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Ann. Geophys., 24, 2901-2909, 2006
www.ann-geophys.net/24/2901/2006/
© European Geosciences Union 2006
The relationship between small-scale and large-scale ionospheric electron density irregularities generated by powerful HF electromagnetic waves at high latitudes
E. D. Tereshchenko1, B. Z. Khudukon1, M. T. Rietveld2, B. Isham3, T. Hagfors4, and A. Brekke5
1Polar Geophysical Institute, 15 Khalturina St. 183010 Murmansk, Russia
2EISCAT Scientific Association, 9027 Ramfjordbotn, Norway
3Interamerican University, Bayamón, Puerto Rico 00957, USA
4Max Planck Institute for Solar System Research, 37191 Katlenburg-Lindau, Germany
5University of Tromsø, 9037 Tromsø, Norway
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.
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