Field‐aligned scattering from a heated region of the ionosphere—Observations at HF and VHF

Abstract

Radio scattering measurements have been made of a region of the ionosphere above the Institute for Telecommunication Sciences' ionospheric heating facility at Platteville, Colorado. Strong scattering was observed at frequencies in the HF and VHF bands. The scattering has been shown to originate from electron density fluctuations that are highly elongated in the direction of the geomagnetic field. Radar scattering cross sections of 70 to 80 dbsm are measured in the HF and low VHF bands. The scattering cross section decreases at a rate of 20 to 30 db per octave at the high end of the VHF band. The scattering irregularities are produced by operation of the heating facility with o‐mode polarization at frequencies that reflect within the ionosphere. The strength of the scattered energy decreases at a rate of at least 10 db per degree for observation geometries that do not match the geometry for specular reflections from long cylinders aligned with the geomagnetic field. The spatial distribution of irregularities has been estimated for both E‐ and F‐region heating. The temporal properties of the scattering have also been determined including the frequency spectrum of the scattered signals. The processes by which ionospheric heating produces these irregularities are not yet fully explained. Several potentially useful applications of radio scattering from such irregularities are discussed.

Theoretical model for long time stimulated electromagnetic emission generation in ionospheric radio modification experiments

“…In the work of Leyser et al [1989] it was reported that several of the prominent spectral features are absent when the applied frequency is near a harmonic of the electron gyrofrequency. After that, emphasis has been put on the possible role of smallscale magnetic field-aligned density striations, which are known to be generated in these experiments [Fialer, 1974;Minkoff et al, 1974;Djuth et al, 1985]. This overall view is strongly supported by later experimental findings, as will be further discussed in the next Gurevich et al, 1995].…”

The thresholds of ionospheric plasma instabilities pumped by high‐frequency radio waves at EISCAT

“…The detection of the striations is highly aspect sensitive and can only be detected when a radar beam is near orthogonal to the magnetic field. Fialer [1974] showed that the backscattered power reduced by 20 dB when pointing 3 ı away from perpendicular.…”

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

“…Consequently, observations of the vertical extent of the irregularities are less numerous and less consistent. Fialer (1974) was able to estimate the vertical extent from radar measurements by allowing the heater interaction height (where irregularities are expected to be strongest) to vary, causing backscatter to be obtained from a range of heights relative to the interaction height and concluded that the Gaussian semi-thickness of the layer was about 7.5 km at Platteville. Korovin et al (1983) determined that the thickness of the layer was about 10 km by performing spatial and frequency correlation measurements on radio waves scattered from the heated region.…”

“…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).…”