Radio frequency scattering from a heated ionospheric volume: 1, VHF/UHF field‐aligned and plasma‐line backscatter measurements

Abstract

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.

Excitation thresholds of field-aligned irregularities and associated ionospheric hysteresis at very high latitudes observed using SPEAR-induced HF radar backscatter

“…The first purpose-built high power transmitters (or heaters), intended for ionospheric studies, appeared in the 1970s and the first reported ionospheric modification (heating) experiments were performed at Platteville, Colorado. These experiments revealed that the high power "pump" wave led to the generation of field-aligned electron density irregularities (FAIs;Fialer, 1974;Minkoff et al, 1974). The FAIs are excited as a result of the coupling of the pump wave to a type of electrostatic plasma waves known as upper hybrid (UH) waves which occur in a narrow altitude region around the upper hybrid height.…”

Effects of multiple scatter on the propagation and absorption of electromagnetic waves in a field-aligned-striated cold magneto-plasma: implications for ionospheric modification experiments

“…These irregularities, which are strongly elongated along the direction of the geomagnetic field, are generated by a high power electromagnetic pump wave in the vicinity of its own upper-hybrid resonance. They have been regularly observed by coherent backscatter radars during ionospheric modification experiments (Fialer, 1974;Minkoff et al, 1974;Hedberg et al, 1983;Robinson et al, 1997). Moreover, Minkoff (1974) has shown that the spectral power of the irregularities produced in heating experiments is primarly confined to crossfield scale sizes in excess of 1 m. This result is also consistent with the estimate of a few tens of km for the scale length of the irregularities along the geomagnetic field line (Jones et al, 1984), since their aspect ratio is expected to be of the order of the ratio of the electron-neutral collision frequency, ν en , to the electron gyro-frequency, .…”

Multi-frequency HF radar measurements of artificial F-region field-aligned irregularities

“…These early observations established that the backscatter was highly aspect-sensitive, with backscatter being observed only when the radar beam was nearorthogonal to the magnetic field lines in the heated region. Fialer (1974) reported that the backscatter power fell by at least 20 dB when the deviation from orthogonality reached 3 • and Minkoff et al (1974) found that attempting to measure the vertical extent of the irregularities by varying the elevation angle of the radar beam resulted in nothing more than a measurement of the radar's radiation pattern in elevation! Further radar backscatter measurements established the field-transverse wave number spectrum of the irregularities (Minkoff, 1974).…”