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ÊCreation of visible artificial optical emissions in the aurora
by high-power radio waves
TODD. R. PEDERSEN-[ 1 ]AND ELIZABETH A. GERKEN-[ 2 ]
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
measurements. 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
wave–particle interactions in the
plasma under conditions difficult to replicate in the laboratory. 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 , 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 6–7
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 10 s in longitudinal
bands running in the magnetic east–west 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 4–6 MHz.
For a period of approximately 10 min between about 06:40 and 06:50,
a number of small speckles {Orbs FRJ } 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
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.
Within a given frame the speckles { Orbs, FRJ } 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
on–off 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. 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 5 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.
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
he 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 on–off 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 on–off 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 1. Bailey, V. A. On some effects caused in the ionosphere
by electric waves, Part II. Phil. Mag. 26(7), 425–453 (1938)
2. Bernhardt, P., Duncan, L. M. & Tepley, C. A. Artificial airglow
excited by high-power radio waves. Science 242, 1022–1027 (1988)
3. Chamberlain, J. W. Physics of the Aurora and Airglow
(International Geophysics Series Vol. 2, Academic, New York, 1961)
4. Rishbeth, H. & Garriot, O. K Introduction to Ionospheric Physics
(Academic, New York, 1969)
5. Djuth, F. T. et al. Large airglow enhancements produced via
wave-plasma interactions in sporadic E. Geophys. Res. Lett. 26,
1557–1560 (1999) | Article |
6. Omholt, A. The Optical Aurora 6 (Physics and Chemistry in Space
Vol. 4, Springer, New York, 1971)
7. Sergienko, T., Kornilov, I., Belova, E., Turunen, T. & Manninen,
J. Optical effects in the aurora caused by ionospheric HF heating. J.
Atmos. Sol-Terr. Phys. 59, 2401–2407 (1997) | Article |
8. Blagoveshchenskaya, N. F. et al. Ionospheric HF pump wave
triggering of local auroral activation. J. Geophys. Res. 106, 29071–29089
(2001) | Article |
9. Bauer, S. J. Physics of Planetary Ionospheres 82–95 (Physics and
Chemistry in Space Vol. 6, Springer, New York, 1973)
10. Davis, N. The Aurora Watchers Handbook 58 (Univ. Alaska Press,
Fairbanks, 1992)
11. Carlson, H. C. Jr & Egeland, A. in Introduction to Space Physics
(eds Kivelson, M. G. & Russell, V.) 459–502
(Cambridge Press, New York, 1995)
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.
Complete HAARP Overview Below...
HAARP Executive Summary
HAARP
HF ACTIVE AURORAL RESEARCH PROGRAM
JOINT SERVICES PROGRAM PLANS AND ACTIVITIES
AIR FORCE
GEOPHYSICS LABORATORY
NAVY
OFFICE OF NAVAL RESEARCH
----------------------------------------------------------------------------
----
HF ACTIVE AURORAL RESEARCH PROGRAM (HAARP)
TABLE OF CONTENTS
EXECUTIVE SUMMARY
1. INTRODUCTION
2. POTENTIAL APPLICATIONS
2.1. Geophysical Probing
2.2. Generation of ELF/VLF Waves
2.3. Generation of Ionospheric Holes/Lens
2.4. Electron Acceleration
2.5. Generation of Field Aligned Ionization
2.6. Oblique HF Heating
2.7. Generation of Ionization Layers Below 90 Km
3. IONOSPHERIC ISSUES ASSOCIATED WITH HIGH POWER RF HEATING
3.1. Thresholds of Ionospheric Effects
3.2. General Ionospheric Issues
3.3. High Latitude Ionospheric Issues
4. DESIRED HF HEATING FACILITY
4.1 Heater Characteristics
4.1.1 Effective-Radiated-power (ERP]
4.1.2 Frequency Range of Operation
4.1.3 Scanning Capabilities
4.1.4. Modes of Operation
4.1.5 Wave Polarization
4.1.6 Agility in Changing Heater Parameters
4.2. Heater Diagnostics
4.2.1. Incoherent Scatter Radar Facility
4.2.2. Other Diagnostics
4.2.3. Additional Diagnostics for ELF Generation Experiments
4.3. HF Heater Location
4.4. Estimated Cost of the New Heating Facility
5. PROGRAM PARTICIPANTS
6. PLANS FOR RESEARCH ON THE GENERATION OF ELF SIGNALS IN THE IONOSPHERE BY
MODULATING THE
POLAR ELECTROJET
6.1. Ionospheric Issues as They Relate to ELF Generation
6.1.1 Ionospheric Research Needs
6.1.2. Ionospheric Research Recommendations
6.2 HF to ELF Excitation Efficiency
6.2.1. Low-Altitude Heating Issues
6.2.2. Low-Altitude Heating Research Recommendations
6.2.3. High-Altitude Heating Issues
6.2.4. High-Altitude Heating Research Recommendations
6.3. Submarine Communication Issues Associated With Exploiting ELF Signals
Generated in the Ionosphere by HF
Heating
6.3.1. General Research Issues
6.3.2. Specific ELF Systems Issuesv 6.4. ELF System-Related Research
Recommendations
7. SUMMARY OF HAARP INITIATION ACTIVITIES
7.1. HAARP Steering Group
7.2. Summary of HAARP Steering Group Activities and Schedule
APPENDIX A HF Heating Facilities
APPENDIX B Workshop on Ionospheric Modification and generation of ELF
Workshop Agenda
Workshop Attendance Roster
HAARP -- HF Active Auroral Research
Program
Executive Summary
As described in the accompanying report, the HF Active Auroral Ionospheric
Research Program (HAARP) is
especially attractive in that it will insure that research in an emerging,
revolutionary, technology area will be focused
towards identifying and exploiting techniques to greatly enhance C3
capabilities. The heart of the program will be
the development of a unique high frequency (HF) ionospheric heating
capability to conduct the pioneering
experiments required under the program.
