Abstract
Abstract. Satellite radio beacons were used in June 2001 to probe the ionosphere modified by a radio beam produced by the EISCAT high-power, high-frequency (HF) transmitter located near Tromsø (Norway). Amplitude scintillations and variations of the phase of 150- and 400-MHz signals from Russian navigational satellites passing over the modified region were observed at three receiver sites. In several papers it has been stressed that in the polar ionosphere the thermal self-focusing on striations during ionospheric modification is the main mechanism resulting in the formation of large-scale (hundreds of meters to kilometers) nonlinear structures aligned along the geomagnetic field (magnetic zenith effect). It has also been claimed that the maximum effects caused by small-scale (tens of meters) irregularities detected in satellite signals are also observed in the direction parallel to the magnetic field. Contrary to those studies, the present paper shows that the maximum in amplitude scintillations does not correspond strictly to the magnetic zenith direction because high latitude drifts typically cause a considerable anisotropy of small-scale irregularities in a plane perpendicular to the geomagnetic field resulting in a deviation of the amplitude-scintillation peak relative to the minimum angle between the line-of-sight to the satellite and direction of the geomagnetic field lines. The variance of the logarithmic relative amplitude fluctuations is considered here, which is a useful quantity in such studies. The experimental values of the variance are compared with model calculations and good agreement has been found. It is also shown from the experimental data that in most of the satellite passes a variance maximum occurs at a minimum in the phase fluctuations indicating that the artificial excitation of large-scale irregularities is minimum when the excitation of small-scale irregularities is maximum.
Highlights
One very effective method for studying physical processes in the ionosphere is to perturb the ionospheric medium using a powerful HF radio wave, or pump
The results of VHF satellite radio probing of the high-latitude ionosphere modified by the powerful transmission from the EISCAT HF heating facility at Ramfjordmoen near Tromsø, Norway, in June 2001 have been described, revealing the generation of both large-scale electron density irregularities and small-scale irregularities during the ionospheric modification
Phase fluctuations of radio signals occur mostly due to large-scale irregularities, whereas small-scale irregularities mainly produce amplitude scintillations
Summary
One very effective method for studying physical processes in the ionosphere is to perturb the ionospheric medium using a powerful HF radio wave, or pump. The irregularities produce artificial spread F seen on HF ionograms, scintillations on HF and VHF signal paths from radio stars and satellite beacons propagating through the modified region, artificial field-aligned scattering, and wide-band attenuation of HF radio waves in the disturbed region (Rietveld et al, 1993; Gurevich et al, 2002) It was suggested by Gurevich (1978) and Gurevich et al (1998, 2002) that the thermal resonance self-focusing instability is the main mechanism for producing electron density irregularities as a result of focusing and defocusing a pump wave in the region of density perturbations caused by the wave. The experimental results obtained during six satellite passes over the receiver chain are presented
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