KAIRA & Kiitos!

The KAIRA project has been running for some time now and, like most major scientific instruments, there has been a peer-reviewed reference paper produced which describes the facility, its technical capabilities, its scientific goals and the initial results. We have written this paper and it has been accepted by the journal IEEE Transactions on Geoscience and Remote Sensing.

TITLE:     KAIRA: the Kilpisjärvi Atmospheric Imaging Receiver Array -- system overview and first results

ABSTRACT:  The Kilpisjärvi Atmospheric Imaging Receiver Array (KAIRA) is a dual array of omnidirectional VHF radio antennas located near Kilpisjärvi, Finland. It is operated by the Sodankylä Geophysical Observatory. It makes extensive use of the proven LOFAR antenna and digital signal-processing hardware, and can act as a stand-alone passive receiver, as a receiver for the European Incoherent Scatter (EISCAT) very high frequency (VHF) incoherent scatter radar in Tromsø, or for use in conjunction with other Fenno-Scandinavian VHF experiments. In addition to being a powerful observing instrument in its own right, KAIRA will act as a pathfinder for technologies to be used in the planned EISCAT_3D phased-array incoherent scatter radar system and participate in very long baseline interferometry experiments. This paper gives an overview of KAIRA, its principal hardware and software components, and its main science objectives. We demonstrate the applicability of the radio astronomy technology to our geoscience application. Furthermore, we present a selection of results from the commissioning phase of this new radio observatory.

DOI: 10.1109/TGRS.2014.2342252

With the publication of this paper, we regard the build and commissioning phase of the KAIRA facility as being complete. As a result, the time has come to close the KAIRA web log, which was written for the purposes of reporting on the build of the instrument. The first post was written on Wednesday 30-Jun-2010 and, today Friday 12-Sep-12104, we have the last.

To all our readers over the years, thank you so much for your support, comments and attention. It has been great fun writing for you and, as our web counters indicate, it has been very popular. We have covered so many topics, ranging from the technical, to the tragic, to the humerous, and the downright bizarre. And, it has been a tough project, with real blood, sweat and tears. Yet, it has all been worth it.

Even though this web log will now halt, it will remain in place for future reference. For those who still want to read about KAIRA, Sodankylä Geophysical Observatory, and Arctic Finland, the story continues on the SGO web log.


However, we will now close here with thanks. In Finnish, of course...

Kiitos!

First Observations of Ionospheric Scintillation in 4-bit mode

On Monday, we reported about experimenting with the 4-bit mode now available at the station.  This allows us to cover up to 190.6 Mhz of continuous bandwidth, four times the amount available using the usual 16-bit mode.  Of course, the low-band and high-band filters only allow up to 80 MHz of continuous bandwidth in any of the 100 MHz Nyquist zones, but it does mean that we can use mode 357 to cover fully the frequency ranges 10-90 MHz, 110-190 MHz and 210-244 MHz simultaneously.  Also, as reported on Monday, there is an issue with saturation across the peak of sensitivity in the low band.

On Sunday night, we carried out the first ionospheric scintillation (as detailed here) observation of Cas A using 4-bit mode.  To alleviate the saturation issue as much as possible, we used the maximum 8 dB attenuation on the signal.  The results look pretty good:

Looking carefully, you can still see a faint band around the 50-60 MHz range, but the scintillation is still clearly visible across this band.  The data themselves also show some very interesting features in the low band, as shown in the 'zoomed-in' image below:

The image is displayed in grey-scale to bring out the features more.  There is a lot of detail to be analysed in these data, particularly as there is no spacing between subbands in 4-bit mode, making it clear that the 4-bit mode is well-enough suited for these kinds of observations.

