Volcanoes are often inconveniently located in isolated and unpopulated regions. Of course, some of these regions are unpopulated precisely because of their volcano, or instead of unpopulated are depopulated, but that is a different story. When an area is devoid of people, there tends to be a reason. Modernity looks for and finds cheap and empty spaces for new housing developments. It doesn’t have time to wonder about the meaning of the words ‘Mississippi flood plain’ or why Ercolano sounds a lot like ‘Herculaneum’. Modernity also demands that we know about even fairly minor eruptions without delay. That is why we need volcano observatories. The case of the commercial airplane that lost its engines when flying into an unexpected volcanic ash cloud comes to mind. But how do you get instant data and messages from these forbidden places? The next newspaper delivery won’t do. In our modern world, it has to be by the fastest means available.
The answer is old-fashioned radio. It travels at the speed of light. Only the financial world needs things to move faster than that. (That also means they travel back in time. Explains a lot, really.) For people with less money, light speed suffices. Volcanology, and volcano observatories, depend on radio, both for hearing about eruptions and informing about them. Even the monitoring satellites communicate to us by radio. Radio is the original ‘wireless’. And now, volcanoes have joined the fray themselves. For the first time, a volcano has been heard announcing its own eruption by radio.
The Aleutian Islands are one of the most active volcanic arcs in the world. 35 active volcanoes are listed on the map above, from the Alaska peninsula to the many volcanic islands. Umnak Island, one of the largest of the Aleutian islands, is known by tourists for its geyser field. The southern part of the island has two rather beautiful stratovolcanoes, Mount Vsevidof and Mount Recheshnoi. The northeast side, almost a separate island, is dominated by the shield volcano Mount Okmok and its 9-km wide caldera, and is most impressive when seen from above.
July 12, 2008, and it is 11:43 am, local time, on Umnak Island. Rather suddenly, Mount Okmok erupts and sends ash 15 kilometer high, in its biggest eruption since the 13th century (at least). It had been a fairly active volcano, with eruptions in 1997, 1958, 1948, and 1817, but the 2008 eruption was bigger. An hour later, an even bigger column forms. In the following weeks the eruption continues with plenty of ashy explosions but little or no lava. The rather wet caldera provides plenty of water for hydro-volcanic explosions. The five week, VEI-4 eruption leaves several Aleutian communities cut off during the height of summer as air travel is curtailed by the airborne ash.
Notably, the eruption got going with very little warning. There were some earthquakes in the hours before the eruption, but that was all and it had not been picked up as a warning. The volcano was monitored with real-time seismic and geodetic networks, but even so, for the real-time monitors it all happened too fast. The Alaska Volcano Observatory (AVO), normally very much on the ball, were told about the on-going eruption by the U.S. Coast Guard, who in turn had heard about it when being contacted by the ranch caretaker family of Fort Glenn (an old US military base) while they were fleeing from the ash fall – they escaped with the help of a fishing vessel. It shows the importance of having people on the ground to report events, albeit in this case unwilling participants who were too close for comfort.
The Okmok eruption was detected 35 minutes later, but only in hindsight. At the time, the watchers did not realize what they were hearing. They were in Dunedin, New Zealand, and hadn’t been looking for volcanic eruptions half a world away. The volcano’s calling went unanswered.
Sferics and whistlers
During the first world war, an inventive German physicist, Heinrich Barkhausen, used a system of tubes and wires to pick up radiation leaking from nearby phone lines. In 1911, Barkhausen had become the first professor in the world in electrical engineering. During the war, he was close to the front, and the phone lines he was spying on were on the other side. But apart from intercepted calls home, he discovered a very different kind of signal. Every now and then, a strange, fast descending tone came from his system. The Germans called them ‘whistlers’ because they sounded like the whistling shell being fired at them. Barkhausen could find no good explanation and suggested they were atmospheric.
Whistlers happen at frequencies around 5 kHz, the VLF band (VLF of course stands for Very Low Frequency). They last a few seconds with a descending frequency. in the 1950’s, after many years of studies, they were traced to lightning activity, confirming Barkhausen’s opinion. The reason that this took so long is that the lightning which caused it was not particularly nearby. Quite the opposite: it could be as far as 15,000 km away! Not quite antipodal, but not far off. What happened?
