SAC Report on the Future Scientific
Direction of
1. Introduction
Following
the last EISCAT Council meeting in June 2003, a questionnaire prepared by the
SAC chairman and the EISCAT director (attached as Appendix 1) was distributed
to the SAC and Council members of all the existing EISCAT countries. The questionnaire asked EISCAT users to
specify their current scientific priorities and to predict what the major
research issues would be during the ten-year period of the next EISCAT
agreement. Users were also asked to
suggest what new facilities EISCAT would need to provide in order to meet their
scientific requirements, as well as to rank the existing facilities in priority
order. It was suggested that national SAC members should distribute the document
among their EISCAT user communities and collate the input into a single
national response on the future of EISCAT.
A deadline of September 19 was suggested for responses to be returned,
so that the issues raised could be discussed at the SAC meeting of October 9
and 10.
As
input to this discussion process, an updated version of the E-Prime document
was circulated to all SAC and Council members, and was made available to all
EISCAT users via the web. It was
stressed that, while the suggestions contained in the E-Prime document might be
a useful input to discussions within the user community, users should not feel
constrained to suggest only facilities identified in the E-Prime document. Rather, they should feel free to suggest any
facility which would help them meet their future scientific aims.
In
addition, many of the EISCAT countries had held meetings of their user
communities during the spring and summer to discuss the future of EISCAT. The suggestions of these meetings were also
incorporated into the responses returned by the national SAC representatives.
2. Science topics for the next 10 years
Although
the responses from the various EISCAT countries contained a wide diversity of
science topics, there was general agreement on many of the most important
issues. The following paragraphs give a
brief outline of each separate research area, and of the major issues which
users are hoping to address in the next ten-year period.
2.1 Ionosphere-Atmosphere
Coupling
This
topic embraced a number of themes. There
was general agreement that understanding the interface between the ionosphere
and middle atmosphere would become an increasingly important topic over the
next decade, as efforts continued to quantify the nature and effectiveness of
the coupling mechanisms between these regions.
Major questions remain concerning the processes which mediate energy
transfer in the mesosphere and lower thermosphere, and the extent to which
these are important in governing the behaviour of the lower atmosphere. One key area is likely to be the study of the
impact which energetic solar and magnetospheric
particles have on the chemistry of the mesosphere and upper stratosphere (e.g.
ozone and nitric oxide abundance) using data from multiple diagnostics in
conjunction with detailed chemical models, such as the Sodankyla
Ion Chemistry Model, which are now under development. The effect of auroral precipitation on the
chemistry at higher altitudes (e.g. via the production of odd nitrogen) also
needs much further study.
A
fundamental question relating to energy coupling through the atmosphere
concerns the effectiveness of planetary waves and other long-term oscillatory
modes in transferring energy from thermospheric
altitudes down to the troposphere. Because such modes can be global in scale,
EISCAT data should form part of a co-ordinated network of long-term
observations in the high-latitude region to study the climatology of such wave
modes. Such measurements would also be
useful in parameterising the long-term behaviour of
atmospheric tides, gravity waves and turbulence in the high-latitude region, which are poorly described in present-day coupled
atmospheric models, but whose contribution to energy transport is likely to be
highly significant.
The
study of phenomena related to “dusty plasmas” is likely to continue to be a
major theme of EISCAT research in the coming decade. EISCAT has been at the forefront of
experiments to establish the nature and behaviour of Polar Mesospheric Summer
Echoes (PMSE). Recently, innovative
experiments using the Tromsø heater to modulate the
intensity of PMSE layers have provided new theoretical understanding of the
microphysical processes responsible for forming these layers. PMSE is of great interest because it is a
sensitive indicator of conditions at the mesopause,
and can also be used in examining the dynamics of this important region. There is also growing interest in the
possible existence and significance of Polar Mesosphere Winter Echoes (PMWE),
which may be observable at EISCAT VHF frequencies.
In
addition to coupling with the lower atmosphere, there is still considerable
interest in studying the electrodynamic coupling
between the ionised and neutral atmospheres at ionospheric
heights. New results suggest that meso-scale structure in the neutral atmosphere might play a
much greater role in determining the overall amount of energy dissipation than
has previously been acknowledged. The
understanding of this process is extremely important, because coupling
processes such as Joule heating and Lorentz forcing
constitute the dominant mechanisms by which the energy of the solar wind is
ultimately dissipated in the upper atmosphere, and are probably not well
described by current models.
In
order to address the above issues, EISCAT would need to develop an improved
observational capability in the lowest part of its altitude range (50-150 km)
and to undertake regular long continuous runs of dedicated experiments. This programme of observations should be
suitable for long-term climatological monitoring, but
must also be flexible enough that observations can be secured during uncommon energetic
phenomena such as PCA events. Well-planned
co-ordinated observations with complementary high-latitude facilities including
rockets, MF/MST radars, lidars and other optical
measurements, as well as other IS radars, will be essential, to optimise the
information obtained from these measurements and their applicability to
global-scale studies.