Ionospheric Scintillation with KAIRA and LOFAR

Yesterday we had a seminar from one of our visiting scientists. The subject was ionospheric scintillation and the speaker was Richard Fallows from ASTRON, Netherlands. The abstract of the presentation was:

 The wide bandwidth of KAIRA (Kilpisjärvi Atmospheric Imaging Receiver Array) and LOFAR (Low Frequency Array) is opening up new perspectives in the study of ionospheric scintillation. Direct observing at station level has enabled observations which combine low-band and high-band modes to cover the full available frequency bandwidth from 10 to 250 MHz. For the first time, the evolution of scintillation from weak to strong scattering regimes has been directly observed in dynamic spectra. "Scintillation arcs", seen previously in two-dimensional power spectra from interstellar scintillation observations, have been noted for the first time using observations of ionospheric scintillation. This offers new methods of studying the plasma structures giving rise to the scintillation.

It was an excellent talk... really interesting, great results and a lively discussion afterwards. Thanks Richard!

Thomas Ulich (standing) introduces the speaker
Richard Fallows (right). Photo: E. Turunen.

That also prompted us to investigate some of the riometry data that we had taken and look at some of the scintillation effects that we see in that. It was certainly good to examine some of the data with an expert at hand to advise us on what we were seeing.

KAIRA presentations at Dalfsen

Just one of the KAIRA talks.

We've now had the Single-Station meeting at Dalfsen, following the Science workshop. The meeting has been extremely well attended, with all the big names in the field present. The fact that several critical meetings have been combined has certainly helped. There is the science workshop, the board meeting, the single-station meeting and the International LOFAR technical operations meeting).

The KAIRA project has also been well-represented at the meeting. In total, there have been five KAIRA presentations made so far, with one more scheduled for tomorrow. They include:

— Scintillation Studies: Updates and New Perspectives
— KAIRA Construction and Commissioning,
— RCU 357 mode observations
— KAIRA Riometry and Incoherent Scatter Radar
— KAIRA outreach
— KAIRA station operations update


It's not over yet (we have the International LOFAR Telescope Technical Operations meeting in the morning), but so far it has been a highly productive meeting with lots of new and interesting science, engineering and ideas!

New radio galaxy discovered

The International LOFAR Telescope has made a new announcement regarding one of their discoveries. Their press-release states that a team of astronomers led by Dr. George Heald (ASTRON) has discovered a previously unknown gigantic radio galaxy using the International LOFAR Telescope (ILT). The discovery was made as LOFAR conducted its first all-sky imaging survey: the Multifrequency Snapshot Sky Survey (MSSS). Using early MSSS images, the astronomers have identified a new, huge source that represents material ejected from the center of the galaxy tens to hundreds of millions of years ago, and now stretching across millions of light years of intergalactic space. If this newly discovered galaxy were visible by eye, it would be larger than the full moon. The MSSS survey is still ongoing, and hopes to discover many new sources like this one.

The new giant radio galaxy (pale blue). Inset the power-source
at the centre of the huge structure. Image: ASTRON

The new galaxy is a member of a class of objects called Giant Radio Galaxies (GRGs). GRGs are a type of radio galaxy with extremely large physical size, suggesting that they are either very powerful or very old. LOFAR’s extreme sensitivity to large objects like this one, combined with its operation at low frequencies that are well suited to observing old objects, make it a powerful tool to find new GRGs like this one.

The centre of the new GRG is associated with one member of a galaxy triplet known as UGC 09555. The central galaxy is located at a redshift of z=0.054536, or 750 million light years from Earth. The central radio source was previously known and has a flat radio spectrum, typical of giant radio galaxies.

LOFAR’s MSSS survey is a concerted effort to image the entire northern sky at very low radio frequencies, between 30 and 160 MHz (wavelengths from 2m to 10m). The primary aim of the survey is to perform an initial shallow scan of the sky, in preparation for deeper observations yet to come.  

Source: ASTRON

LOFAR Science Meeting in Dalfsen

Dalfsen, de Westermolen.
Image: Michiel Verbeek

The LOFAR science meeting, will be held this week (19-22th March 2013) in Dalfsen, Netherlands. The first two days will be for science with the overall LOFAR network. Then there will be a day dedicated specifically to single-station science with LOFAR stations. Finally, the last day will be the technical operations meeting, to discuss issues with the running, maintenance, upgrade and enhancement of the LOFAR systems.

