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.