The
continued availability of the Tromsø heater will be
essential in carrying out controlled experiments to establish the microphysics
of PMSE layers, while the maintenance or enhancement of the mainland remote
sites is also necessary to any detailed work on electrodynamic
coupling, for which reliable measurements of electric field at high
time-resolution are essential.
2.2 Fundamental
plasma physics
This
topic should perhaps be further sub-divided into two headings, corresponding to
plasma processes observed in the artificially modified and natural (unmodified)
high-latitude ionosphere.
The
EISCAT Tromsø heater continues to be the world’s most
powerful HF transmitter for research purposes.
As such it remains the best tool of its kind for carrying out controlled
plasma physics experiments in the ionosphere-magnetosphere system, which is in
itself the only unconstrained plasma environment accessible to such
experimentation. Despite many years of
operation, the heater continues to provide some of the most innovative science
coming out of EISCAT. The recent use of
the heater to modulate the charge state of PMSE layers has already been
referred to above. Other examples of new
techniques include field-line tagging to facilitate ground-satellite studies,
and the use of APIs (Artificial Periodic Irregularities) to probe the
temperature structure of the lower atmosphere.
There have even been attempts to use the heater as a magnetospheric radar, which have suggested that
backscatter can be received from magnetospheric
altitudes, though its nature and interpretation are not yet understood.
It
is clear that ionospheric heating, using both the Tromsø heater and the forthcoming SPEAR facility, currently
being deployed by the
Despite
producing a large amount of truly world-class science, the Tromsø
heater has not had any extensive maintenance in recent years. In order to ensure that the facility remains
viable for the coming decade, a programme of modernisation is needed to invest
it with some of the more advanced capabilities (e.g. more flexible modulation
patterns, electronic beam-forming, greater steerability)
which can be found on more modern (though less powerful) facilities such as
SPEAR and HAARP. Because many of the
most interesting experiments are dependent on the existence of particular ionospheric conditions or satellite conjunctions, there is
a need to establish a flexible observing programme that will enable
experimenters to take advantage of such conditions whenever they occur.
In
the natural (i.e. unmodified) ionosphere, there are also a number of important
plasma physics issues. Possibly the most
exciting field of research opened up by EISCAT in recent years concerns the
importance of filamentary field aligned current structures, and their ability
to carry large amounts of energy and stimulate plasma waves. Section 2.3 on auroral physics discusses
these issues in greater detail. The
physical interpretation of radar spectra at other wavelengths (e.g. in VHF and
HF coherent scatter) continues to be an important unresolved question,
especially the relationship of spectral width to the open/closed field line
boundary, particle precipitation regions and E-field turbulence. Finally, studies of the plasma physics of
particle precipitation are applicable equally to the natural and heated
ionosphere, in that observation of relativistic precipitation in the
unperturbed ionosphere can give important information on magnetospheric
processes such as pitch angle scattering and diffusion, while such studies can
also be furthered by attempts to use SPEAR and the Tromsø
heater to modulate particle precipitation artificially.
2.3 Auroral
physics
The
aurora is the best known of all phenomena in solar terrestrial physics, but the
detailed understanding of auroral processes is far from complete. In addition to the continuing debate about
the processes responsible for accelerating precipitating particles to the
observed energies, EISCAT has recently provided dramatic demonstrations of the
importance of small-scale structure in auroral arcs, and of the processes such
as plasma wave coupling which occur in connection with these. Because many of the unresolved questions in
auroral physics relate to fundamental processes in plasma physics, there is
substantial overlap of this topic with 2.2 above.
In
recent years, there has been increasing understanding of the importance of
small-scale structures and to the overall energetics
of the aurora. It is becoming clear, for
example, that auroral rays are of particular interest in this respect, and that
their incoherent scatter signature cannot be understood in terms of
conventional Maxwellian plasmas, but necessitates consideration of the role
played by small-scale plasma waves. The understanding of current flow in
auroral structures means that it is also necessary to understand “black
aurora”, the regions of no emission which may contain large electric fields
and/or substantial field-aligned currents.
Because of the large energy densities and small spatial scales involved,
these highly structured regions are likely to be rich in plasma physics
phenomena. Because these structures have scale sizes much smaller than the
radar beam width, the emerging new technique of radar interferometry, pioneered
at EISCAT, clearly has an important role to play.