This will be the first meeting of the International LOFAR Telescope since formal operations began last year. There will be over 100 attendees at the meeting, making it a very exciting event.

KAIRA will be strongly represented on the single-station day, as this is our primary mode of operations. It will also be a great opportunity for us to show-case some of our initial results and outline our plans for the future work.

This is a fantastic chance to bring more attention to the science results already coming out of LOFAR, and the fact that full operations are now happening. No doubt there will be a number of significant discoveries and results announced during the next few days, and we'll be reporting them here and on twitter (@KairaProject) when we can.

Link:  http://www.astron.nl/lofarscience2013/

LOFAR observations of Virgo A

Today, ASTRON announced some stunning new results of the radio galaxy Virgo A (also referred to as Vir A, M87, etc.) along with a great image taken by the International LOFAR Telescope:

This false colour image shows the galaxy M87. Optical light is shown in white/blue (Credits: SDSS), the radio emission in yellow/orange (LOFAR). At the centre, the radio emission has a very high surface brightness, showing where the jet powered by the supermassive black hole is located. Credits: Francesco de Gasperin, on behalf of the LOFAR collaboration.

From their press release...

Using a brand-new radio telescope, astronomers have produced one of the best images ever made at the lowest frequencies of giant bubbles produced by a super-massive black hole. The observations were performed at frequencies ranging from 20 to 160 MHz which are normally used for communications by {aeroplane} pilots. The picture shows what looks like a giant balloon filled with radio emitting plasma, which exceeds the size of an entire galaxy.

Refer to the full press-release in the links section below for details.

Links

• ASTRON press release
• Article on VirA (M87)

Using LOFAR LBA and HBA arrays simultaneously

Most LOFAR stations have two antenna fields. These are the Low-Band Antenna (LBA) array and the High-Band Antenna (HBA) array. The LBA and HBA antennas are capable of receiving a range of 10-90 and 110-270 MHz respectively. However, the signal processing is arranged slightly differently. Each "channel" in the signal processing system has three inputs into which the antennas are connected. These receiver units (RCUs), then have several signal paths that can be used, each of which switch in different filters. Thus, the different antennas and frequency bands can be selected and sampled.

Because each individual receiver unit has both an LBA and an HBA polarisation connected to it, it is not possible to observe with both of these simultaneously. Furthermore, because of the filters used in the RCUs, it is not possible to observe across the entire frequency range of the HBA simultaneously either. You must select one of these "RCU modes" for each given channel.

•  RCU mode 3 = LBA input, 10-90 MHz filters
•  RCU mode 5 = HBA input, 110-190 MHz filters
•  RCU mode 7 = HBA input, 210-270 MHz filters

There are other modes as well, such as RCU mode 6, which uses not just different filters, but also a different clock rate (160, rather than 200 MHz) in order to sample the frequencies around 200 MHz (which would normally be aliased in the other modes). The point is that each RCU is limited to a single mode. This has led some to believe that LOFAR stations cannot observe with the LBA and HBA simultaneously.

Actually... they can.

Here at KAIRA, we have been using combinations of modes. In what we refer to as "RCU mode 357", we have been observing with RCU modes 3, 5 and 7 simultaneously. The modes are interleaved, so that there is still a distribution of antennas for each; this allows beam-forming to take place.

Of course, this is not for the faint-hearted, and one must be careful in mapping the channels and powering-up the HBA tiles in such a way so as not to cause power supply failures. However, when done correctly, multiple beams across all bands can be formed giving frequency coverage over nearly the entire VHF band. Here is one of our first results:

A small sample of data from KAIRA, ranging from ~15 to ~275 MHz. Each frequency channel has been divided by the median to bring out the radio features. The bright arcs are ionospheric scintillation during observations of Cas A. (D. McKay-Bukowski, J. Vierinen, and R. Fallows.)

Already we have seen some interesting things and we will be reporting these over the next few days.

