- The first author, who had been employed at EISCAT
since 1987, became responsible for running the facility from
1993 until December 2020, and, initially, two engineers were
dedicated with the operation and maintenance of the Heating
division of EISCAT. Gradually, the staff at Ramfjordmoen
shared the tasks necessary to operate the IS radars and Heating as required by the changing user demands of the different facilities. For example, much effort was spent on building
the EISCAT Svalbard Radar (ESR) in the early to mid 1990s
which diverted scientific interest and operations to the polar
cap region.
The transfer to EISCAT resulted in a larger user group
continuing heating experiments that nominally used 200 h
of heater time per year but which varied between 100 and
300 h depending partly on the solar cycle. Fewer experiments
were possible when the F region critical frequency was low
during solar minimum. Most experiments were in conjunction with the IS radar as the major diagnostic instrument
(in many cases) or as one of several diagnostics. Improvements in diagnostic instrumentation; the deployment of new
instruments; and improved radar coding, modulation, and
data storage associated with advances in computing technology led to new discoveries that were impossible or difficult to achieve in the first decade of operation. Some examples are (1) the discovery that heating the lower ionosphere
can weaken or suppress polar mesospheric summer echoes
(PMSE) observed by the VHF (224 MHz) radar (Chilson et
https://doi.org/10.5194/hgss-13-71-2022 Hist. Geo Space Sci., 13, 71–82, 2022
78 M. T. Rietveld and P. Stubbe: History of the Tromsø ionosphere heating facility
al., 2000) and (2) the production of light emission from the
heated ionosphere (Brändström et al., 1999). These two very
different areas of research, namely study of the mesospheric
dusty plasma and energetic electron acceleration, have remained as major topics of research up to the present time.
A technique developed in the former Soviet Union that
uses only the powerful HF waves to measure ionospheric and
atmospheric parameters by the production of artificial periodic irregularities (API) (Belikovich et al., 2002) was successfully applied in the auroral ionosphere for the first time
by combining the Heating facility as a transmitter and the
Dynasonde as a receiver (Rietveld et al., 1996b). Later, more
sophisticated experiments by Vierinen et al. (2013) showed
how this technique is particularly interesting and particularly
promising for studies of the mesosphere.
The Heating facility remained essentially unchanged
through the 1990s. An operating licence was obtained to
allow frequency-stepping experiments around harmonics of
the gyro-frequency. There was an upgrade of the computer
from the original Commodore PET to a Microsoft Windowsbased personal computer system in 1999, when the original
BASIC control program was converted to Hewlett Packard
BASIC, and the more modern computer allowed RF synthesizer and transmitted HF parameters to be stored in a
digital log. Previously, the transmitter settings had been
recorded largely in a hand-written log book. There were minor changes made to some of the control system, such as a
programmable step change in the control grid bias voltage
during long (> ca.1 s) “RF off” intervals such that the quiescent current in the transmitter tubes dropped from about 6
to 1 A (at 10 kV in each of 12 transmitters!) to save on electricity power consumption. One saves most on power consumption when the high voltage is switched off so that no
quiescent current flows through the tube, but this had to be
done manually by pushing 12 buttons that actuated 12 large
relays, something that is undesirable for non-transmitting intervals of a few seconds, tens of seconds, or a few minutes,
which are rather commonly used modulation periods. The
cost of electric power for Heating operation, especially when
experiments required Heating and the VHF and UHF radars,
was a major economic concern in the 1990s at a time when
there were some EISCAT associates who were advocating
for the closure of the Heating facility to save money.
