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.

HF spectrogram observed by DEMETER on 2/12/2010

• 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.

O+ ion density observed by DEMETER on 2/12/2010

Analogy with optics

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:

22pe /ωω1η

zn

ρz,Δn

/ωω1

ω/ω

2

1Δn

nΔ

22pe

22pe

2

1i d

r

rc

ωi,φiexp

2eE o

z

icA 1z

odzΔ

c

ωΔφ

0z

ρ

2c

ωdz

ρ

Δn

n

ρ

/ωω1

/ω

2c

ω

c2z

ωΔφ

f

2

2

2z

22pe

22pe

f1

f

o

Lens distance:

Δn/nΔz

ρz

2o

f

n

ΔnΔz

c2

ω

z

ρ

c2

ω

A

E

f

2o

Theoretical analysis - focal point Based on the figure:

1. 0 = 70 km2. 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.

HF spectrogram observed by DEMETER on 10/21/2009

O+ ion density observed by DEMETER on 10/21/2009

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.