Applications
An exciting and challenging aspect of ionospheric enhancement is its
potential to control ionospheric processes in
such a way as to greatly improve the performance of C3 systems. A key goal
of the program is the identification
and investigation of those ionospheric processes and phenomena that can be
exploited for DOD purposes, such as
those outlined below.
Generation of ELF waves in the 70-150 Hz band to provide communications to
deeply submerged submarines. A
program to develop efficient ELF generation techniques is planned under the
DOD ionospheric enhancement
program.
Geophysical probing to identify and characterize natural ionospheric
processes that limit the performance of C3
systems, so that techniques can be developed to mitigate or control them.
Generation of ionospheric lenses to
focus large amounts of HF energy at high altitudes in the ionosphere, thus
providing a means for triggering
ionospheric processes that potentially could be exploited for DOD purposes.
Electron acceleration for the generation of IR and other optical emissions,
and to create additional ionization in
selected regions of the ionosphere that could be used to control radio wave
- propagation properties.
Generation of geomagnetic-field aligned ionization to control the
reflection/scattering properties of radio waves.
Oblique heating to produce effects on radio wave propagation at great
distances from a HF heater, thus
broadening the potential military applications of ionospheric enhancement
technology.
Generation of ionization layers below 90 km to provide, radio wave
reflectors (mirrors) which can be exploited for
long range, over-the-horizon, HF/VHF/UHF surveillance purposes, including
the detection of cruise missiles and
other low observables.
Desired HF Heater Characteristics
A new, unique, HF heating facility is required to address the broad range of
issues identified above. However, in
order to have a useful facility at various stages of its development, it is
important that the heater be constructed in
a modular manner, such that its effective-radiated-power can be increased in
an efficient, cost effective manner as
resources become available.
Effective-Radiated-Powers (ERP) in Excess of 1 Gigawatt
One gigawatt of effective-radiated-power represents an important threshold
power level, over which significant
wave generation and electron acceleration efficiencies can be achieved, and
other significant heating effects can
be expected.
Broad HF Frequency Range
The desired heater would have a frequency range from around 1 MHz to about
15 MHz, thereby allowing a wide
range of ionospheric processes to be investigated.
Scanning Capabilities
A heater that has rapid scanning capabilities is very desirable to enlarge
the size of heated regions in the
ionosphere Continuous Wave (CW) and Pulse Modes of Operation. Flexibility in
choosing heating modes of
operation will allow a wider variety of ionospheric enhancement techniques
and issues to be addressed.
Polarization
The facility should permit both X and O polarization in order to study
ionospheric processes over a range of
altitudes.
Agility in Changing Heater Parameters
The ability to quickly change the heater parameters is important for
addressing such issues as enlarging the size of
the heated region the ionosphere and the development of techniques to insure
that the energy densities desired in
the ionosphere can be delivered without self-limiting effects setting-in.
HF Heating Diagnostics
In order to understand natural ionospheric processes as well as those
induced through active modification of the
ionosphere, adequate instrumentation is required to measure a wide range of
ionospheric .parameters on the
appropriate- temporal and spatial scales. A key diagnostic these
measurements will be an incoherent scatter radar
facility to provide the means to monitor such background plasma conditions
as electron densities, electron and ion
temperatures, and electric fields, all as a function of altitude. The
incoherent scatter radar facility, envisioned to
complement the planned new HF heater, is currently being funded in a
separate DOD program, as part of an
upgrade at the Poker Flat rocket range, in Alaska.