Around 2005, plans were made to upgrade the synthesizers
to direct digital synthesis (DDS) and the associated computer
control to a unix-based system. Apart from replacing ageing hardware, a major motivation was to allow fast frequency
changes of the HF pump wave which were increasingly requested in order to examine the ionospheric response to HF
pumping at and near harmonics of the ionospheric gyrofrequency. Previously, frequency changes required a severalminute-long tuning and phasing procedure under computer
control, as the HP synthesizers started with a random phase
value for any frequency change. The digital system would
allow setting of phases to any desired values practically inFigure 6. The Heating facility console in the control room as it was
in 2007, with the first author at the controls. Six columns of meters,
lights, and push buttons on each side show the status and are used
to control each of the 12 transmitters. The 12 commercial RF synthesizers in the middle of the console have been replaced by digital
synthesizers in the transmitter hall, and the space is now filled with
large computer screens. (Photo credit: Michael T. Rietveld)
stantaneously. The final system, which was taken into regular
use in 2009, used some hardware and much software that the
EISCAT IS radars had implemented in the mid-1990s when
the ESR was built. This upgrade was a major effort involving the expertise of EISCAT staff from Tromsø as well as
the other two mainland sites, the EISCAT headquarters in
Kiruna, Sweden, and Sodankylä in Finland. This upgrade and
further improvements to the Heating system are described in
Rietveld et al. (2016). From 2012, even more functionality
has been developed such that the status of many transmitter
and array parameters that were only indicated by lights or
controllable by buttons are now monitored and set by computer. Figure 6 shows the console in the control room before
the upgrade. The main difference after the upgrade is the replacement of the 12 original synthesizers in the central part
with large computer screens that are now used to control and
monitor the facility’s operation and observe some scientific
results in real time.
In 2013, a modification was made to the coaxial switches
that fed Array 3 to allow receivers to be connected to that
array. The motivation for this was to try and receive magnetospheric echoes, for example, from ion acoustic turbulence
excited by auroral processes such as has been observed by
the VHF and UHF IS radars (Rietveld et al., 1996a). Previously, related experiments had been tried using Heating
as a transmitter and the large HF radio telescope, UTR2, in
Ukraine as a receiver, although without results. As the modified Array 1 and Array 3 cover the same frequency range, one
could transmit on Array 1 and receive on Array 3, albeit with
different antenna gains and, hence, beam widths. The first
Hist. Geo Space Sci., 13, 71–82, 2022 https://doi.org/10.5194/hgss-13-71-2022
M. T. Rietveld and P. Stubbe: History of the Tromsø ionosphere heating facility 79
version of a receiver connected to Array 3 for radar work
is described in Rietveld et al. (2016), where fixed-length
phasing cables were used to combine the signals from the
six rows of orthogonal antennas into two receiver channels.
Since 2017, each individual row of antennas is connected to
a digital receiver allowing beam forming of the received signal in the north–south plane. This receiving system seems to
work well for mesospheric echoes, but echoes from the magnetosphere have not been detected to date. Using Array 3 as
a receiving antenna has some weaknesses, such as the aluminium connectors in the feeder lines where an oxide layer
may adversely affect weak radio signals, although this layer
is burnt through by the powerful radio wave on transmission
for which it was designed.
The Heating facility was only intended to operate for a
limited time of about 10 years; thus, 40 years after construction, it was inevitable that some parts of the system had aged
to a critical point or that spare parts had become unobtainable. One key component is the transmitting tube in each
of the 12 power amplifiers. The original tube was no longer
produced after 1980, but a good number of spares allowed
operation at near-full-power level until recently. Very few
tubes failed completely, but after about 12000 h of filamenton time over the lifetime of Heating, several were slowly delivering less power. A few tubes were sent to firms in the
USA to be rebuilt, but the success rate was poor. In searching for an alternative tube that required minimal modification
to the transmitters, it was found that the RS2054SK tetrode
was almost compatible with the existing transmitter, and this
tube type was still manufactured in Europe and in China. Although it was a drop-in replacement in the tube socket, the
new tube required a different filament voltage and a slow
ramping up and down of the voltage so that several modifications to the transmitter had to be made. In 2018, the
first of the new tubes entered operation, and two transmitters
presently use the new tubes with a third ready to be similarly
modified.
