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HF Focusing due to Field Aligned Density Perturbations. A. Vartanyan 1 , G. M. Milikh 1 , K. Papadopoulos 1 , M. Parrot 2 1 Departments of Physics and Astronomy, University of Maryland, College Park, Maryland, USA
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HF Focusing due to Field Aligned Density Perturbations • A. Vartanyan1, G. M. Milikh1, K. Papadopoulos1, M. Parrot2 • 1 Departments of Physics and Astronomy, University of Maryland, College Park, Maryland, USA • 2 Laboratoire de Physique et Chimie de l’Environnement et de l’Espace, CRNS, Orleans, France
HF heating • Experiments are conducted by injecting HF radio-waves into the ionosphere’s F-region plasma using the HAARP facility. • Heating causes plasma density perturbations that travel along field lines, called ionospheric ducts. • Effects of heating on quantities such as plasma density and temperature, and ULF/VLF field values can be measured with the DEMETER satellite during a close flyby to HAARP’s magnetic zenith.
Observed HF focusing • During a heating experiment conducted at HAARP on 2/12/2010, DEMETER observed a multiple frequency band structure which is characteristic of a strong HF signal exceeding the detector’s saturation level. • Analysis of the O+ density measured by DEMETER along its orbit shows that the strong HF signal coincides with the presence of a “negative” duct in the ionosphere.
The observed spectral “line” at 2.8 MHz is over 2,800 km long. • The bandwidth of the spectral line is about 20 kHz, while HAARP’s original beam is on the order of Hz. • It is generated by radio emission stimulated by the interaction of the injected HF with the F-region plasma, rather than by the direct “free space” HAARP signal.
Theory of focusing of HF waves by ducts based on Gurevich et al. [1976] E field is given by: Expanding and r by powers of 2 and setting exponent equal to zero, we obtain the focusing distance and magnification: Lens distance: Lens Magnification:
Theoretical analysis - focal point • Based on the figure: • 0 = 70 km • n ≈ 500 cm^-3, while n ≈ 2800 cm^-3, thus Δn/n ≈ .17 • On the basis of these measurements and focal length equation, we can estimate the focal length of the focusing duct as about 300 km. • The duct lower boundary is located near the F2 peak at 300 km, the duct thus provides optimal focusing at about 300 + 100 + 300 = 700 km. • Focal length of 700km is close to DEMETER’s orbit of 670km.
Magnification • We find that a wave of frequency f = 2.8 MHz is magnified by 150 times at the focal point. • Considering that the power density detected outside of the duct was 25 (µV/m)^2/Hz, and that the half bandwidth of the signal is about 12 kHz, we find that the strongest signal outside of the perturbed region was about 0.5 mV/m. Since calibration tests of detector gave a 10 mV/m saturation level at 2.8 MHz, the observed detector saturation requires a magnification of at least by 20. • This is consistent with the observations.
Possibility of focusing due to artificial ducts • On 10/21/2009 a heating experiment was conducted with the intention of creating an artificial ionospheric duct. DEMETER observed successful duct creation. • In addition, DEMETER observed what may be focusing due to fine structure inside the duct. • Areas of instrumental overload seem to correspond to small negative ducts. • While this case is more speculative, a theoretical analysis similar to before leads to positive results.
Theoretical analysis • Δn/n ≈ .06 and radius 0 = 20 km. • Focal length of the focusing duct is about 70 km, and magnification is by 100 times at the focal point of this lens and its focal nodes. • The first node occurs at altitude 220 + 100 + 70 = 390 km, the second at 390 + 140 = 530 km, and the third at 530 + 140 = 670 km, close to DEMETER’s orbit.
Conclusions • Possibility of HF focusing via ionospheric plasma irregularities was demonstrated. • This phenomenon can be used for satellite communication via subcritical frequencies.