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.
Soure: http://www.viewzone.com/haarp.exec.html