For ELF generation experiments, the diagnostics complement would include a
chain of ELF receivers, a digital HF
ionosonde, a magnetometer chain, photometers, a VLF sounder, and a VHF
riometer. In other experiments, in situ
measurements of the heated region in the ionosphere, via rocket-borne
instrumentation, would also be very
desirable. Other diagnostics to be employed, depending on the nature of the
ionospheric modifications being
implemented, will include HF receivers, HF/VHF radars, optical imagers, and
scintillation observations.
HF Heater Location
One of the major issues to be addressed under the program is the generation
of ELF waves in the ionosphere by
HF heating. This requires location the heater where there are strong
ionospheric currents, either at an equatorial
location or a high latitude (auroral) location. Additional factors to be
considered in locating the heater include other
technical (research) needs and requirements, environmental issues, future
expansion capabilities (real estate),
infrastructure, and considerations of the availability and location of
diagnostics. The location of the new HF heating
facility is planned for Alaska, relatively near to a new incoherent scatter
facility, already planned for the Poker Flat
rocket range under a separate DOD program.
In addition, it is desirable that the HF heater be located to permit rocket
probe instrumentation to be flown into the
heated region of the ionosphere. The exact location in Alaska for the
proposed new HF heating facility has not yet
been determined.
Estimated Cost of the New HF Heating Facility
It is estimated that eight to ten million dollars ($8-10M) will provide a
new facility with an effective-radiated-power of
approximately that of the current DOD facility (HIPAS), but with
considerable improvement in frequency tunability
and antenna-beam steering capability. The facility will be of modular design
to permit efficient and cost-effective
upgrades in power as additional funds become available. The desired
(world-class) facility, having the broad
capabilities and flexibility described above, will cost on the order of
twenty-five to thirty million dollars ($25-30M).
Program Participants
The program will be jointly managed by the Navy and the Air Force. However,
because of the wide variety of issues
to be addressed, active participation of the government agencies,
universities, and private contractors is
envisioned.
HF Active Auroral Research Program
The DOD HF Active Auroral Research Program (HAARP) is especially attractive
in that it will insure that research in
an emerging, revolutionary, technology area will be focused towards
identifying and exploiting techniques to greatly
enhance C3 capabilities. The heart of the program will be the development of
a unique ionospheric heating
capability to conduct the pioneering experiments required to adequately
assess the potential for exploiting
ionospheric enhancement technology for DOD (Dept. of Defense) purposes. As
outlined below, such a research
facility will provide the means for investigating the creation, maintenance,
and control of a large number and wide
variety of ionospheric processes that, if exploited, could provide
significant operational capabilities and advantages
over conventional C3 systems. The research to be conducted in the program
will include basic, exploratory, and
applied efforts.
1. Introduction
DoD agencies already have on-going efforts in the broad area of active
ionospheric experiments, including
ionospheric enhancements. These include both space- and ground-based
approaches. The space-based efforts
include chemical releases (e.g., the Air Force's Brazilian Ionospheric
Modification Experiment, BIME; the Navy's
RED AIR program; and multi-agency participation in the Combined Release and
Radiation Effects Satellite,
CRRES). In addition other, planned, programs will employ particle beams and
accelerators aboard rockets (e.g.,
EXCEDE and CHARGE IV), and shuttle- or satellite-borne RF transmitters
(e.g., WISP and ACTIVE). Ground-based
techniques employ the use of high power, radio frequency (RF), transmitters
(so-called "heaters") to provide the
energy in the ionosphere that causes it to be altered, or enhanced. The use
of such heaters has a number of
advantages over space-based approaches.
These include the possibility of repeating experiments under controlled
conditions, and the capability of conducting
a wide variety of experiments using the same facility. For example,
depending on the RF frequency and effective
radiated power (ERP) used, different regions of the atmosphere and the
ionosphere can be affected to produce a
number of practical effects, as illustrated in Table 1. Because of the large
number and wide variety of those.
effects, and because many of them have the potential to be exploited for
important C3 applications, the program is
focused on developing a robust program in the area of ground-based, high
power RF heating of the ionosphere.
To date, most DoD ionospheric heating experiments have been conducted to
gain better understanding of
ionospheric processes, i.e., they have been used as geophysical-probes. In
this, one perturbs the ionosphere, then
studies how it responds to the disturbance and how it ultimately recovers
back to ambient conditions. The use of
ionospheric enhancement to simulate ionospheric processes and phenomena is a
more recent development, made
possible by the increasing knowledge being obtained on how they evolve
naturally. By simulating natural
ionospheric effects it is possible to assess how they may affect the
performance of DoD systems. From a DoD point
of view, however, the most exciting and challenging aspect of ionospheric
enhancement is its potential to control
ionospheric processes in such a way as to greatly enhance the performance of
C3 systems (or to deny accessibility
to an adversary), This is a revolutionary concept in that, rather than
accepting the limitations imposed on
operational systems by the natural ionosphere, it envisions seizing control
of the propagation medium and shaping
it to insure that a desired system capability can be achieved. A key
ingredient of the DOD program is the goal of
identifying and investigating those ionospheric processes and phenomena that
can be exploited for such purposes.
