There is an Interesting paper in Nature (#433,
page 498-500, 3 Feb, 2005) in which high power HF
radio transmissions from HAARP were used to cause
visible changes in the aurora, where speckles
appeared. They were using the full 960 kW beam,
from 4 -6 MHz, with 7.5 seconds on and then 7.5
second off time. Operating QRO. Look it up, its
pretty neat. I have a text version I can forward
upon request.
73
John
K5PRO
-----------------------------------------
Nature 433, 498 - 500 (03 February 2005); doi:10.1038/nature03243
Creation of visible artificial optical emissions
in the aurora by high-power radio waves
TODD. R. PEDERSEN1 AND ELIZABETH A. GERKEN2
1 Space Vehicles Directorate, Air Force Research
Laboratory, Hanscom Air Force Base, Massachusetts
01731, USA
2 Department of Electrical and Computer
Engineering, Cornell University, Ithaca, New York
14853, USA
Correspondence and requests for materials should
be addressed to T.R.P.
(todd.pedersen@hanscom.af.mil).
Generation of artificial light in the sky by
means of high-power radio waves interacting with
the ionospheric plasma has been envisaged since
the early days of radio exploration of the upper
atmosphere, with proposed applications ranging
from regional night-time street lighting to
atmospheric measurements1. Weak optical emissions
have been produced for decades in such
ionospheric 'heating' experiments, where they
serve as key indicators of electron acceleration,
thermal heating, and other effects of
incompletely understood waveñparticle
interactions in the plasma under conditions
difficult to replicate in the laboratory2. The
extremely low intensities produced previously
have, however, required sensitive instrumentation
for detection, preventing applications beyond
scientific research. Here we report observations
of radio-induced optical emissions bright enough
to be seen by the naked eye, and produced not in
the quiet mid-latitude ionosphere, but in the
midst of a pulsating natural aurora. This may
open the door to visual applications of
ionospheric heating technology or provide a way
to probe the dynamics of the natural aurora and
magnetosphere.
The most readily observed emissions produced in
ionospheric heating are the 'forbidden' red and
green lines from atomic oxygen at 630.0 and 557.7
nm, both common components of the natural aurora
and airglow3. In almost all past experiments,
artificial emissions have been produced by the
interaction of radio waves with the ionospheric F
region, the long-lived primary ionospheric layer
composed of atomic ions at an altitude of several
hundred kilometres4. Only rarely have optical
effects been reported from the ionospheric E
region5, an ephemeral layer created from
occasional meteoric ions or continuous solar
illumination or auroral precipitation near an
altitude of 100 km (ref. 4), where increased
collisions with neutral molecules alter the
behaviour of the plasma, and the proximity to the
transmitter provides a large inverse-square
increase in power density5. Emission intensities
achieved previously have typically ranged up to
several hundred Rayleighs (1 R = 106 photons cm-2
s-1 integrated along a line of sight) for the
more easily excited red line and tens of
Rayleighs for the higher-energy green line. In
all cases, intensities have remained far below
the threshold for detection by the human eye,
which is given as 20 kR at 630 nm (ref. 2)
towards the red end of the visible wavelength
range, and 1 kR for 558 nm (ref. 6) near the peak
sensitivity of the eye.
We recently produced dramatically stronger
artificial optical emissions bright enough to be
visible to the naked eye in an experiment
targeting the ionospheric E layer created by the
natural aurora. The experiment was conducted on
10 March 2004, between approx6ñ7 UT, using the
960-kW transmitter array at the High Frequency
Active Auroral Research Program (HAARP) facility
near Gakona, Alaska (62.4? N, 145.15? W). The
HAARP transmitter was run in a 15-s cycle
alternating between 7.5 s of full power and 7.5 s
off. Four filtered optical imaging systems
ranging from all-sky to telescopic were operated
in synchronization with the transmitter on and
off intervals. Background conditions during the
experiment period were characterized by aurora
pulsating with apparent periods of approx10 s in
longitudinal bands running in the magnetic
eastñwest direction over most of the sky,
including the region within the transmitter beam
(Fig. 1). The auroral precipitation created a
blanketing E layer near an altitude of 100 km
with critical frequencies ranging from 4ñ6 MHz.
Figure 1 A pair of all-sky images taken 5
s apart showing the dynamic natural background
aurora pulsating in long bands over most of the
sky. Full legend
High resolution image and legend (37k)
For a period of approximately 10 min between
about 06:40 and 06:50, a number of small speckles
of enhanced green emission were observed with the
HAARP telescope wide-field camera, which provided
high-resolution images of the region within the
transmitter beam near magnetic zenith (Fig. 2).