On
a larger scale, a combination of new ground-based and space-based imagers has
made increasingly clear the importance of proton precipitation in transferring
energy from the magnetosphere to the ionosphere. Understanding the occurrence and relative
importance of proton and electron precipitation is an undertaking that will
rely on a combination of radar results and multi-wavelength optical measurements,
underpinned by modelling and spectroscopic theory. The availability of high-quality optical data
on a variety of spatial scales is a vital prerequisite for understanding the
chemistry and dynamics of auroral processes. The existing EISCAT countries have
a very strong tradition of investment in auroral instrumentation, both on the
mainland and at
In
order to fully exploit the world-leading position which EISCAT has acquired in
the area of radar interferometry, an obvious step would be to provide
purpose-built interferometry capabilities at the active radar sites. Detailed plans on how to achieve this at the
ESR, using a network of small receiving antennas in the vicinity of the
existing dishes, have already been produced in
Successful
auroral experiments are clearly heavily dependent on conditions, needing both
clear skies and appropriate levels of magnetic activity. A flexible radar operating schedule based on
near-continuous observations would allow the possibility to implement auroral
modes whenever conditions were appropriate and would thus maximise the
scientific return on observations. In
addition, because the availability of high-quality supporting diagnostics,
particularly optical systems, is so important, strenuous efforts should be made
to ensure that a full suite of such instruments is available in the vicinity of
the radar and is available to carry out observations whenever conditions are
suitable.
2.4 Magnetosphere-Ionosphere
coupling
This
topic is heavily bound up with both 2.2 and 2.3, since naturally occurring
plasma instabilities and aurora can both be considered as important aspects of
magnetosphere-ionosphere energy transfer.
Under this heading, one should also include large-scale phenomena such
as ion outflow, whose causes and overall importance are still a matter of
debate. Neither the acceleration
mechanisms which produce ion outflow, nor the source of the free energy which
makes it possible, are well defined.
Indeed, ion outflow seems to arise under a range of conditions, so that
there may be no convenient single explanation.
Ion outflow is certainly an important mechanism in transferring mass and
energy from the ionosphere to the magnetosphere, but the degree of this
importance (e.g. whether it represents a significant long-term loss process) is
unclear. Part of the difficulty arises from a relative paucity of observations
in the region of interest (1000-3000 km altitude) where well-known ionospheric phenomena such as ion outflow evolve into well-known
magnetospheric phenomena such as ion beams and conics,
such that the detailed connection between them has not been established.
This
part of the high-altitude ionosphere, or near-Earth magnetosphere, is a region
which has not been extensively studied, but contains a number of interesting
phenomena, such as the upper boundary of the Alfven
resonance region. Processes of critical importance to auroral acceleration may
also manifest themselves at these altitudes. The rarefied nature of the
atmosphere at these heights and the presence of light ions such as hydrogen and
helium, give rise to a regime where different ion species can take up their own
flow characteristics and thermal velocity distributions, and where heat flow,
advection and Coulomb collisions are increasingly important in understanding
the energy budget. The EISCAT community
has had a long-standing interest in this region, but this has never really been
supported by high-quality observations, in part because of the substantial
difficulties inherent in measuring at these altitudes.
The
study of large-scale (or even global-scale) structure is another aspect of
magnetosphere-ionosphere coupling which has developed rapidly in recent years. This reflects the increasing use of networks
of ground-based instruments in concert with spacecraft data. An obvious examples is the use of the GPS
satellite beacons for global studies of Total Electron Content and tomographic imaging of electron density structures, for
which EISCAT has provided important “ground truth” measurements. This technique will continue to be important
during the next decade, when similar capabilities will also be provided by the
European network of Galileo satellites.
The availability of such global-scale images of the ionospheric
response to solar and magnetospheric forcing
mechanisms are of immense interest in themselves, but provide added value to
data from individual ground-based radars, as they enable observations made
within a relatively small field-of-view to be interpreted in a much broader
global context. Incoherent scatter data are a vital complement to such
measurements. For example, the
availability of accurate electron density profiles from radar measurements is
absolutely essential to the correct interpretation of TEC data.
Complementary
ground-based data also currently provides key parameters in support of spacecraft
missions. EISCAT has implemented a
comprehensive programme of experiments in support of the Cluster mission, and
it is anticipated that this pattern will be replicated for future missions such
as Double Star, Themis and MMS. In this role, EISCAT will be part of a
well-organised network of ground-based diagnostics whose data will be
invaluable for putting the spacecraft data into context and quantifying the ionospheric effect of the magnetospheric
processes which they are observing.
For
EISCAT to maximise its supporting role for spacecraft missions, or to play a
major role in the types of global instrument networks described above, it is
clearly desirable for it to operate as near continuously as possible, with sufficiently
flexible scheduling to exploit possible ground-satellite co-ordinations to
their best advantage. For studies of
ionosphere-magnetosphere coupling, the requirement is for high power or high
gain and relatively low (VHF) frequencies suited to low density plasmas with a
long Debye length.
Support from other diagnostics such as rockets or satellites is essential
to the exploration of this altitude region, and a strong connection between
observation and modelling is also needed.