First international experiment

This week, KAIRA is participating for the first time in a large international experiment with baselines of several thousands of kilometres. Apart from hoping to discover some new science, this is a perfect opportunity to link EISCAT, LOFAR and, of course, KAIRA itself. As the week goes on, we will be incorporating more and more stations into this experiment network. However, on this first night of observing the following stations are involved (from North to South): 

• ESR (EISCAT Svalbard Radar, Norway)
• KAIRA (Kilpisjärvi Atmospheric Imaging Receiver Array, Finland)
• Kiruna (EISCAT receiver station, Sweden)
• SE607 (LOFAR station at Onsala, Sweden)
• DE604 (LOFAR station at Potsdam, Germany)
• UK608 (LOFAR station at Chilbolton, UK)
• FR606 (LOFAR station at Nançay, France)

 Putting them on the map, the layout is as follows:

The observations are being carried out at approximately 224 MHz (thus allowing us to use the Kiruna receiver station). The ESR is at a different frequency, but is providing concurrent observing. And both KAIRA and UK608 are using special observing modes. Kiruna and the ESR are conventional, mechanically-steered, parabolic dishes. The others are all LOFAR-design phased arrays.

People often ask if KAIRA is part of the LOFAR network. The answer to this is: "not directly". Although there are plans to one day link KAIRA directly to LOFAR via a high speed link, we are currently running as an independent station. While this gives us the flexibility to pursue our own science programme, it certainly does not preclude us from participating in large, important, observing experiment — like this one.

Oh... and we should also point out that although this particular experiment is being run by ASTRON, this work is being done by ASTRON staff at the KAIRA site!

Stay tuned as the week goes on as, no doubt, we will have some very interesting things to report!

First pulsar observation at KAIRA

PSR J0332+5434 observed in single station mode with 31 subbands. The dedispersed signal is shown on the left hand side, while the original incoming signal is shown on the top right hand side. The lower left plot shows the power averaged across all 31 subbands after dedispersion. 

Today we tried something different. Instead of looking at our local atmospheric plasma, we looked at interstellar plasma, this time with the help of a pulsar. In this case, PSR J0332+5434, which is the brightest pulsar in the northern hemisphere.

Pulsars are highly magnetized neutron stars that spin very fast. This spinning movement causes broadband radio emissions due to acceleration of charged particles in the magnetic field of the pulsar. Due to the fact that the interstellar space contains charged particles (although typically less than one electron per cubic centimeter), the broadband pulse arrives to us at different delays for different frequencies. This is a familiar phenomena for radio waves travelling in our own local ionosphere, although the scales are much larger. In order to improve signal statistics, this dispersion has to be measured and also corrected for before averaging over wider bandwidths.

Using the LBA as a solar radio emission receiver

A five minute dynamic spectrum recorded using a single LOFAR LBA antenna element. Apart from the HF radio band, the most prominent features are several type 3 solar radio bursts and the FMCW over-the-horizon radar signal transmitted from Cyprus.

We have now started comissioning the LBA antennas. Although there still are several problems, we have managed to get over half of the antennas up and running. Here is a plot from one of the first spectral measurements done using this new antenna field. Here is a spectrum recorded using a single LBA antenna element over a 100 MHz band with one second time resolution. We happened to catch several solar radio bursts. We also regularly see the FMCW over-the-horizon radar located in Cyprus, so we are confident that the system is working as expected.

Fine tuning FFT LPI

Going from initial first light to calibrated and cross validated ionospheric plasma parameter measurements is not an overnight step. However, the KAIRA team is making good progress. We have now managed to get fast Fourier transform lag-profile inversion working to some extent with real world signals contaminated with space debris and meteor head echos. We can now estimate ionospheric plasma incoherent scatter autocorrelation functions and spectra with a fairly nice accuracy and computational speed.

Here is an example of a 512 second integration combined from all of the 30 beams intersecting the Tromsø VHF beam at different altitudes. With this experiment that has a 128 microsecond baud length, we can reach altitudes up to 400 km.

(Click image to enlarge)

The plots are fairly similar with monostatic VHF spectra, although there are still lots of challenges with absolute calibration of electron density. These issues can hopefully be solved by comparison with monostatic Tromsø measurements, or with future plasma line measurements with KAIRA.

Lag-profile inversion

We have been extremely busy developing new analysis methods for efficient side-station inversion of the incoherent scatter measurements. Here is a preview of what we have accomplished so far. The follow plot combines lag-profile inversion results from 20 different beams simultaneously pointing at different altitudes along the EISCAT VHF beam. With this configuration, we can perform a bi-static measurement simultaneously along all the different altitudes. Measurements like this will be extremely useful in the future for determining e.g., ionospheric drift velocities. The plot shows standard E- and F-region incoherent scatter autocorrelation functions, along with what seems to be tropospheric propagation from the Tromsø antenna sidelobes.

Averaged lag-profile matrix (incoherent scatter autocorrelation functions for different ranges) measured with KAIRA.  Ranges given as round-trip time, i.e., half of the time that light travels from VHF to KAIRA given in range, assuming speed of light in vacuum. 

EISCAT_3D Science Case

The second revision of the EISCAT_3D Science Case has been publicly announced via the EISCAT_3D website. This science case document was prepared as a part of the EISCAT_3D Preparatory Phase. It is updated regularly and is an excellent summary of the scientific goals. Obviously, apart from EISCAT_3D itself, much of this will impact on the work being done by KAIRA — both as a pathfinder for the EISCAT_3D project and as a stand-alone experiment.

Source: EISCAT_3D

Ref: http://www.eiscat3d.se/drupal/content/second-version-eiscat3d-science-case-available

Understanding the cosmic radio noise background

When it comes to understanding the data, we are going to need to carefully calibrate KAIRA to ensure that our radar readings are meaningful. For radar science, we are interested in the echoes from the ionosphere. However, beyond the realm of our own solar-terrestrial environment there is also the radio signals from deep space sources. These are typically radio galaxies, supernova remnants and other such astrophysical phenomena.

Knowing the flux (signal strength) of these sources are particular frequencies, and knowing the distribution of radio noise from our own galaxy, is crucial for our measurements.

Two recent works that assist with this are 'A model of diffuse Galactic Radio Emission from 10 MHz to 100 GHz' and 'A broadband flux scale for low frequency radio telescopes'.

For the first, de Oliveira-Costa et al. have looked at the diffuse emission from not just discrete sources, but also our own Galaxy. This allows a complete sky model to be produced, which aids our calibration.

For the second, Scaife & Heald pick six well studied low-frequency radio sources and collate years of studies that have included them. From this, flux curves can be produced that provide a useful spectral comparison.

The references for these papers are:

• http://arxiv.org/abs/0802.1525v3 -- de Oliveira-Costa et al. 2008
• http://arxiv.org/abs/1203.0977 -- Scaife & Heald, 2012

De Oliveira-Costa also has a great webpage which displays this data. The link is: https://www.cfa.harvard.edu/~adeolive/gsm/

Updated Science Image

Today I updated the figure (on the right panel) that shows the different science cases that can be addressed with KAIRA. I have added two more cases: Relative TEC tomography, and tropospheric echos.

Relative TEC tomography involves several different LOFAR stations simultaneously measuring the total electron content, which is basically the difference in ionospheric propagation delay. Because LOFAR can do this with radio stars, in theory it should be possible to perform these kinds of measurements on a huge number of directions simultaneously. This is a huge advantage compared to satellite tomography, which can only measure the total electron content between ground based stations and satellites. This large number of rays might make it possible to do extremely high resolution 3D ionospheric tomography with LOFAR. Currently the best location for doing this would be near the core of the array, where there are many stations located nearby.

The tropospheric echo science case was added after discussions with Ingemar Häggström of EISCAT, who told me that he often sees tropospheric echos in Kiruna, both from the VHF and UHF. With multistatic receivers, this would potentially allow tropospheric wind and scattering spectrum measurements.