Spatial Spectrum of Artificial Ionospheric Irregularities Induced by Powerful HF Radiowaves

Abstract

We present measurement results and spatial-spectrum shapes of the dependence of the relaxation time on the scale across the geomagnetic field for artificial ionospheric irregularities (AIIs) induced in the upper ionosphere by powerful HF radiation of the SURA heating facility. The irregularity diagnostics was based on observing amplitude scintillations of a 243-MHz beacon signal from a quasi-geostationary, solar-synchronous satellite and on measuring field-aligned scattering at frequencies 15 and 20 MHz. The satellite signal was received at the Kazan State University Observatory. The field-aligned scattering signals were received and analyzed by a bistatic HF radar based on the UTR-2 radio telescope located near Kharkov (Ukraine). It is shown that irregularities of the electron density, whose two-dimensional spectrum in the plane perpendicular to the geomagnetic field is the power law æ_⊥ ^-p with index p≲2, are developed in the scale range 30–60 ≲ l_⊥ ≲ 200–400 m. In this case, the relative fluctuations (〈δN²〉)^1/2 of the electron density increase with decreasing scale l_⊥=2π/æ_⊥. The estimate 〈δ N²〉)^1/2 ≲ 1–1.5% is obtained for a heating power of 150 MW and irregularity scales l_⊥ ∼ 30–60 m at which the fluctuations are maximum. The measured dependence of the AII relaxation time, defined as the e-folding time of the amplitude-scintillation intensity, has the form τ_r ∝ l_⊥ ^α. If l_⊥ ≲ 30–60 m, then the index α is close to 2, whereas the effective diffusion coefficient D_⊥ ≃ (2–3)· 10^-1 m²/s corresponds to the ambipolar cross-field diffusion coefficient in a magnetized plasma. The time τ_r for scales l_⊥≳ 60–100 m is independent of l_⊥ and increases with decreasing velocity of regular drift of the plasma. The Doppler-spectrum broadening (〈Δν²〉)^1/2 ≲ 0.6 Hz observed when receiving field-aligned scattered signals can be related to chaotic motions of plasma-density disturbances whose random drift velocities amount to (〈Δv²〉)^1/2 ≃ 2–3 m/s for scales l_⊥ ∼ 20 m at which the power-law index changes drastically.