2.5 Solar
wind studies
For
many years, EISCAT has had a small group of users working on the study of the
solar wind using measurements of interplanetary scintillation (IPS) of the
radio signals from distant objects such as quasars. EISCAT is acknowledged within the solar
physics community as being the best instrument of its kind in the world for
such studies. This is because the high
frequencies used in the most recent EISCAT experiments (1.4 GHz since 2002)
enable it to probe regions closer to the solar limb than any other current
diagnostic. This allows EISCAT to study
directly the region in which the solar wind is being accelerated, and in which
turbulent interaction regions between wind streams of different speed can
develop. In addition, EISCAT can study the spectrum of irregularity scale sizes
present within the solar wind plasma, permitting inferences to be drawn on the
role of waves in transferring and dissipating energy within the solar wind. Because
the nature of the acceleration process is still unknown, such observations are
extremely important, especially when coupled with results from other
ground-based systems which allow the development of the solar wind further from
the sun to be investigated.
As
powerful as the IPS technique currently is, it will become still more powerful
and important in the period beyond 2006, when it will be possible to use EISCAT
measurements in conjunction with observations by the STEREO spacecraft. Currently, interpretation of IPS measurements
is constrained by a number of ambiguities (such as the lack of solar wind
density measurements) which EISCAT alone cannot resolve. STEREO will provide some of this missing
information, substantially aiding the interpretation of IPS data and reducing
the current reliance on models. Because
EISCAT IPS and STEREO will measure different properties of the solar wind (e.g.
spectral content and electron density), they should be considered as strongly
complementary data sets, for which the value of each would be decreased by the
absence of the other. The continued
existence of EISCAT’s IPS capability during the
STEREO mission would therefore add substantially to the scientific value of the
STEREO data for modest marginal cost.
This would also be true for the Solar Orbiter
mission, due for launch in 2012, when it would be possible for the first time
to compare EISCAT measurements of solar wind speed and structure in the
acceleration region with actual in-situ observations made by spacecraft
instruments.
In
order to fully exploit the potential of interplanetary scintillation studies,
EISCAT will need to retain its three mainland sites, and make a number of other
technological developments. For example,
the ability to make IPS measurements at 1.4 GHz, currently only available at Kiruna and Sodankyla, should be extended to Tromsø,
allowing EISCAT to take advantage of the longest baseline in its mainland
system. Also, VLBI receivers of the kind
conventionally used in radio astronomy should be incorporated into all EISCAT
sites. This would extend EISCAT’s capabilities for IPS observation by allowing phase
and amplitude scintillation to be measured simultaneously. It would also allow EISCAT the possibility to
act as a northward extension of the existing European VLBI network for radio
astronomy, with the aim of provoking interest and funding from the astronomical
community.
2.6
Meteor Studies
As
with solar wind studies, this area is not in the mainstream of EISCAT research.
It represents, however, an interesting application of the radar technique with
potential for future extension. Because
of its tristatic nature, the EISCAT UHF system is
currently the only high-latitude radar facility with the capability to measure
the 3-d velocity of meteors entering the radar beam. Because of its location, EISCAT provides a
unique chance to study the off-ecliptic component of interplanetary and
interstellar dust distribution, and to study the effects of meteor influx on
the high-latitude atmosphere and ionosphere.
While
it is known that asteroids and comets are the main contributors to the dust
population around Earth orbit, the relative importance of these sources has not
been quantified. While spacecraft
observations have provided good measurements of the dust cloud close to the
ecliptic plane, the nature of the dust cloud at high latitudes is not well
known. Long period comets could be an
important source for this high-latitude dust, and establishing their role is
important to understanding dust production in the inner solar system, where they
are likely to be an even more significant source. In addition, EISCAT results
have provided important insights into the plasma physical processes occurring
in the heads and trains of vaporising meteors.
In particular, EISCAT data have enabled the construction of the best
current model to explain and predict the characteristics of meteor head echoes.
The
mass range of detectable meteors largely depends on the radar frequency, being
much larger at VHF than at UHF. An
upgraded VHF system with tristatic capabilities would
increase the observable rate of meteors whose vector could be determined by an
order of magnitude with respect to present-day UHF observations, allowing a
much better statistical characterisation of the high-latitude distribution.
2.7 New Techniques and Technologies
Throughout
its operational lifetime, EISCAT has acted as a spur for the development of new
experimental techniques for incoherent scatter radar, as users have sought ways
to maximise the spatial and temporal resolution of their data. Modern experiments, with spatial resolutions
as low as 300m and temporal resolutions down to 0.2s, represent the culmination
of years of work on coding theory, experiment design, signal processing and
data analysis. Despite this progress,
the development of new techniques is still a strong element of EISCAT work, the
progress in radar interferometry referred to above being perhaps the most
striking recent example.