2. Potential Applications
A brief description of a variety of potential applications of ionospheric-
enhancement technology that could be
addressed in the DOD program are outlined below.
2.1. Geophysical Probing
The use of ionospheric heating to investigate natural ionospheric processes
is a traditional one. Such-research is
still required in order to develop models of the ionosphere that can be used
to reliably predict the performance of
C3 systems, under both normal and disturbed ionospheric conditions. This
aspect of ionospheric enhancement
research is always available to the investigator; in effect, as a by-product
of any ionospheric enhancement
research, even if it is driven by specific system applications goals, such
as discussed below.
2.2. Generation of ELF/VLF Waves
A number of critical DOD communications systems rely on the use of ELF/VLF
(30 Hz-30kHz) radio waves. These
include those associated with the Minimum Essential Emergency Communications
Network (MEECN) and those
used to disseminate messages to submerged submarines. In the latter,
frequencies in the 70-150 Hz range are
especially attractive, but difficult to generate efficiently with
ground-based antenna systems. The potential exists for
generating such waves by ground-based heating of the ionosphere. The heater
is used to modulate the
conductivity of the lower ionosphere, which in turn modulates ionospheric
currents. This modulated current, in
effect, produces a virtual antenna in the ionosphere for the radiation of
radio waves. The technique has already
been used to generate ELF/VLF signals at a number of vertical HF heating
facilities in the West and the Soviet
Union. To date, however, these efforts have been confined to essentially
basic research studies, and few attempts
have been made to investigate ways to increase the efficiency of such
ELF/VLF generation to make it attractive for
communications applications. In this regard, heater generated ELF would be
attractive if it could provide
significantly stronger signals than those available from the Navy's existing
antenna systems in Wisconsin and
Michigan. Recent theoretical research suggests that this may be possible,
provided the appropriate HF heating
facility was available. Because this area of research appears especially
promising, and because of existing DOD
requirements for ELF and VLF, it is already a primary driver of the proposed
research program.
In addition to its potential application to long range, survivable, DOD
communications, there is another potentially
attractive application of strong ELF/VLF waves generated in the ionosphere
by ground-based heaters. It is known
that ELF/VLF signals generated by lightning strokes propagate through the
ionosphere and interact with charged
Particles trapped along geomagnetic field lines, causing them, from time to
time, to precipitate into the lower
ionosphere. If such processes could be reliably controlled, it would be
possible to develop techniques to deplete
selected regions of the radiation belts of particles, for short periods,
thus allowing satellites to operate within them
without harm to their electronic components, any of the critical issues
associated with this concept of radiation-belt
control could be investigated as part of the DOD program.
2.3. Generation of Ionospheric Holes/Lens
It is well known that HF heating produces local depletions ("holes") of
electrons, thus altering the refractive
properties of the ionosphere. This in turn affects the propagation of radio
waves passing through that region. If
techniques could be developed to exploit this phenomena in such a way as to
create an artificial lens, it should be
possible to use the lens as a focus to deliver much larger amounts of HF
energy to higher altitudes in the
ionosphere than is presently possible, thus opening up the way for
triggering new ionospheric processes and
phenomena that potentially could be exploited for DOD purposes. In fact, the
general issue of developing
techniques to insure that large energy densities can be made available at
selected regions in the ionosphere, from
ground-based heaters, is an important one that must be addressed in the DOD
program.
2.4. Electron Acceleration
If sufficient energy densities are available in the ionosphere it should be
possible to accelerate electrons to high
energies, ranging from a few eV to even KeV and MeV levels. Such a
capability would provide the means for a
number of interesting DOD applications.
Electrons in the ionosphere accelerated to a few eV would generate a variety
of IR and optical emissions.
Observation and quantification of them would provide data on the
concentration of minor constituents in the lower
ionosphere and upper atmosphere, which cannot be obtained using conventional
probing techniques. Such data
would be important for the development of reliable models of the lower
ionosphere which are ultimately used in
developing radio-wave propagation prediction techniques. In addition, heater
generated IR/optical emission, over
selected areas of the earth could potentially be used to blind space-based
military sensors.
Electrons accelerated to energy levels in the 14-20 eV range would produce
new ionization in the ionosphere, via
collisions with neutral particles. This suggests that it may be possible to
"condition" the ionosphere so that it would
support HF propagation during periods when the natural ionosphere was
especially weak. This could potentially be
exploited for long range (OTH) HF communication/surveillance purposes.
