observed as a pair of sidebands at f ± fh where fh is the heater frequency. The two scattering modes are observed to have markedly dissimilar characteristics. Center‐line scattering is highly aspect sensitive with respect to the direction of the geomagnetic field, B; plasma‐line scattering is found to be much less aspect sensitive, if at all. The region of maximum backscatter for the center‐line mode is found from these measurements to consist of the locus of points within the heated volume over which perpendicularity between the radar line of sight and B is achieved, independent of the location of maximum heating. The backscattering region for the plasma‐line mode is found from these measurements to be determined by the altitude of maximum heating, independent of geometrical considerations involving B. The longitudinal coherence length, L, for center‐line scattering is found from these measurements to be greater than the maximum antenna diameter of 85 ft; no more exact estimate for L is possible. A striking reversal in frequency dependence is found between the center‐line and plasma‐line modes. The per‐unit‐volume center‐line backscatter cross section is found to be about 7 db greater at VHF than at UHF; the per‐unit‐volume plasma‐line backscatter cross section is found to be at least 11 db less at VHF than at UHF. Preliminary results concerning time‐dependent behavior are presented. For both modes the scattering cross section is found to be effectively turned on and off very rapidly in response to the heater excitation; the spectral width of the scattering for both modes is found to be quite narrow (about 10 Hz). The spatial configuration of the heated volume is investigated; significant differences are observed depending on whether fh/f0F2 is greater or less than unity.
The proper normalization procedure for determining per‐unit‐volume properties of aspect‐sensitive scattering media from experimental measurements is discussed. This is complicated by the fact that, because of the aspect sensitivity, it is not clear a priori what the per‐pulse scattering volume actually is. In general, the correct normalization requires quantitative knowledge of the aspect sensitivity which may not be possible to determine because of fundamental limitations imposed by the spatial‐resolution capability of the measurement system on the extent to which this property of the scattering medium can actually be determined. The identical problem exists in radio‐auroral measurements. This problem is considered here, and an exact expression from which the desired per‐unit‐volume quantities of interest can be obtained from measurements is given. It is shown that proper choice of the experimental parameters leads to a simplification whereby the normalization can be carried out without knowledge of the aspect sensitivity being necessary. The results are applied to calculating from experimental data the per‐unit‐volume scattering properties of a heated ionospheric volume as a function of frequency, from which a transverse scale size for the scattering medium of 3 m is estimated. For these experiments, under maximum heating conditions, a representative value of 1% for the rms fractional electron density deviation is calculated; an upper bound is also established, showing that values as large as 4 or 5% were probably never achieved. It is shown that, for an electron density distribution axially symmetric with respect to the geomagnetic field, B, the cross section for backscatter within a plane containing B uniquely determines the cross section for bistatic scattering axially around B. Experimental results for small axial‐bistatic angles are presented showing good agreement between calculated and measured values.
Spectral analysis of field‐aligned VHF backscatter from a heated ionospheric volume in the F layer over Platteville, Colorado, reveals a complex frequency‐space picture for both center line and plasma line scattering modes. The spectra are observed to fall roughly into one of three categories: (1) narrow band (in relation to the 100‐Hz unambiguous frequency interval), (2) noiselike (essentially uniform over at least 100 Hz), and, (3) composite structure, consisting of a narrow‐band peak and a noiselike wide‐band component. For the field‐aligned plasma line observations the presence of a mechanism different from that responsible for the Arecibo Ionospheric Observatory plasma line observations is strongly indicated. Signal bandwidths are found to be range dependent, with the narrowest spectral widths consistently being observed at the southern periphery of the heated volume. Measurements of ionospheric drift velocities along a nominally north‐south axis are also presented. Values are found to range from 0 to 30 m/s. The result of a measurement of cross‐section decay in response to step function heater power variation is found to admit a simple interpretation, i.e., that the Fourier component of the electron density fluctuations at half the radar wavelength smooths out by a diffusion process in planes perpendicular to B, with a time constant proportional to λ². This interpretation is consistent with other observations in which, after sudden turn‐off of heater excitation, the scattering is always found to decay slower at longer radar wavelengths.
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