The
work done by the group at the
The
Finnish EISCAT community, which has been responsible for many of the most
profound technical innovations, is now working on an entirely new measurement
technique, in which scattering amplitudes are recorded directly from the
measured echoes without correlation calculation. Using this method, the measurement errors of
each data point become independent, and can be easily processed by inverse
problem solvers. While the data sets
obtained by this technique are very large, the result should reflect the
optimum measurement which it is possible to make, with good characterisation of
errors and no biases introduced by post-processing of the measured data.
A
further area in which EISCAT has been leading the way in the implementation of
new techniques is the field of space debris detection. Specialised signal processing hardware has
been developed, which can be “piggy-backed” without interference onto normal
incoherent scatter measurements, giving EISCAT the ability to detect space
debris fragments down to a scale size of 2cm at 1000 km range. Although not strictly a science application,
the ability to monitor space debris is of commercial importance, and EISCAT
currently has a contract with ESA to continue this monitoring work.
2.7
Studies of Long-Term Change
By
the end of the next EISCAT agreement in 2016, EISCAT will have been in
existence for more than three solar cycles.
This will not only allows permit EISCAT users to carry out much improved
studies of solar cycle related change, but allows the possibility of searching
for longer-term trends. Of particular
interest are the possibilities to measure long-term variability in the dynamics
of the mesosphere and lower thermosphere and in ionospheric
(and by implication thermospheric) composition and
temperature. Any secular changes in the
prevalence and behaviour of mesospheric phenomena such as PMSE or PMWE (see
above) are of particular interest, as these effects are sensitive indicators of
mesopause conditions and have been suggested as being
“miner’s canaries” for possible anthropogenic effects on the polar upper
atmosphere.
In
order to optimise the usefulness of EISCAT data for long-term studies, future
observing schedules must continue to ensure an even coverage of suitable
observing modes with respect to time and season. In addition, the integrity of the older data
sets needs to be maintained and the provision of well-ordered archives,
allowing the potential for data mining and statistical studies, needs to be
adequately supported.
3.
Requirements
for new facilities
As
can be seen from Section 2, the EISCAT user community has identified a very
strong scientific programme which addresses many of the most important
unresolved issues in solar-terrestrial physics, while building on EISCAT’s traditional areas of strength and expertise. In order to secure the observations necessary
to carry out this programme, however, some significant changes will be needed
to EISCAT’s existing complement of instruments as
well as to its operational philosophy.
The following paragraphs outline the most important of these.
3.1 Continuous
Operations
There
are a number of compelling scientific reasons why
EISCAT should move to continuous operations, or at least to a much higher level
of operation than is undertaken at present.
Firstly,
areas of study such as ionosphere-atmosphere coupling clearly imply a
requirement for regular monitoring, in order to measure seasonal and solar
cycle changes in the mesosphere and lower ionosphere, to monitor long-period
phenomena such as planetary waves and tides and to develop climatologies
of these and other wind and wave modes.
Secondly,
much of the most interesting physics can only be studied on an opportunistic
basis. For example, studies of auroral
physics can only be done given the correct level of geomagnetic activity, and
some of the most interesting science in this area relies on the appearance of
the most interesting types of auroral structure (e.g
discrete arcs or active rays, as opposed to diffuse precipitation). Since there is an important requirement for
co-ordinated optical support, the need for clear skies is also an important
consideration and when such a combination of conditions exists it should be
exploited, rather than the opportunity being wasted because no experiment
happens to be scheduled. The same logic
applies to heating experiments, which also often have particular requirements
for ionospheric conditions (maximum plasma frequency,
shape of the density profile, observability of plasma
lines, availability of optical support etc.)
Extreme cases of condition-dependent phenomena include Polar Cap
Absorption Events, an energetic proton phenomenon often associated with
Earthward-directed Coronal Mass Ejections, of which only a few examples occur
per year. There is strong interest in
what happens at the onset of these events, due in part to the almost complete
lack of good observations.
Thirdly,
the possibility of continuous operations would make it much easier for EISCAT
to provide observational support for satellites, rocket campaigns and other
types of diagnostic instrument.
Currently, EISCAT support for satellite missions relies on choosing in
advance a very limited number of overpasses during which to run and hoping that
interesting conditions arise at these times.
The availability of near-continuous radar observations would ensure that
data were available whenever conditions were interesting, and allow the EISCAT
community to play a full role in the ground-based support of future missions. Examples include Double Star (spacecraft to
be launched in 2003 and 2004), Resonance (2008), MMS and GEC (both 2009) and
proposed missions such as MagCon and APEX/MAXWELL.
