The relationship between small-scale and large-scale ionospheric electron density irregularities generated by powerful HF electromagnetic waves at high latitudes

摘要

Satellite radio beacons were used in June 2001 to probethe ionosphere modified by a radio beam produced by the EISCAT high-power,high-frequency (HF) transmitter located near Troms? (Norway). Amplitudescintillations and variations of the phase of 150- and 400-MHz signals fromRussian navigational satellites passing over the modified region wereobserved at three receiver sites. In several papers it has been stressedthat in the polar ionosphere the thermal self-focusing on striations duringionospheric modification is the main mechanism resulting in the formation oflarge-scale (hundreds of meters to kilometers) nonlinear structures alignedalong the geomagnetic field (magnetic zenith effect). It has also beenclaimed that the maximum effects caused by small-scale (tens of meters)irregularities detected in satellite signals are also observed in thedirection parallel to the magnetic field. Contrary to those studies, thepresent paper shows that the maximum in amplitude scintillations does notcorrespond strictly to the magnetic zenith direction because high latitudedrifts typically cause a considerable anisotropy of small-scaleirregularities in a plane perpendicular to the geomagnetic field resultingin a deviation of the amplitude-scintillation peak relative to the minimumangle between the line-of-sight to the satellite and direction of thegeomagnetic field lines. The variance of the logarithmic relative amplitudefluctuations is considered here, which is a useful quantity in such studies.The experimental values of the variance are compared with model calculationsand good agreement has been found. It is also shown from the experimentaldata that in most of the satellite passes a variance maximum occurs at aminimum in the phase fluctuations indicating that the artificial excitationof large-scale irregularities is minimum when the excitation of small-scaleirregularities is maximum.