Finally, the use of an HF heater to
accelerate electrons to KeV or MeV energy levels could be used, in
conjunction with satellite sensor
measurements, for controlled investigations of the effects of high energy
electrons on space platforms. There
already is indication that high power transmitters on space-craft accelerate
electrons in space to such high energy
levels, and that those charged particles can impact on the spade- craft with
harmful effects. The processes which
trigger such phenomena and the development of techniques to avoid or
mitigate them could be investigated as part
of the DOD program.
2.5. Generation of Field Aligned Ionization
HF heating of the ionosphere produces patches of ionization that are aligned
with the geomagnetic field, thus
producing scattering centers for RF waves. Natural processes also produce
such scatterers, as evidenced by the
scintillations observed on satellite-to-ground links in the equatorial and
high latitude regions. The use of a HF
heater to generate such scatterers would provide a controlled way to
investigate the natural physical processes
that produce them, and could lead conceivably to the development of
techniques to predict their natural
occurrence, their structure and persistence, and (ultimately) the degree to
which they would affect DOD systems.
One interesting potential application of heater induced field-aligned
ionization is already a part of an on-going DOD
(Air Force/RADC) research program, Ducted HF Propagation. It is known that
there are high altitude ducts in the E-
and F-regions of the ionosphere (110-250 km altitude range) that can support
round-the-world HF Propagation.
Normally, however, geometrical considerations show that it is not possible
to gain access to these ducts from
ground-based HF transmitters, From time-to time, however, natural gradients
in the ionosphere (often associated
with the day-night terminator) provide a means for scattering such HF
signals into the elevated ducts. If access to
such ducts could be done reliably, interesting very long range HF
communications and surveillance applications
can be envisioned.
For example, survivable HF propagation above nuclear disturbed ionospheric
regions would be possible; or, the
very long range detection of missiles breaking through the ionosphere on
their way to targets, could be achieved.
The use of an HF heater to produce field-aligned ionization in a controlled
(reliable) way has been suggested as a
means for developing such concepts, and will be tested in an up-coming
satellite experiment to be conducted
during FY92. The experiment calls for a heater in Alaska to generate
field-aligned ionization that will scatter HF
signals from a nearby transmitter into elevated ducts. A satellite receiver
will record the signals to provide data on
the efficiency of the field-aligned ionization as an RF scatterer, as well
as the location, persistence, and HF
propagation properties associated with the elevated ducts.
2.6. Oblique HF Heating
Most RF heating experiments being conducted in the West and in the Soviet
Union employ vertically propagating
HF waves. As such the region of the ionosphere that is affected is directly
above the heater. For broader military
applications, the potential for significantly altering regions of the
ionosphere at relatively great distances (1000 km
or more) from a heater is very desirable. This involves the concept of
oblique heating. The subject takes an added
importance in that higher and higher effective radiated powers are being
projected for future HF communication
and surveillance systems. The potential for those systems to inadvertently
modify the ionosphere, thereby
producing self-limiting effects, is a real one that should be investigated,
In addition, the vulnerability of HF systems
to unwanted effects produced by other, high power transmitters (friend or
foe) should be addressed.
2.7. Generation of Ionization Layers Below 90 Km
The use of very high power RF heaters to accelerate electrons to 14-20 eV
opens the way for the creation of
substantial layers of ionization at altitudes where normally there are very
few electrons. This concept already has
been the subject of investigations by the Air Force (Geophysics Lab), the
Navy (MU), and DARPA. The Air Force, in
particular, has carried the concept, termed Artificial Ionospheric Mirror
(AIM), to the point of demonstrating its
technical viability and proposing a new initiative to conduct
proof-of-concepts experiments. The RF heater(s) being
considered for AIM are in the 400 MHz-3 GHz range, much higher than the HF
frequencies (1.5 MHz-15 MHz)
suitable for investigating the other topics discussed in this summary. As
such, the DOD program (HAARP) will not
be directly involved with AIM-related ionospheric enhancement efforts,
3. IONOSPHERIC ISSUES ASSOCIATED WITH HIGH POWER RF HEATING
As illustrated in Figure 1, as the HF power delivered to the ionosphere is
continuously increased the dissipative
process dominating the response of the geophysical environment changes
discontinuously, producing a variety of
ionospheric effects that require investigation. Those anticipated at very
high power levels (but not yet available in
the West from existing HF heaters) are especially interesting from the point
of view of potential applications for DOD
purposes,
3.1. Thresholds of Ionospheric Effects
At very modest HF powers, two RF waves propagating through a common volume
of ionosphere will experience
cross-modulation, a superposition of the amplitude modulation of one RF wave
upon another. At HF effective
radiated powers available to the West, measurable bulk electron and ion gas
heating is achieved, electromagnetic
radiation (at frequencies other than transmitted) is stimulated, and various
parametric instabilities are excited in the
plasma. These include those which structure the plasma so that it scatters
RF energy of a wide range of
wavelengths.
