Very Low Frequency Experiments

2007-04-07

Very Low Frequency Happenings

Up in space not very much has been happening. The Sun is quiet and hasn't spurted any energetic plasma towards us for a few days. This leaves the ionosphere (a layer of ions and electrons close to the edge of the atmosphere) intact and generates the most interesting days for our Very Low Frequency radio experiment. This receives electro-magnetic waves, much like those that carry Radio 4, but with a much lower frequency, between zero and around forty thousand Hertz. So low, in fact, that we can directly translate the received signal into sound and listen. Normal radio sets do something clever to get sounds out of the signals. The background is silence slashed by a constant spatter of clicks like the vague applause that follows a good talk, or the crackling of water droplets on a shower curtain. These are the signals given off as thunderbolts cross from cloud to ground. As VLF signals travel well between the surface of the Earth and the bottom of the ionosphere we hear lightning strikes from Africa and the Amazon, normally at a rate of somewhere between 50 and 500 zaps each minute.

Along with this constant crackling, a strong ionosphere also provides sinking whistles like the retreating whine of a screeching jet engine. Also linked with lightning but this time from bolts in the North Eastern United States. These send unseen electrons shooting up into space which then race around the Earth's magnetic field - curving high over the equator - to then smash back down into the ionosphere above us and generate the whistle. Sometimes the bursts are reflected back again, and can bounce back and forth five or more times, triggering a stretched shriek here every four or so seconds.

As well as natural emissions, VLF waves are generated by electric transmission lines (listen in the UK and you get a fat buzzing at 50Hz and its higher harmonics), electronic equipment and vehicles (from their spark plugs and coils). When I'm doing work on the mast the receiving loop is so sensative that it can pick up the ticking of my cheap digital watch. For this reason the mast is located a long ski away from the station. Down at Halley we don't have much electronic kit, and try our best to shield the stuff we do run, which cuts out almost all the unnatural stuff and leaves us to concentrate on the interesting signals.

There are also two man made sets of transmissions on the VLF band. Many years ago the forerunner to GPS used pulsed VLF transmissions from a number of stations around the globe, each tranmitting a unique pattern, much like lighthouses, and a time code, which could be used by ships to locate themselves on the ocean fairly well. Today these are all turned off. Finally, and still being transmitted, nations with submarines deployed around the globe use VLF transmissions to send orders to their fleet. The signals travel well and penetrate a little way under water, which higher frequency signals from satellites could not do, making them pretty much ideal. Because these military transmitters are well known, blast out at constant power, and seem to be well run, we have an experiment which listens to the carrier waves for the signals. As the waves reflect off the ionosphere on their way around the world, they have their properties changed very slightly, depending on the state of the ionosphere at the time. By monitoring these signals carefully we can follow those changes, and speculate on the space physics that drives them.

We detect the VLF signals using a pair of square loops hung on a fifteen metre mast. These are then amplified and fed to the Piggott. Here we record portions of the signals on DAT tapes, and feed a suite of instruments that average important aspects of the signals all the time. This forms part of a data set that stretches back for over twenty years, and is collected to see if the environment changes in any unexpected way. