HF Focusing due to Field Aligned Density Perturbations

1
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
  lEnvironnement et de lEspace, CRNS, Orleans,
  France

2
HF heating

• Experiments are conducted by injecting HF
  radio-waves into the ionospheres 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 HAARPs magnetic zenith.

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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 detectors
  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.

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HF spectrogram observed by DEMETER on 2/12/2010
5

• 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 HAARPs 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.

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O ion density observed by DEMETER on 2/12/2010
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Analogy with optics
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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 Magnification
Lens distance
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Theoretical analysis - focal point

• Based on the figure
• ?0 70 km
• ?n 500 cm-3, while n 2800 cm-3, thus ?n/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 DEMETERs orbit
  of 670km.

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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.

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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.

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HF spectrogram observed by DEMETER on 10/21/2009
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O ion density observed by DEMETER on 10/21/2009
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Theoretical analysis

• ?n/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
  DEMETERs orbit.

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Conclusions

• Possibility of HF focusing via ionospheric plasma
  irregularities was demonstrated.
• This phenomenon can be used for satellite
  communication via subcritical frequencies.