Another ageing problem that first appeared around 2008
was in the modified Array 1. The feed cables to the antennas
in this array were different from the original design in that,
instead of towers made of aluminium coaxial cable feeding
each antenna, commercial twin-wire flexible cable was used.
Starting in 2008, an increasing number of the 288 feed points
at the antenna failed during transmission through burning insulation and fire, often resulting in the collapse of the whole
feed point and antenna to the ground and sometimes resulting in a dramatic grass fire. The cause of this failure was a
mystery for a long time, and a total of 17 feed cables and the
centre parts of the antennas had required laborious repairs
by 2017. In 2015, the cause of this failure was found to be
metal fatigue in the flexible twin-wire braided-copper cable
where it was anchored to the fixed centre of the antenna, with
the rest of the feed cable from near the ground having been
free to move slightly in the wind for about 20 years. Many
of the anchoring guy ropes which should have minimized
movement of the cable in the wind had broken and were not
repaired. The wind-induced movement of the cable caused
many of the braids to break such that the cross-sectional area
of the cable was reduced to the point that the high-power RF
caused overheating or arcing across the final break, resulting in burning of the insulation. The solution was to bypass
the anchor point with a short piece of wire crimped to the
feed wires on each of the 144 antenna centres on the 12 m
tall masts, a job which took several summer seasons and required EISCAT to buy a lift to safely implement the repairs.
It is hoped that this solution is robust enough for the remaining lifetime of the Heating facility. Figure 7 shows the repair
work in Array 1, where the different feed cables (compared
with those in Fig. 3) are clearly visible.
7 Present status and future
The main hardware of Heating, the transmitters, feed lines,
and antennas, have remained essentially unchanged since
1990, apart from the computer control and RF synthesizer
upgrades described above. The user community has changed
with time, with some users and groups changing field, but
there are also new users entering the field. With the closure of other facilities like the HIPAS (HIgh Power Auroral Stimulation) Observatory in Alaska (Wong et al., 1990)
and SPEAR (Space Plasma Exploration by Active Radar)
on Svalbard (Robinson et al., 2006) as well as the hopefully temporary closure of the Arecibo heating facility, the
only sites with working HF ionospheric heating facilities are
HAARP (High-frequency Active Auroral Research Program)
in Alaska (Pedersen and Carlson, 2001) and Sura in Russia (Belikovich et al., 2007). On 1 December 2020, after
57 years of usage, Arecibo Observatory’s 900 t platform containing transmit–receive feeds fell ∼ 150 m and crashed into
the 305 m diameter reflector dish. This halted IS radar, HF
heating, planetary radar, and radio astronomy observations
at the observatory. Full or partial recovery plans are currently
under consideration by the US National Science Foundation.
Complete decommissioning appears unlikely, and a modest
HF facility is currently being constructed at Arecibo to keep
HF heating research moving forward. However, none of these
other installations will have an IS radar as a diagnostic instrument in the foreseeable future, which makes the Tromsø HF
heater a unique and valuable facility for the world.
Groups from all the EISCAT Associates have been regular users of the Heating facility. In recent years, researchers
from China – the China Research Institute of Radio Wave
Propagation (CRIRP) became an EISCAT Associate in 2007
– have become important regular users. A large international community of scientists have been able to use the
Tromsø HF facility, especially in the last decade, as nonEISCAT Associates could either buy time or apply for a
limited number of free hours on either the IS radar, heater,
or both via a peer-review programme. An excellent examhttps://doi.org/10.5194/hgss-13-71-2022 Hist. Geo Space Sci., 13, 71–82, 2022
80 M. T. Rietveld and P. Stubbe: History of the Tromsø ionosphere heating facility
Figure 7. Repair work to one of the 12 m high antennas out of
144 such antennas in Array 1 to bypass existing and potential weak
points in the twin-feed cable connection to the antenna centre. The
original 23 m wooden masts are visible in the background. Note the
different support mast and the different feed lines compared with
the original design as used in Array 2 and Array 3, shown in Fig. 3.