The speckles were present only during the image
frames when the transmitter was on and were
absent from exposures taken during the off
periods. There is evidence of dynamic pulsations
in the background aurora within this narrower
field of view as well, such as the auroral bands
that appear and disappear in the lower left
corner of the images. The largest speckles are
approximately one degree across.
Figure 2 Images from the HAARP telescope
wide field camera showing speckle-like artificial
optical emissions superimposed on the background
aurora only during frames when the transmitter
was on. Full legend
High resolution image and legend (48k)
Within a given frame the speckles appear to be
randomly distributed, but upon closer examination
of successive 'on' frames, some of the speckles
often appear to be correlated with but displaced
from those seen in the previous 'on' period. This
suggests that some speckle features are in motion
but may still retain coherence across multiple
onñoff cycles of the transmitter.
Calibrated average intensities for the background
aurora within the transmitter beam were obtained
from another imager, which made measurements at
several different wavelengths once each minute
(Fig. 3). In spite of the rapid pulsations in
narrow bands and on 10-s timescales, the average
auroral brightness at 557.7 nm remained near 4
kR, with an increase to approx5 kR near the time
the speckles were observed. This intensity
calibration, applied to the high-resolution data
in Fig. 2, indicates that the brightest speckles
were approximately 4 kR in total intensity, well
above the threshold for visibility and 1 kR or
more above the darker auroral regions.
Figure 3 Calibrated measurements from the
HAARP imager showing the intensities of various
auroral emissions including the oxygen green
line, which was well above the threshold for
detection by the naked eye during the experiment.
Full legend
High resolution image and legend (57k)
Given the unprecedented brightness of the
speckles, we carried out a number of tests to
rule out potential artefacts, including:
repeating the transmission pattern at a different
time to rule out radio-frequency (r.f.)
interference with the camera electronics,
verifying from radar records that no aircraft
were in the area, and measuring the periods of
white-light sources such as nearby communications
towers and airport beacons. We attempted to
reproduce the results whenever an aurora moved
into range, but auroral events later in the
experiment window never produced E layers of
sufficient density to support significant
transmissions at the original frequency again.
More detailed analysis of the data revealed
additional weak speckles earlier in the original
6-UT hour, at about 06:20 UT, when the
transmitter was operated at a lower frequency
(and lower effective power), and some of the
brightest speckles were also identified in data
from one of the other lower-resolution camera
systems operated from a separate building,
eliminating any doubt that the speckles represent
actual light from the sky.
Although visible levels of artificial optical
emissions have not been reported previously,
there have been other attempts made to stimulate
the auroral E layer with radio waves. A similar
experiment that used low-light television cameras
and a 2-s onñoff cycle but different polarization
reported an estimated modulation of less than 10
R, interpreted as radio-induced decreases in the
green line emission7. Large-scale structural
changes in the overhead aurora have been reported
in conjunction with E-layer heating8, but the
extremely small number of cases and the close
similarity of the observed effects to naturally
occurring processes make it difficult to assess
the true influence of the radio waves on the
auroral events. In contrast, the recent HAARP
observations demonstrate clear on-off control of
the speckles over 50 or more complete cycles.
Potential sources of the observed bright speckles
fall into two categories: production in the local
E-region ionosphere by the transmitter beam, or
indirect creation by modification of the auroral
particle precipitation, which then produces the
optical speckles in the same way as the
background aurora. If the speckles are locally
generated, the role of the natural aurora would
probably be limited to creation of the E layer
for the radio waves to interact with, and it
might be possible to generate similar phenomena
in non-auroral E layers independent of any
specific on-off cycling, a potentially desirable
condition for creation of visible artificial
light. If, on the other hand, the speckles result
from modification of the auroral particle
population, perhaps through perturbations to
currents flowing in the E layer or a wave
resonance, we expect that the specific frequency
of the onñoff cycling relative to the natural
pulsation frequencies might be a critical
parameter, and experiments of this type could
potentially become a new tool for exploration of
time-dependent processes in the aurora and
magnetosphere.
Received 13 September 2004;accepted 6 December 2004
------------------
References
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the ionosphere by electric waves, Part II. Phil.
Mag. 26(7), 425ñ453 (1938)
2. Bernhardt, P., Duncan, L. M. & Tepley, C.
A. Artificial airglow excited by high-power radio
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Article |
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Acknowledgements. HAARP is a Department of
Defense programme operated jointly by the US Air
Force and US Navy. We thank E. Mishin for his
contributions to the experiment planning and P.
Ning for operating the all-sky imager.
Competing interests statement. The authors
declare that they have no competing financial
interests.
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