Figure 1. Thresholds of Ionospheric Effects as a function of Heater ERP
(unavailable)
There is also evidence in the West that at peak power operation parametric
instabilities begin to saturate, and at
the same time modest amounts of energy begin to go into electron
acceleration, resulting in modest levels of
electron-impact excited airglow. This suggests that at the highest HF powers
available in the West, the instabilities
commonly studied are approaching their maximum RF energy dissipative
capability, beyond which the plasma
processes will "runaway" until the next limiting process is reached. The
airglow enhancements strongly suggest that
this next process then involves wave-particle interactions and electron
acceleration.
The Soviets, operating at higher powers than the West, now have claimed
significant stimulated ionization by
electron-impact ionization. The claim is that HF energy, via wave-particle
interaction, accelerates ionospheric
electrons to energies well in excess of 20 electron volts (eV) so that they
will ionize neutral atmospheric particles
with which they collide. Given that the Soviet HF facilities are several
times more powerful than the Western facilities
at comparable mid-latitudes, and given that the latter appear to be on a
threshold of a new "wave-particle" regime
of phenomena, it is believed that the Soviets have crossed that threshold
and are exploring a regime of
phenomena still unavailable for study or application in the West.
The Max Planck HF facility at Tromso, Norway, possesses power comparable to
that of the Soviet high power
heaters, yet has never produced airglow enhancements commonly produced by US
HF facilities at lower HF power,
but at lower latitudes. This is attributed to a present inadequate
understanding 'f how to make the auroral latitude
ionosphere sustain the conditions required to allow the particle
acceleration process to dominate, conditions which
are achieved in the (more stable) mid- latitude regions.
What is clear, is that at the gigawatt and above effective radiated power
energy density deposited in limited regions
of the ionosphere can drastically alter its thermal, refractive, scattering,
and emission character over a very wide
electromagnetic (radio frequency) and optical spectrum, what is needed is
the knowledge of how to select desired
effects and suppress undesired ones. At present levels of understanding,
this can only be done by: identifying and
understanding what basic processes are involved, and how they interplay,
This can only be done if driven by a
strong experimental program steered by tight coupling to the interactive
cycle of developing
theory-model-experimental test.
3.2. General Ionospheric Issues
When a high-power HF radio wave reflects in the ionosphere, a variety of
instability processes are triggered. At
early times (less than 200 ms) following HF turn-on, microinstabilities
driven by ponderomotive forces are excited
over a large (1-10 km) altitude interval extending downwards from the point
of HF reflection to the region of the
upper hybrid resonance. However, at very early times (less than 50 ms) and
at late times (greater than l0 s) the
strongest HF-induced Langmuir turbulence appears to occur in the vicinity of
HF reflection. The Langmuir
turbulence also gives rise to a population of accelerate electrons. Over
time scales op 100's of milliseconds and
longer, the microinstabilities must coexist with other instabilities that
are either triggered or directly driven by the
HF-induced turbulence. Some of these instabilities are believed to be
explosive in character. The dissipation of the
Langmuir turbulence is thought to give rise to meter-scale irregularities
through several different instability routes.
Finally, over time scales of tens of seconds and longer, several thermally
driven instabilities can be excited which
give rise to kilometer-scale ionospheric irregularities. Some of these
irregularities are aligned with the geomagnetic
field, while others are aligned either along the axis of the HF beam or
parallel to the horizontal.
Recently, ionospheric diagnostics of HF modification have evolved to the
point where individual instability
processes can be examined in detail. Because of improved diagnostic
capabilities, it is now clear that the
wave-plasma interactions once thought to be rather simple are in fact rather
complex. For example, the latest
experimental findings at Arecibo Observatory suggest that plasma processes
responsible for the excitation of
Langmuir turbulence in the ionosphere are fundamentally different from past
treatments based on so-called "weak
turbulence theory".
This theoretical approach relies on random phase approximations to treat the
amplification of linear plasma waves
by parametric instabilities. Research in HF ionospheric modification during
the period 1970-1986 commonly
focused on parametric instabilities to explain observational results. In
contrast, there is in increasing evidence that
the conventional picture is wrong and that the ionospheric plasma undergoes
a highly nonlinear development,
culminating in the formation of localized states of strong plasma
turbulence. The highly localized state (often
referred to as cavitons) consists of high-frequency plasma waves trapped in
self- consistent electron density
depletions.
It is important to realize that many different instabilities are
simultaneously excited in the plasma and that one
instability process can greatly influence the development of another.
