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

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.