Variations in scintillation

Ionospheric scintillation is the variation in the amplitude and phase of radio signals as they pass through the ionosphere. What is happening is small-scale variations in the electron density result in differences in the ionosphere's refractive index at radio-frequencies. It is sort of the radio equivalent of the "twinkle of stars". The localised variations in refractive index cause momentary focusing, defocusing or shifting of the incoming radio waves. For broad, diffuse structures in the sky (like the synchrotron emission from the Galaxy) all of these effects average out for different directions. But for a point-source (such as bright "radio stars") the singular line -of-sight can result in dramatic differences.

One of KAIRA's regular experiments has a number of "beams" on the sky. These are sort of like pixels in an image and they monitor the variation in power levels coming from that direction. Most of these have been set up for specific geodetic directions. But we also have some that track astronomical sources.

The following plot shows the radio power coming from the bright radio object Cassiopeia A (or "Cas A" for short).

As can be seen, there are some huge variations. There is a natural variation as Cas A rises higher and lower in the sky (it never sets completely as it is circumpolar at these latitudes). However, this is only a variation between approx. 96.5 and 97.5 on the arbitrary scale used above. The point where the strong scintillation occurs shows huge variations.

Compare the difference between, say, samples around 25000 and around 35000. Not only can the effect be very great, but it can change very quickly too.

The remains

Following the great Kilpisjärvi Storm of 2014, the telescope rescue crew scoured the surrounding landscape trying to find and recover debris. Part of this was recovering as much scattered electronics as possible... some of which may be able to be cleaned up and re-used as spares.

This sad crate of fragments are some of the parts salvaged during that operation. (Click on the photograph to enlarge.)

Salvaged parts from the destroyed LOFAR HBA tile at KAIRA. (Photo: L. Roininen)

Film editing

This week two of the SGO staff were in the United Kingdom to sit in on part of the final edit of the KAIRA film. As we have mentioned before, we're producing a film about KAIRA, its construction and its science. The main production team is from a company called Site-Eye. They specialise in construction site filming, time-lapse and other specialist filming work. They have been excellent and we've been really happy with the work they've done for us.

During our visit to their film-production offices we went through the KAIRA film and tweaked some sequences and cut/extended some scenes to get the balance just right. Site-Eye are now working on the sound engineering and sub-titles, and we hope to have it ready for public release very soon now.

Alex from Site-Eye working on the KAIRA film. (Photo: D. McKay-Bukowski)

Have a nice weekend, everyone!

JIVE ASTRON Daily Image -- Storm Report

For those who didn't see it, KAIRA was featured yesterday on the JIVE/ASTRON Daily Image. This long-running web-log features images and news from these two organisations. As KAIRA is built of LOFAR technology, which is from ASTRON, and yours truly used to work for JIVE, we have always been keen to support their long-running outreach efforts. And yes, it is always fun to get an article posted there!

Yesterday's article was about the recent storm damage. For those who follow the KAIRA web log, this will be nothing new, as we've covered the topic in detail here. However, we did put together a nice montage and we have that here today.

However, the JIVE/ASTRON Daily Image article does provide a great summary of the recent problems (and solutions!), so go check it out if you haven't already done so.

Direct link: http://www.astron.nl/dailyimage/main.php?date=20140326

Radio bright sun -- Part 2

Yesterday we showed some KAIRA data of a very active sun. It was so bright that it easily outshone Cas A and Cyg A. Given this, we surmised it should be possible to see the solar signal with a single dipole.

Sure enough, that was the case.

The following plot shows the subband statistics for a single antenna (LBA aerial #45, Y-polarisation).

The vertical axis is frequency, shown as both the subband number (left-hand side) and frequency in MHz (right-hand side). The broad band of noise between 8 and 25 MHz (approx.) are the sum of countless shortwave radio transmissions. Below 8 MHz and about 80 MHz, there is little signal due to the LOFAR RFI suppression filters and the weaker amplifier and aerial responses.

Radio bright sun

On 20 March 2014, the Sun was particularly active. As it passed over the southern horizon, KAIRA detected strong radio emission. We had the station correlator running at the time and managed to capture this all-sky image sequence.

