"Acknowledgments. We acknowledge support from HAARP,
Office of Naval Research (ONR), Air Force Research Laboratory, and
Defense Advanced Research Programs Agency, via ONR grant
N0001405C0308 to Stanford University. We thank Mike McCarrick for
operation of the HAARP array."
GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L12101, doi:10.1029/2008GL034061, 2008
Geometric modulation: A more effective method of steerable ELF/VLF
wave generation with continuous HF heating of the lower ionosphere
[1] ELF/VLF radio waves (300 Hz–30 kHz) are difficult
to generate with practical antennae, because of their
extraordinarily long (10–1000 km) wavelengths, and the
lossy nature of the Earth’s surface at these frequencies.
ELF/VLF waves have been successfully generated via
amplitude modulated (AM) HF (2–10 MHz) heating of
the lower ionosphere. Through the temperature-dependent
conductivity of the lower ionospheric plasma, a patch of
the ionospheric current becomes a large radiating
‘antenna’. We implement a new method of ELF/VLF
wave generation, herein named ‘geometric modulation’,
involving scanning the HF heating beam in a geometric
pattern without modulating its power. Utilizing results
from the upgraded 3.6 MW radiated HAARP HF antenna
array, we show that geometric modulation can enhance ELF/
VLF wave generation by up to 11 dB over the conventional
AM method. Geometric modulation also allows directional
launching of the signal into the Earth-ionosphere waveguide,
forming an unprecedented steerable large-element ELF/VLF
ionospheric phased array. Citation: Cohen, M. B., U. S. Inan,
and M. A. Golowski (2008), Geometric modulation: A more
effective method of steerable ELF/VLF wave generation with
continuous HF heating of the lower ionosphere, Geophys. Res. Lett.,
35, L12101, doi:10.1029/2008GL034061.
[exerpt] Page 2 (11) [11] The four modulation schemes discussed here are
summarized in Figure 2, top panel. For amplitude modulation
(AM), we utilize 50% duty cycle, 100% depth square
wave modulation. Three types of GM ‘sweep’ schemes are
introduced, each of which is compared to a typical AM
scheme. The GM schemes are herein labeled line-sweep,
where the heating beam scans back and forth along a
chosen azimuth, completing a full back-forth scan (in this
case ±15 ) in one ELF/VLF period); sawtooth-sweep,
where the heating beam scans along one chosen azimuth,
completing one sweep across the path in one ELF/VLF
period and starting back at the initial end; and circle-sweep,
where the heated beam follows a circular pattern with some
radius (in this case 15 ). In these experiments, we utilize an
HF carrier frequency of 3.25 MHz, with X-mode polarization,
and with ERP of 575 MW.
summarized in Figure 2, top panel. For amplitude modulation
(AM), we utilize 50% duty cycle, 100% depth square
wave modulation. Three types of GM ‘sweep’ schemes are
introduced, each of which is compared to a typical AM
scheme. The GM schemes are herein labeled line-sweep,
where the heating beam scans back and forth along a
chosen azimuth, completing a full back-forth scan (in this
case ±15 ) in one ELF/VLF period); sawtooth-sweep,
where the heating beam scans along one chosen azimuth,
completing one sweep across the path in one ELF/VLF
period and starting back at the initial end; and circle-sweep,
where the heated beam follows a circular pattern with some
radius (in this case 15 ). In these experiments, we utilize an
HF carrier frequency of 3.25 MHz, with X-mode polarization,
and with ERP of 575 MW.
FULL PDF VERSION OF THIS STANFORD RELEASE CAN BE FOUND:
**************************
GEOPHYSICAL RESEARCH LETTERS,
VOL. 35,
L12101,
6 PP., 2008
doi:10.1029/2008GL034061
doi:10.1029/2008GL034061
Geometric modulation: A more effective method of steerable ELF/VLF wave generation with continuous HF heating of the lower
ionosphere
ELF/VLF radio waves (300 Hz–30 kHz) are
difficult to generate with practical antennae, because of their
extraordinarily long
(10–1000 km) wavelengths, and the lossy nature
of the Earth's surface at these frequencies. ELF/VLF waves have been
successfully
generated via amplitude modulated (AM) HF (2–10
MHz) heating of the lower ionosphere. Through the temperature-dependent
conductivity
of the lower ionospheric plasma, a patch of the
ionospheric current becomes a large radiating ‘antenna’. We implement a
new
method of ELF/VLF wave generation, herein named
‘geometric modulation’, involving scanning the HF heating beam in a
geometric
pattern without modulating its power. Utilizing
results from the upgraded 3.6 MW radiated HAARP HF antenna array, we
show
that geometric modulation can enhance ELF/VLF
wave generation by up to ∼11 dB over the conventional AM method.
Geometric modulation
also allows directional launching of the signal
into the Earth-ionosphere waveguide, forming an unprecedented steerable
large-element
ELF/VLF ionospheric phased array.
Received 19
March
2008;
accepted 8
May
2008;
published 18
June
2008.
Citation:
(2008),
Geometric modulation: A more effective method of steerable ELF/VLF wave generation with continuous HF heating of the lower
ionosphere,
Geophys. Res. Lett.,
35,
L12101,
doi:10.1029/2008GL034061.
Cohen, M. B., U. S. Inan, and M. Gołkowski (2009), Reply to comment by
R. C. Moore and M. T. Rietveld on “Geometric modulation: A more
effective method of steerable ELF/VLF wave generation with continuous HF
heating of the lower ionosphere”, Geophys Res Lett, 36, L04102.[CrossRef]
Cohen, M. B., U. S. Inan, M. Gołkowski, and M. J. McCarrick (2010),
ELF/VLF wave generation via ionospheric HF heating: Experimental
comparison of amplitude modulation, beam painting, and geometric
modulation, J Geophys Res, 115, A02302.[CrossRef]
Cohen, M. B., U. S. Inan, M. Gołkowski, and N. G. Lehtinen (2010), On
the generation of ELF/VLF waves for long-distance propagation via
steerable HF heating of the lower ionosphere, J Geophys Res, 115, A07322.[CrossRef]
Cohen, M.B., U.S. Inan, and E.W. Paschal (2010), <![CDATA[Sensitive
Broadband ELF/VLF Radio Reception With the AWESOME Instrument]]>, GEOSCIENCE AND REMOTE SENSING IEEE TRANSACTIONS ON, 48(1), 3.[CrossRef]
Cohen, M. B., U. S. Inan, D. Piddyachiy, N. G. Lehtinen, and M.
Gołkowski (2011), Magnetospheric injection of ELF/VLF waves with
modulated or steered HF heating of the lower ionosphere, J Geophys Res, 116, A06308.[CrossRef]
Golkowski, M., U. S. Inan, and M. B. Cohen (2009), Cross modulation of
whistler mode and HF waves above the HAARP ionospheric heater, Geophys Res Lett, 36, L15103.[CrossRef]
Gołkowski, M., M. B. Cohen, D. L. Carpenter, and U. S. Inan (2011), On
the occurrence of ground observations of ELF/VLF magnetospheric
amplification induced by the HAARP facility, J Geophys Res, 116, A04208.[CrossRef]
Moore, R. C., and M. T. Rietveld (2009), Comment on “Geometric
modulation: A more effective method of steerable ELF/VLF wave generation
with continuous HF heating of the lower ionosphere” by M. B. Cohen, U.
S. Inan, and M. A. Golkowski, Geophys Res Lett, 36, L04101.[CrossRef]
Streltsov, A. V., and T. R. Pedersen (2010), An alternative method for generation of ULF waves by ionospheric heating, Geophys Res Lett, 37, L14101.[CrossRef]