Lightning is caused by electric charges building up in clouds. Ice crystals in the cloud become charged by friction. Small crystals are positively charged and large hail stones acquire negative charge. Hail stones tend to fall downward, but the small crystals are blown upward. This gives a tremendous charge separation, with the top of the cloud positively charged and the bottom layer negative. Below this, the ground becomes positively charged. The charge difference finally exceeds the insulating capacity of air. A spark begins to fly from one end. The heat of the spark ionises the air and suddenly the air forms a conducting tube. Once the spark reaches the opposite charge (often trying out various paths until one works), an instantaneous current flows, typically around 10-100 kilo-Amp. The lightning flashes, and the over-heated air causes a sonic boom. Cloud-to-cloud lightning is called a lightning flash, and cloud-to-ground a lightning strike, but there is no fundamental difference between them; note that you can’t get strikes from the top of the cloud directly to the ground through the bottom cloud layer as they have the same charge. (A third type is cloud-to-space, traveling upward. These are called sprites and elves, common but difficult to spot.) Lightning blinds and thunder deafens: people lose their two most important senses simultaneously. The sound can be felt as a disorientating pressure wave. The smell of ozone completes the quartet. It is a frightening experience. Being hit is even worse, of course, and involves a significant fatality risk as the current interferes with the heart and can stop it. A nearby strike can have the same effect: the current travels through the ground especially when it is wet (not uncommon during thunderstorms) until it reaches your feet, and if those feet are some distance apart, it causes a voltage difference between them. A current flows through your body. After a personal experience with a particularly bad thunderstorm, I now know that cattle are particularly prone to this: their legs are very far apart so they attract a lot of current – a bug in their design.
The instantaneous current flowing through the lightning path causes an electromagnetic pulse to travel outward in all directions. Our bodies are not equipped to detect this radio wave, but it travels the furthest of all lightning signals. Whilst light is limited by the horizon and thunder sound waves curve away from the earth’s surface and can’t be heard more than some kilometers away, the low-frequency radio pulse can curve around the Earth, by continuous reflection between the ground and the ionosphere. In this way it can reach up to 2000 km before fading below detectability. At that distance, there is no indication of any thunderstorm: literally a bolt from the blue.
The radio emission, called sferics (for radio atmospheric signal), is strongest in the VLF band. They have broadband frequencies of around 10-100 kHz and wavelengths of 3-30 km. Modern lightning detectors use these sferics. They use a network of sensors over a region, and from the difference in arrival times at each sensor can calculate where the lightning occured. Four sensors are needed for an accurate position but in practice five are used. Detection rates vary, but for the world-wide network typically 15-30% of detected lightning strikes can be located, mostly for lightning currents in excess of 30 kA. A denser grid of sensors helps, and therefore national networks can perform better and also detect weaker lightning.
But if the wave can travel upward and reach the ionosphere, 100 km above the Earth, it can travel along the magnetic field lines – and these bend around the horizon to the other hemisphere. The lines connect pole to pole, and initially go out, reaching high altitudes over the equator. Across the equator they begin to curve down again. Finally, they enter the Earth’s surface on the way to the centre. The point where a field line leaves the Earth’s surface and where it re-enters will be at the same distance from the magnetic pole (but the opposite pole, obviously). The point of re-entry is called the conjugate of the outward crossing. Because of symmetry, the outward crossing point is itself the conjugate of the re-entry point: each point has a unique magnetic conjugate.
Thus, the electromagnetic wave which reaches the ionosphere will eventually approach the conjugate point of its origin. Here a fraction of the wave leaks through to the surface, and interferes with the radio enthusiast. The higher frequency travel faster, and thus the
interference begins at the highest frequency and descends as the lower frequency parts of the wave arrive. Thus the whistler is born. When you are hearing a whistler, it is your magnetic conjugate calling, from a world away. Listen carefully.
In practice the waves can first travel across the Earth’s surface before finding their way into the ionosphere. Thus, when you hear whistler, it may be from lightning not at your conjugate point, but some distance from it. The real location of the lightning could be as far as 2000 km from the point you are listening to. This is what makes research frustrating – and exciting.
There are many VLF receivers around the world which detect sferics and whistlers. The signals are automatically analyzed, by what is called an Automatic Whistler Detector and Analyzer, or AWDA for short. (This slightly disappointing acronym seems open for improvement. How about Analyzing Detections of Whistlers by Automatons!, or AnD WhAt!.) This has shown that Hungarian whistlers come from the East coast of South Africa. Whistlers at the Antarctic Peninsula come from the Gulf Stream. But in both cases, there are also correlations with other regions with high lightning activity, closer to the equator. Apparently equatorial lightning has a separate path, not via the conjugate point, to travel long distances but the details of this are not understood. Lightning is much more frequent in tropical climates, and even a minor leakage here can become noticeable.