The video covers the period from approx. 10:05 until 10:15 UTC. Each frame is a 1-second integration, however it has been sped up to reduce the duration. The date/time (UTC) is shown in the filename at the top-left of the image sequence. The observing frequency is subband 300 (= approx. 58.6 MHz, with 195 kHz bandwidth).

The object near the centre of the image is Cas A and to its right is Cyg A. These are normally very bright radio sources. However, the sun (on the lower edge) easily outshines them... especially around 10:09 UTC (about 0:24 in the video). For the purposes of making this sequence, we really had to turn down the contrast to prevent complete saturation.

When the sun is radio-bright, strange artefacts can be seen in other parts of the image. These are the "sidelobe" responses of the telescope and are not real sources.

It says something about the strength of the solar radio bursts, when the sidelobe response can outshine even Cas A!

Astronomical Association URSA meeting in Inari

The Finnish astronomical association, URSA, held the event called "Tähtipäivät" - "The Star Days" in the Sámi cultural centre Sajos in Inari, 20-23 March 2014. Esa Turunen, the director of the Sodankylä Geophysical Observatory, gave a public lecture in the event. The talk was titled "Avaruusalus planeetta Maa" - "Spaceship Planet Earth". The audience consisted of Finnish professional and amateur astronomers as well as on a number of other interested persons (including myself). Of course, Esa talked about KAIRA as it is, in a sense, the biggest radio telescope on Finnish soil. Hence, today we feature two photos from Esa's talk!

"

The last sweet

In August 2013, we attended the EISCAT symposium in the United Kingdom. At the conference we were all issued with a tin of "rock"... a type of British sweet or candy, where a design is run through the sweet using pulled sugar.

In this case the design was "EISCAT 2013".

Well, I've finally (almost) finished that tin of sweets.

The last one!

Of course the tin was accompanied by a bag of about 500 others, so it was quite a challenge. I suspect I've now got some form of Type-43 Diabetes too.

We had considered posting the sweets back to Mike Kosch (the coference organiser). We'd send them one at a time. One each week. After week. After week.

But no... we just ate them all.

(Hmm... however, I wonder if the other bag of 500 sweets is still around. When I last heard it was in Tromsö, Norway.)

In any case, I guess it really is 2014 now.

Storm damage at MAARSY

Ralph Latteck from IAP sent me some pictures of the damage at MAARSY caused by the same winter storm that disintegrated one of KAIRA's HBA tiles and fell down an antenna mast of the EISCAT Tromsø ionospheric heater.

Even though MAARSY is right by the seaside in Andoya, it survived the storm with only minor scratches. The main damage was in the form of twisted cable trays and a few bent antenna elements. While the some of the cable trays look pretty mangled, only a few of the cables feeding the antennas were severed, allowing the radar to continue operations pretty much normally. Good thing that the antennas were fairly rugged and firmly weighed down with slabs of concrete.

One mangled cable tray. The picture also shows the new configuration of the array with the dual polarization capability. This is helpful when observing the ionosphere, where the 53.5 MHz radio wave experiences significant Faraday rotation.   

The worst of the damage. The cable trays have caught some wind, causing two antenna elements to bend out of shape.

This was also a reminder for me that I should write a posting about MAARSY at some point. It is truly a remarkable instrument. Not only is it a state of the art instrument for observing the mesosphere, stratosphere, and troposphere, it can also be used to observe meteors and the ionosphere.

Storm damage to the EISCAT Heating Facility

The storm that caused the KAIRA damage recently, probably also caused damage at the EISCAT site at Ramfjordmoen. This Heating antenna mast (which is also four coaxial feed lines) to tilt as one of the antenna-fixing wires broke.

The antenna was sound electrically on 23 February but was found faulty on 11 March, and a ski tour of inspection on sunny Sunday 16th March found this.

We suspect that the holding cable was also weak due to faulty construction about 30 years ago based on a similar failure once before in 1995, and that the extra forces from the wind finally broke the connection.

The second photo shows the broken insulating cable hanging from the wooden support mast. It will take some time to rebuild this. It means that two out of twelve Heating transmitters cannot be used on this low frequency (4-5.5 MHz) array until the summer.