(Photo credit: Michael T. Rietveld)
ple of fruitful scientific results from heating experiments by
a non-EISCAT Associate is a 25-year collaboration with a
group from the Russian Arctic and Antarctic Research Institute (Blagoveshchenskaya et al., 2020). Other long-term
users were researchers from the Polar Geophysical Institute, in Murmansk, Russia, and from the Institute of Radio Astronomy in Ukraine. A description of the various
scientific results that have been obtained from the Heating facility is beyond the scope of this paper. The number of accumulated publications from the Tromsø heating
facility amounts to more than 490, and these papers are
listed on the EISCAT publications web page (https://eiscat.
se/scientist/publications/heating-publications/, last access: 3
March 2022). Streltsov et al. (2018) discuss many of the
physical problems that are topics of present and future research in the field of active experiments using high-power
radio waves. Some of the interesting phenomena to explore
are narrowband SEE (stimulated Brillouin scatter), artificial
ionization, unexplained X-mode effects, and the irregularities postulated to explain wide-altitude ion line enhancements sometimes known by the acronym WAILES (Rietveld
and Senior, 2020).
The Tromsø heating facility has not been overly troubled
by adverse publicity or conspiracy theories. The experiments
conducted at the Tromsø facility were always open, and all
publications resulting from it appear in the open literature;
we believe this is true for nearly all of the experiments performed at the other facilities as well. Nevertheless, over the
years, there have been exaggerated and false claims and conspiracy theories made about some of the experiments that are
possible with heating facilities like HAARP in Alaska and
Heating in Tromsø.
The site in Ramfjordmoen is about to undergo a major
change when the EISCAT UHF and VHF radars are decommissioned and EISCAT_3D, the next-generation IS radar
(McCrea et al., 2015), starts operation, with the core site in
nearby Skibotn. Since the retirement of the first author, the
Heating facility has been led and run by Erik Varberg. The
Heating facility is planned to remain in operation for experiments with the new radar which will offer unprecedented
insights into HF-induced phenomena. The improved spatial
resolution and the ability to quickly steer the beam of the new
radar electronically or to have multiple beams should help
probe the horizontal spatial properties of HF-induced irregularities. There is one disadvantage to not having the HF facility and the radar co-located, namely the radar cannot probe
in the field-aligned direction along the heater beam in the F
region. This problem may be overcome by resurrecting, in a
slightly modified form, the east–west tilting hardware built
for the HERO rocket campaign in the 1980s mentioned earlier. Most of the switching hardware still exists, but new aluminium coaxial phasing cables would need to be made and
installed. The possibility of building a new heater nearer Skibotn is also being investigated.
Data availability. No data sets were used in this paper. All citations appear in the reference list.
Author contributions. MTR wrote most of the paper, and PS provided additions and corrections.
Hist. Geo Space Sci., 13, 71–82, 2022 https://doi.org/10.5194/hgss-13-71-2022
M. T. Rietveld and P. Stubbe: History of the Tromsø ionosphere heating facility 81
Competing interests. The contact author has declared that neither they nor their co-authors have any competing interests.
Disclaimer. Publisher’s note: Copernicus Publications remains
neutral with regard to jurisdictional claims in published maps and
institutional affiliations.
Acknowledgements. In addition to the people mentioned in this
paper, we thank the large number of unnamed staff from MPAe and
EISCAT as well as other collaborators, who helped build, operate,
and maintain this remarkable scientific facility. EISCAT is an international scientific association presently supported by research organizations in China (CRIRP), Finland (SA), Japan (NIPR and STEL),
Norway (NFR), Sweden (VR), and the United Kingdom (NERC).
Review statement. This paper was edited by Kristian Schlegel
and reviewed by two anonymous referees.
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Historien om EISCAT Tromsø
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