Dunedin and the Okmok whistlers
Dunedin in New Zealand is geographically opposite (antipodal) to Coruna, Spain, a cultural highlight. But its magnetic conjugate is not there. The local magnetic field line connects to 55.84°N 174.70°W, just north of the Aleutian Islands. This should make Dunedin a good place to avoid whistlers, as lightning is fairly rare in the Aleutians. Still, whistlers are heard. After detailed studies, the conclusion was reached that the Dunedin whistlers are best correlated with lightning activity at the west coast of Central and North America. The lightning rates here are 2000 times higher that those in the Aleutian Islands, and so even if only a small fraction makes it to Dunedin, they will still dominate the numbers. But the area is 6000 km from the conjugate point. Apparently an alternative path exists which allows the waves at this location to travel below the ionosphere towards Dunedin. The details of this connection are not clear. Getting to New Zealand can involve mysterious ways.
On 12/13 July 2008, an enormous number of 21,021 whistlers were detected at Dunedin. They started suddenly and ceased after nine hours. This was one of the highest number of whistlers ever recorded here: usual numbers are less than 1000 per day. Over an 8 year period there were only two days with a higher count (both in the second half of 2012). Even stranger, the spike happened during daylight hours when whistlers don’t travel well. The spike was noticed at the time (it was hard to miss!) but the cause was not known. There was no thunderstorm activity in the usual suspect region of North/Central America. The answer was only found six years later when people looked for activity near the conjugate point. It was the volcano radio of Mount Okmok.
Mount Okmok is located 350 km from Dunedin’s conjugate point. Two other rather active volcanoes are also within range of Dunedin’s magnetic conjugate: Mount Redoubt is 870 km away, and Mount Kasatoch 815 km. But the clear culprit was Okmok and its significant eruption that day.
Two other possible cases of volcano radio were however found in the Dunedin records, related to these two. Mount Redoubt had several eruptions during 2009, and its eruption from 26 to 28 March coincided with a high count of whistlers. Other Redoubt eruptions did not lead to higher whistler counts.
Mount Kasatochi (Kasatochi Island) erupted on 7 August 2008. The 14 km ash cloud led to 36 whistlers. An eruption a few hours later did not add to the whistler count. So both Redoubt and Kasatochi are radio-active – but Okmok far outstripped them.
The clear cause of the whistlers was lightning activity in the ash clouds. The eruptions that showed whistlers had lightning – the others didn’t. The World Wide Lightning Location Network (WWLLN – I would have gone for the acronym www.lightning.net) showed that lightning at Okmok began 35 minutes after the onset of the eruption, mostly within 10 km of the volcano. The Dunedin whistlers began at the same time. WWLDN measures the time of a lightning strike to 30 micro-seconds (although it tends to see only the strong strikes), allowing individual whistlers to assigned a particular lightning strike.
We have many images of volcanic lightning in explosive ash clouds. But how does volcanic ash generate lightning? How does it build up the electric charge and how and how does it discharge the charge? Part of this is not difficult to answer. Even in normal thunderstorms, adding a bit of dust to the atmosphere greatly increases the lightning activity. Saharan sand over Europe can coincide with very impressive thunderstorms. Part of this may be due to the sand itself being charged, in the same way as the ice, by friction. But dust grains can also act as condensation nuclei: adding a bit of dirt to air makes it much easier for the water to condense.
Volcanic ash is not the same as sand. Does it work the same way? Or is the crucial difference the water mixed in with the ash? Wet eruptions are more ashy, so the two often go together. Recent studies of Sakurajima have shown that the volcanic lightning occurs in the lower parts of the ash plumes, relatively close to the ground. The electric charge apparently is already created in the initial explosion. This is different from thunderstorms where it is created at higher altitude.
It is a strange thought that VLF radio can be used to detect -and locate- distant eruptions. Mount Okmok was first heard 11,000 km away, across the entire Pacific. If the Falklands (or Malvinas, depending on your preference) erupts, England could be the first to know, via radio whistling. (The lack of a volcano on the Falklands may make this scenario less likely, of course.) But of course the whistlers really detect the lightning, and lightning can also be detected directly. If you monitor the WWWLLN you could already know about the eruption. Whistlers are more sensitive: the WWWLLN detects only a fraction of the strongest strikes, and you can detect whistlers from lightning missed by the WWWLLN. But the whistlers also tell you other things. With volcanic whistlers, you know exactly where the lightning occured, and therefore how the radio wave traveled. This tells you how well the lightning connects to the ionosphere – and that is a poorly understood problem which may also tell us a lot about volcanic ash in the stratosphere. Scientists have found themselves a new tool – what they will eventually use it for can be hard to predict. Who could have expected that Heinrich Barkhausen’s early experiment in government snooping would lead to real-time monitoring of thunderstorms and volcanoes on the other side of the world?
And the title of this post? Lightning Of Volcanic Eruptions uses whistlers to send messages from secretive and hidden volcanoes around the world to their conjugate admirers. Volcanoholics need more LOVE.
Based on the article: Investigating Dunedin whistlers using volcanic lightning, by ￼Claire Antel and collaborators. Geophysical Research Letters, Volume 41, Pages 4420–4426 (2014)