In
order to best exploit the possibilities offered by continuous operations, a
flexible and intelligent scheduling mechanism would be needed, with clearly-defined
and agreed procedures for running particular experimental modes under specified
conditions. Such a schedule would need
to accommodate the need for regular synoptic monitoring, while retaining the
facility to run user-driven Special Programmes as at present. Although such a system might not be trivial
to establish, it should be possible to arrive at a model which optimally meets
the scientific needs of the user community within the framework of continuous
operations. Indeed, such a scheduling
system is necessary if EISCAT users are to exploit the opportunities which will
be open to us during the next decade.
3.2 Lower
Operating Frequencies for the mainland radars
While
much of the core science coming from the mainland EISCAT radars up to now has
been produced at UHF frequencies, many of the science priorities defined in
Section 2 imply a need for lower operating frequencies. Also there are compelling practical reasons
why EISCAT may need to move away from its present frequency band to guarantee
the security of long-term operations.
Part
of the reason for moving to lower frequencies arises from the desire to study
the lower density plasmas of the lower ionosphere and the ionospheric
topside. The effectiveness of the UHF
radar at these altitudes is limited by the requirement that the sounding
wavelength should be somewhat greater than the Debye
length of the ionospheric plasma. If this condition is not satisfied, the detectability and interpretation of the incoherent scatter
signal are severely compromised. For the current mainland UHF system at 928
MHz, Debye length effects can become significant for
altitudes below 80 km and above 1500 km.
At EISCAT’s current VHF frequency of 224 MHz,
the longer sounding wavelength means that incoherent scatter can be received
from altitudes below 70 and up to over 3000 km.
In
addition, EISCAT’s current UHF bandwidth has been
subject to ever-increasing pressure from commercial users. The advent of GSM mobile phone systems, as
well as use by other broadcasters such as the Swedish rail network, has meant
that it has been necessary to seriously limit the range of frequencies which
can be received, using bandpass filters at the remote
sites. While EISCAT currently has
temporary agreements with local mobile phone companies which prevent the
deployment of base stations broadcasting within EISCAT’s
UHF band within a certain distance of each site, the agreement which EISCAT has
with the Scandinavian telecommunications authorities offers no long-term
assurance that EISCAT will be able to continue working at UHF frequencies in
the long term. Conversely, there is
little competition from other broadcasters in EISCAT’s
current VHF band, and while the possibility of future use (e.g. for DAB radio)
certainly exists, the long-term outlook for using this band is certainly much
better than at UHF frequencies.
Finally
it is also the case that lower frequency operation would offer practical
benefits in terms of transmitter and receiver technology. While EISCAT’s
existing VHF system uses a klystron-based transmitter, improvements in
transmitter technology over the last 20 years now make it possible to use a
large number of solid-state transmitter modules in this frequency range. Such a distributed transmitter system would
minimise the current dependence on individual tubes, and provide a much more
suitable basis for continuous operations.
In addition it would be well-suited for deployment in conjunction with phased
array radars, which have a number of potential advantages over the current
dish-based systems (see below).
3.3 Phased
Array Radars
Historically,
EISCAT has been based on dish or trough antennas with large collecting
areas. However there are a number of
reasons why a phased array system should be strongly considered as the basis for
the future mainland EISCAT system, especially given the requirement for
continuous or near-continuous operations.
Modern
phased arrays have a number of appealing properties from the viewpoint of
incoherent scatter radar operations.
Firstly they are inherently modular.
This both removes any dependence on a single dish structure and allows
the system to be expanded incrementally as resources become available. Because the modularity of the system allows a
graceful degradation in the event of a module failing, this configuration is
also much better suited to continuous operations. By using facilities such as
electronic beam-forming and beam-steering, phased array systems are not only
capable of steerability comparable with dish systems
(depending on the design of the array) but also of forming simultaneous
multiple beams in flexible configurations, for applications such as large-scale
imaging. A VHF phased array at Tromsø would allow the possibility of making field-aligned
observations in the topside ionosphere, or bistatic measurements with the ESR,
both of which have been very difficult to achieve up to now. The ability to form multiple simultaneous
beams at the remote sites would, for example, allow tristatic
velocities to be measured simultaneously at a range of altitudes, which is of
great importance for the study of ion-neutral coupling.
Phased
array technology would also allow the flexibility to split the receiver array
to carry out applications such as interferometry, which has already been
specified as a key requirement of a future system. Phased array radars often have a reasonably
good response at certain multiples and integer fractions of the nominal
operating frequency. The possibility therefore exists to use the same array for
purposes other then incoherent scatter (e.g. for riometry
or MST radar studies). The possibility
of providing an active radar capability to the remote site phased arrays,
either for incoherent scatter or heating, would greatly increase their
usefulness. Finally, phased arrays can be re-locatable (at least in principle)
allowing the possibility to move them to a different site, or to bring
dispersed arrays together to form a single radar with
greater power and/or higher gain.