Studies of competition between similar types
of instability processes and the interaction between dissimilar wave-plasma
interactions are in the earliest stages of
development. However, it is clear that the degree to which one instability
is excited in the plasma may severely
impact a variety of other HF-induced processes through HF-induced pump wave
absorption, changes in particle
distribution functions, and the disruption op other coherently-driven
processes relying on smooth ionospheric
electron density gradients. Because the efficiency of many instability
processes is dependent on geomagnetic dip
angle, the nature of instability competition in the plasma is expected to
change with geomagnetic latitude. Indeed,
observational results strongly support this notion. consequently, it may be
very difficult to extrapolate the
observational results obtained at one geomagnetic latitude to another.
Moreover, even at one experimental station,
physical phenomena excited by a high-power HF wave is strongly dependent
upon background ionospheric
conditions. A classic illustration of this point may be found in Arecibo
observations made when local electron energy
dissipation rates are low. In this case, the ionospheric plasma literally
overheats due to the absence of effective
electron thermal loss processes.
The large (factor of four) enhancement in electron temperature that
accompanies this phenomenon gives rise to a
class of instability processes that is completely different from others
observed under "normal" conditions where the
ionospheric thermal balance is not greatly disrupted. At ERPs greater than a
gigawatt (greater than 90 dBW),
ponderomotive forces are no longer small compared to thermal forces. This
may qualitatively change the nature of
the instability processes in the ionosphere. Experimental research in this
area, however, must wait until such
powerful ionospheric heaters are developed.
3.3. High Latitude Ionospheric Issues
Radio wave heating of the ionosphere at mid-latitudes (e.g., Arecibo and
Platteville) has occurred under conditions
where the background ionosphere (prior to turning on the heater) was fairly
laminar, stable, fixed, etc. However, at
high latitudes (i.e., auroral latitudes such as HIPAS and Tromso) the
background ionosphere is a dynamic entity.
Even the location of the aurora and the electrojet are changing as a
function of latitude, altitude and local time.
Moreover, the background E- and F-region ionosphere may not be laminar on
scale sizes less than 20 km and less
than 100 km, respectively. Rather, there is the possibility of E- and F-
region irregularities (with scale sizes from
cms to kms) occurring at various times due to (for example) electrojet
driven instabilities in the E-region, and
spread F or current driven instabilities in the F-region. High energy
particles, e.g., from solar flares, may also lead
to D-region structuring. In addition, connection to the magnetosphere via
the high conductivity along magnetic field
lines can play an important role. The theoretical understanding of high
latitude ionospheric heating processes has
been improving; however, given the dynamic nature of the high latitude
ionosphere, it is important to diagnose the
background ionosphere prior to the inception of any heating experiments.
This diagnostic capability aids in
determining long term statistics, as well as real-time parameters. While
such diagnostics have been an integral part
of the heating experiments at Arecibo and Tromso, HF heating experiments at
HIPAS have been severely
hampered by a lack of similar diagnostics.
4. DESIRED HF HEATING FACILITY
In order to address the broad range of issues discussed in the previous
sections, a new, unique, HF heating facility
is required. An outline of the desired capabilities of such a heater, along
with diagnostic needed for addressing
these issues are given in Table 2.
(Table 2 not available in this document)
4.1. Heater Characteristics
The goals for the HF heater are very ambitious. In order to have a useful
facility at various stages of its
development, it is important that the heater be constructed in a modular
manner, such that its effective-
radiated-power can be increased in an efficient, cost effective manner as
resources become available. Other
desired HF heater characteristics are outlined below.
Effective-Radiated-Power (ERP)
One gigawatt of effective-radiated-power (90 dBW) represents an important
threshold power level, over which
significant wave generation and electron acceleration efficiencies can he
achieved, and other significant heating
effects can be expected. To date, the Soviet Union has built such a powerful
HF heater. The highest ERPs
achieved by US. facilities is about one-fourth of that. Presently, a heater
in Norway, operated by the Max Planck
Institute in the Federal Republic of Germany, is being reconfigured to
provide 1 gigawatt of ERP at a single HF
frequency. The HAARP is to ultimately have a HF heater with an ERP well
above 1 gigawatt (on the order of 95-100
dBW); in short, the most powerful facility in the world for conducting
ionospheric modification research. In achieving
this, the heated area in the F-region should have a minimum diameter of at
least 50 km, for
diagnostic-measurement purposes.
4.1.2. Frequency Range of Operation
The desired heater would have a frequency range from around 1 MHz to about
15 MHz, thereby allowing a wide
range of ionospheric processes to be investigated. This incorporates the
electron-gyro frequency and would permit
operations under all anticipated ionospheric conditions. Multi-frequency
operation using different portions of the
antenna array is also a desirable feature. Finally, frequency changing on an
order of milliseconds is desirable over
the bandwidth of the HF transmitting antenna.