Text and photographs by Mike Rietveld

North Celestial Pole

In a LOFAR system, a "beamlet" is a pointing direction and a frequency channel (which is called a subband). In some ways, you can think of it as a  "radio pixel" on the sky. These can be static, pointing at a fixed direction, or tracking, where they follow a particular astronomical object as it rises, travels across the sky, and then sets.

This rising and setting, and the motion of the stars generally, is dominated by the rotation of the Earth. Although some objects move about in the sky, due to orbits or proper motion, it is the 24-hour cycle of the Earth that is the dominant effect.

However, there are two points on the sky which do not move very quickly nor very much (although they do still move!). These are the Celestial Poles. These are the points at infinity that are projections of the Earth's rotation axis. Check the Wikipedia article for a full description.

So, in principle, if you were to point a beamlet at the North Celestial Pole (which is the one we see from KAIRA), then you would expect the power level measured to be absolutely flat. There is radio power coming from that direction, but it should remain constant, as it is only rotating within that radio pixel.

It is sort of flat... but not quite!

Here's an example:

The variations come from several things. Tall vertical spikes are radio-frequency interference. And the fuzzy-noisy patches can be from scintillating sources going through the side lobes.

More generally, gradual variations can be from larger areas of Galactic emission and discrete sources passing through the sidelobes of the beam pattern. And then there are subtle variations caused by snow on the antennas, temperature variations, receiver stability, etc.. These are much smaller, but are still present.

As a result, it can be quite a challenge to calibrate data from the instrument.

Such is the fun of radio astronomy!  :-)

KAIRA in the snow

Today we just have a photograph of KAIRA in the snowy conditions typical for the early part of the year. After all the trauma of last couple of weeks, let's hope things remain calm now.

KAIRA in the snow (Photo: D. McKay-Bukowski)

Mapping the RCU number to the LBA/HBA number

Each polarisation from each antenna is processed by a separate channel. The electronics on this channel is called a Receiver Unit (RCU). KAIRA has 96 RCUs (numbered 0..95).

When looking at the signals coming into the system, they are labelled by their RCU number in the first instant. One needs to map this to the antennas to work out which antenna/polarisation is being shown.

The polarisation mapping is straight forward... even numbers for X-polarisation and odd numbers of Y-polarisation. However the match between the RCU numbers and the antenna numbers is a bit more confusing. Yes, there is a logic, but it is not obvious. Sometimes it is simply easier to look it up!

So, here is the mapping of RCU to antenna number for KAIRA (the scheme is different for other LOFAR stations).

For where those antennas are on the actual site, refer to the array layout map.

Map of the 2014 KAIRA storm damage

Today, we've plotted where there has been damage to KAIRA. The map is a bit scary, but actually as we have seen from yesterday things are not so bad and we've come out of it fairly well.

As can be seen, we have lost two antennas completely. These are tile #H75 and aerial #L34.

We've had two aerials (#L05 and #L21) seriously damaged. These will need to be replaced.

There are three aerials (#L22, #L35 and #L36) which have been damaged but have been repaired to the point where they are operating normally again. However additional work will be required in the summer when the snow has melted and we can access them properly.

There are many aerials with minor damage. The aerials are still operational, but will require some attention in the summer to ensure they remain robust for the following winter. The problem has been buffeting to the aerial wires causing cracks in the PVC moulding and coatings. These can be patched, but it does require warmer weather.

System status

Following the storm and the repair work, we carried out remote system checks for the bandpasses for all antennas. Here are the results, starting with the High-Band Antenna (HBA) array:

As seen, RCUs 80 and 81 (= tile #H75) remain dead. These are the two channels from this destroyed tile.

Here is the Low-Band Antenna (LBA) array results:

Here you can see RCUs 68 and 69 (= aerial #34) remain dead. This aerial was destroyed and we have not even found the remains of it! Following repairs, there remain two other damaged signal channels. These are RCU 11 (= aerial #L05, Y-polarisation) and RCU 42 (= aerial #L21, X-polarisation). These two aerials have suffered broken wires and are thus impared in this polarisation. Temporary repairs have been effected, but more work remains to be done.

However, in total, we only have lost 6 out of 192 signal channels. Throughout the storm and the repair work, KAIRA has continued to observe our riometry and interferometry programmes. That's pretty good.