Phased
array technology was chosen by the
The
way in which a phased array system would be implemented is open to
discussion. One possibility might be to
construct new ground-plane arrays at the mainland remote sites, while mounting
a new array assembly on the existing VHF dish at Tromsø. Because phased arrays would in principle be
re-locatable, the possibility of operating remote arrays at geophysically
interesting locations such as ESRANGE and Andoya
should also be strongly considered, in addition to the present mainland remote
sites, as this would offer the prospect of even closer collaboration with
rocketry and optical instruments.
3.4 Operations
on
The
discussion regarding lower frequency phased arrays in sections 3.2 and 3.3
related to the future development of EISCAT’s
mainland radar systems. On
One
upgrade is, however, felt to be a compelling priority by a large number of
users. As stated in section 2.7,
researchers at the
3.5 High-Frequency
IPS capabilities
Although
there are probably compelling reasons why future active EISCAT experiments on
the mainland should be carried out at lower frequencies in future, there is one
important aspect of EISCAT’s operation which
currently uses the highest frequencies accessible to the system, and which
should not be forgotten in future planning.
As
stated in section 2.5, EISCAT experiments for the study of the solar wind by
interplanetary scintillation exploit a capability which is presently unique in
the world. No other ground-based system
offers the possibility for long baseline operations at such high frequencies,
meaning that EISCAT is able to make high resolution observations at distances
closer to the Sun than other radars. The
ability to probe the solar wind close to the Sun is crucial, because this is
the region where the solar wind is accelerated and where turbulent interaction
between solar wind streams from different surface features occurs.
EISCAT
IPS measurements can be performed at two frequencies. All three sites can make passive measurements
at the nominal incoherent scatter frequency of 928 MHz, though these are now
badly compromised by the limited bandwidth available. Because of this, EISCAT has invested in new
receiver hardware to make it possible to observe at frequencies around 1.4 GHz.
This frequency range is an internationally protected band, reserved for radio
astronomy, so there is sufficient bandwidth for IPS measurements, with no
prospect of competition from terrestrial broadcasters. The ability to measure at 1.4 GHz is
currently implemented only at Kiruna and Sodankyla, and the changeover from 928 MHz to 1.4 GHz
requires a manual reconfiguration of each system which takes several hours.
Interplanetary
scintillation measurements are a highly regarded part of EISCAT’s
scientific programme, and such measurements will provide invaluable support to
missions such as STEREO and Solar Orbiter during the
period of the next EISCAT agreement.
Because of this, the IPS capability should be retained, with steps being
taken to facilitate the switching between incoherent scatter and IPS
experiments, and to extend to Tromsø the ability to
observe at frequencies around 1.4 GHz.
Ideally a capability would be evolved to carry out IPS measurements
independently of incoherent scatter experiments, so that an extensive programme
of IPS measurements could be undertaken every summer. It should also be noted
that a limited programme of other radio astronomy experiments, including
studies of Jovian radiation, is currently being
carried out at EISCAT. The model for the future development of EISCAT’s mainland systems put forward by SAC (see Section
4) would make it possible to retain such activities, while also expanding the
programme of IPS measurements.
3.6 Supporting
Diagnostics
Progress
in solar terrestrial physics, as in many other branches of observational
science, is increasingly dependent on the combination of data from different
instruments and observing techniques.
The importance of supporting observations from satellites, rockets,
optical systems and other types of radars has already been referred to several
times in this document. Examples of
complementary instrumentation highlighted by the user community included VHF
coherent radars, MST radars, lidars, photometers and
all-sky cameras. While the user community clearly has a strong interest in
ensuring that a good network of high-quality diagnostic instruments should
continue to be available close to the radar sites, it is not necessarily seen
as being appropriate for EISCAT to provide such supporting instrumentation from
its own resources.
There
is, however, general support for EISCAT to form close links with instrument
teams and national funding agencies, encouraging them to deploy their
instruments in the vicinity of the radar sites.
One way of doing this would be for EISCAT to help by providing
infrastructure such as instrument housing, power, computing support and data
access, together with more practical help such as routine maintenance, tape
changing etc. Compared to the data
volumes produced by modern IS radar experiments, the data sets produced by some
of these supporting instruments would be small, so that the storage and
distribution of these data would not constitute a large overhead on EISCAT’s existing data-related activities.
3.7 Data
Analysis, Archiving and Distribution
During
the twenty years since the beginning of EISCAT, procedures for analysing,
distributing and archiving data have steadily improved in line with the
increasing availability of computing power, network access and high capacity
storage media. EISCAT was an early
adopter of web technology, for example, and has been a leading participant in
the Madrigal database which, by providing on-line archives of analysed data
from multiple radars and other STP instruments forms a very powerful resource
for scientists in the international incoherent scatter community. As the
capacity of storage media steadily increased, it has become possible to store
an increasing fraction of the EISCAT raw data on-line, and EISCAT is presently
engaged in setting up a multi-Terrabyte archive of
raw data from all of its most recent experiments.