4.3. Scanning Capabilities
A heater that has scanning capabilities is very desirable in order to
enlarge the size of heated regions in the
ionosphere. Although a scanning range from vertical to very oblique (about
10 degrees above the horizon) would
be desirable, engineering considerations will most likely narrow the
scanning range to about 45 degrees from the
vertical. The capability of rapidly scanning (microseconds time scale) in
any direction, is also very desirable.
4.1.4. Modes of Operation
Flexibility in choosing heating modes of operation, including continuous-
wave (CW) and pulsed modes, will allow a
wider variety of ionospheric modification techniques and issues to be
addressed.
4.1.5. Wave polarization
The heater should permit both X and O polarizations to be transmitted, in
order to study ionospheric processes
over a range of altitudes.
4.1.6. Agility in Changing Heater Parameters
The ability to quickly change heater parameters, such as operating
frequency, scan angle and direction, power
levels, and modulation is important for addressing such issues as enlarging
the size of the modified region in the
ionosphere and the development of techniques to insure that the energy
densities desired in the ionosphere can
be delivered from the heater without self-limiting effects setting-in.
4.2. Heating Diagnostics
In order to understand natural ionospheric processes as well as those
induced through active modification of the
ionosphere, adequate instrumentation is required to measure a wide range of
ionospheric parameters on the
appropriate temporal and spatial scales.
4.2.1. Incoherent Scatter Radar Facility
A key diagnostic for these measurements will be an incoherent scatter radar
facility to provide the means to monitor
such background plasma conditions as electron densities, electron and ion
temperatures, and electric fields, all as
a function of altitude. In addition, the incoherent scatter radar will
provide the means for closely examining the
generation of plasma turbulence and the acceleration of electrons to high
energies in the ionosphere by HF
heating. The incoherent scatter radar facility, envisioned to complement the
planned new HF heater, is currently
being funded in a separate DOD program, as part of an upgrade at the Poker
Flat rocket range, in Alaska.
4.2.2. Other Diagnostics
The capability of conducting in situ measurements of the heated region in
the ionosphere, via rocket-borne
instrumentation, is also very desirable. Other diagnostics to be employed,
depending on the specific nature of the
HF heating experiments, may include HF receivers for the detection of
stimulated electromagnetic emissions from
heater induced turbulence in the ionosphere; HF/VHF radars, to determine the
amplitudes of short-scale (1-10 m)
geomagnetic field-aligned irregularities; optical imagers, to determine the
flux and energy spectrum of accelerated
electrons and to provide a three-dimensional view of artificially produced
airglow in the upper atmosphere: and,
scintillation observations, to be used in assessing the impact of HF heating
on satellite downlinks and in diagnosing
large- scale ionospheric structures.
4.2.3. Additional Diagnostics for ELF Generation Experiments
These could include a chain of ELF receivers to record signal strengths at
various distances from the heater; a
digital HF ionosonde, to determine background electron density profiles in
the E- and F-regions; a magnetometer
chain, to observe changes in the earth's magnetic field in order to
determine large volume ionospheric currents and
electric fields; photometers, to aid in determining ionospheric
conductivities and observing precipitating particles; a
VLF sounder, to determine changes in the D-region of the ionosphere; and, a
riometer, to provide additional data
in these regards, especially for disturbed ionospheric conditions.
4.3. HF Heater Location
One of the major issues to be addressed under the program is the generation
of ELF waves in the ionosphere by
HF heating. This requires locating the heater where there are strong
atmospheric currents, either at an equatorial
location or at a high latitude (auroral) location. Additional factors to be
considered in locating the heater include
other technical (research) needs and requirements, environmental issues,
future expansion capabilities (real
estate), infrastructure, and considerations of the availability and location
of diagnostics. The location of the new HF
heating facility is planned for Alaska, relatively near to a new incoherent
scatter facility, already planned for the
Poker Flat rocket range under a separate DOD program. In addition, it is
desirable that the HF heater be located to
permit rocket probe instrumentation to be flown into the heated region of
the ionosphere. The exact location in
Alaska for the proposed new HF heating facility has not yet been determined.
4.4. Estimated Cost of the New HF Heating Facility
It is estimated that eight to ten million dollars ($8-10M) will provide a
new HF heating facility with an
effective-radiated-power of approximately that of the current DOD facility
(HIPAS), but with considerable
improvement in frequency tunability and antenna-beam steering capability,
The new facility will be of modular
design to permit efficient and cost-effective upgrades in power as
additional funds become available. The desired
(world-class) facility, having the broad capabilities and flexibility
described above, will cost on the order of
twenty-five to thirty million dollars ($25-30M).
5. PROGRAM PARTICIPANTS
The program will be jointly managed by the Navy and the Air Force. However,
because of the wide variety of issues
to be addressed, substantial involvement in the program by other government
agencies (DARPA, DNA, NSF, etc.),
universities, and private contractors is envisioned.