What a week!

It has been a nerving week with all the KAIRA storm damages! Fortunately, most of the site has survived and we can continue operations! The damaged aerials and the destroyed HBA panel will be serviced or replaced during the coming weeks or next summer!

Hence, even though we had bad luck, we are very happy to see that KAIRA is back in business!

Have a nice weekend! ... and let's hope that there won't be another storm on Saturday :) !!!

Thursday morning photo - service operations in progress!

Day 2: Robust fixes - storm repairs

Our team in Kilpisjärvi had a long day in the KAIRA site on Thursday fixing bits and pieces here and there. The repairs, maybe not elegant, but they are robust and should give us time to plan a full-scale repair operations in the site! We let the photos to speak for themselves! ... And of course - many thanks to all the staff up in Kilpisjärvi!

We will post more pics in the coming days!

Correlating gusts and failures

From the last few days, we've been looking at the loss of signal from the LBA aerial #L34 and today we've correlated that with the wind-gust speeds. Although the average speed is often of interest, it is the gusts that usually determine points of failure or direct damage.

In the graph we have plotted the 10-minute gust maxima for Kilpisjärvi village and Saana mountain (the two nearest official weather stations to KAIRA). Also plotted is a vertical line showing the point at which signal was suddenly lost from LBA aerial #L34.

Not surprisingly, it corresponds to a strong peak. However it is good to note that we tolerated up to that value and that the failure did not occur earlier at lower gust speeds.

The failure time is slightly ahead of the peak. This also not surprising, as the weather stations were both downwind of the KAIRA site (by a few km) at the time of the failure.


Data courtesy from the Finnish Meteorological Institute (www.fmi.fi)

KAIRA storm damage repairs - Day 1

A three-person SGO team travelled up to Kilpisjärvi today. They have investigated the storm damages as well as the protection cover constructed by local craftsmen on Monday. The cover is of good quality and seems to protect the HBA field fully, despite we have one HBA tile totally destroyed. 

The LBA field has been damaged during the storm. We suspect that this is due to the HBA debris which has flown through the LBA field. Altogether around 8 antennas require different levels of maintenance. One antenna is flat-lining!

Below are a set of photos from the Day 1 of the SGO team damage inspection and emergency repair expedition! The weather is continuing to be challenging in Kilpisjärvi! Despite of that, our courageous team is doing their best with the repairs! 

Good work chaps! … and good luck for Thursday!

Moment of death

We trawled back through the subband statistics (SST) data to find the exact instant that LBA aerial #L34 was destroyed. The critical moment seemed to be around 18:50:20 UTC.

As can be seen, there is a possible preliminary event some 12 seconds earlier. Not sure what this could have been. Flying debris, perhaps?

In any case, this was the point when we lost aerial #L34, which probably was due to flying debris  from tile #H75.

It was a hurricane-force storm!

In order to analyse the causes of damages at KAIRA, we need to see the wind directions and wind speeds during the storm. Here are the FMI 10 minute averages from the Kilpisjärvi village and Saana top. It was indeed a hurricane-force storm in the Beaufort scale!

LBA aerial problems

Along similar lines to yesterday's web log report, we've also carried out tests for all the aerials of the Low-Band Array (LBA). Here, we also have problems, although the situation is more complicated. Like yesterday, we shall start with a plot of all subbands for all RCUs configured for the LBA aerials.

As can be seen, there are a number which show abnormalities. Most of these are suppressed signals, sometimes causing a shift in the peak amplifier response frequency. This is likely to be caused by snow, as seen in one of our previous reports.

However, the effect on RCUs 68 and 69 is more serious. This looks like a destroyed aerial.

If we plot these onto a map of the array, the positions of the affected aerials are as follows:

One thing to note is that the signal processing chains can be cleared of any faults. The problem with the HBA was on RCUs 80 and 81, but this was absolutely fine for the LBA. The same applies the other way; for example, RCUs 68 and 69 are down here, but were okay on the HBA.

What needs to be done now is to evaluate the situation, especially with aerial #34.

KAIRA Gale Damage

We report the damages at KAIRA site due to the high winds during the weekend. HBA tile 75 is destroyed! Damages are self-explained by photographs!