In
the period of the next EISCAT agreement, this evolution will continue in ways
that it might not be easy to predict at present. It seems probable, however, that there will
be an increasing emphasis on the concept of making data available as part of “virtual
observatories”, large-scale distributed data facilities allowing users to access
data resources on a global scale and to compare, combine and manipulate them
using advanced tools. Examples might
include the combination of EISCAT and other data into value-added products
describing both the ionised and neutral atmosphere, and the assimilation of
real-time data into models. Although it
is difficult to be specific, it will clearly remain a high priority for EISCAT
to continue to adhere to the latest data standards and to make use of emerging
new technologies for data dissemination and combination as the field develops. It
is clear, however, that the function of producing high-quality, well-calibrated
data in a timely manner will continue to be of extreme importance, as will the
active and secure maintenance of EISCAT’s long-term
data archive.
4. SAC Recommendations on
Future Priorities
The
following is SAC’s recommendation on the development
of the EISCAT systems in the period 2007-2016, based on the user inputs and discussions which took place prior to and
during the SAC meeting in Uppsala on October 9 and 10 2003.
4.1
The EISCAT
The
ESR should be retained in its existing form throughout the period of the next
agreement, during which time it will operate on a continuous or near-continuous
basis, making key contributions to every area of EISCAT science (with the
exception of solar wind and meteor studies).
An upgrade should be carried out to permit improved interferometer
measurements, by deploying small dishes or low-gain antenna arrays (each with
its own receiver) in conjunction with the existing dishes, along the lines
suggested by the Tromsø group. This could probably be done at relatively
modest cost (1-2 MSEK).
4.2
The Mainland Radars
The
existing mainland radars are not well-suited to the principle of continuous
operations. The VHF radar in particular might
have a very limited future, especially in the light of present concerns as to
whether the klystrons can be repaired in the event of failure. The UHF klystrons are relatively new, and
several more years of operation can probably be expected. Intense competition for bandwidth at UHF
frequencies, the increasing age of the antennas and a scientific requirement
for operation at lower frequencies, mean however, that the existing UHF radars cannot
be expected to provide the optimum basis for mainland operations throughout the
period of the next agreement. SAC
recommends that EISCAT should continue to run the VHF radar as long as it is
practical to do so, and should operate the UHF on a quasi-continuous basis
during the initial phase of the next agreement.
EISCAT should, however, immediately begin a design study for a multi-static
mainland phased array system at VHF frequencies based on deployment at the
existing mainland sites, but including the possibility to deploy arrays at
other locations such as ESRANGE and Andoya. Serious consideration should be given to the
extent to which other radar capabilities, such as the ability to perform
interferometry and make MST measurements, could be factored into the design of
these arrays.
4.3
The Tromsø Heater
The
HF heater at Tromsø remains the leading facility of
its kind and delivers an excellent scientific return for relatively modest
operating costs. The heater continues to
produce excellent new science and heating users have innovative plans to use
the facility in a number of novel ways.
The study of fundamental plasma physics will continue to be one of the
central aims of EISCAT during the period of the next agreement. The heater and Dynasonde
should therefore be retained and upgraded to a standard whereby their continued
operation can be assured.
4.4
High-Frequency IPS Capabilities
The
use of the tristatic UHF system to study the solar
wind via interplanetary scintillation measurements at 1.4 GHz is seen as being
a unique capability of EISCAT, which should be retained to exploit the synergy
with forthcoming space missions such as STEREO and Solar Orbiter. In the short term, measures should be taken
to facilitate a faster changeover from 928 MHz to 1.4 GHz operation at the
EISCAT remote sites, and to extend the high-frequency observing capability to Tromsø. In the
longer term, when the mainland phased array becomes available, the existing UHF
dishes should continue to be operated passively at 1.4 GHz as part of an
expanded programme of IPS and radio astronomy studies. Consideration should be given to adding VLBI
receivers at each of the existing mainland EISCAT sites, especially if this
would lead to increased usage by the radio astronomy community.
4.5
Supporting Diagnostics
In
view of the major benefits to be gained by combining radar data with data from
other ground-based instruments, EISCAT should strongly encourage the deployment
of supporting diagnostics at each of its radar sites. While it might not be appropriate for the
association to provide these instruments from its own resources, EISCAT should
negotiate with instrument providers and funding agencies on the basis that it
could offer help with instrument accommodation, infrastructure, routine
maintenance etc. Appropriate support
should also be given to these instruments in terms of computing and data
dissemination, as long as this does not become too large an overhead on EISCAT’s core activities.
Ian
McCrea
Rutherford
Appleton Laboratory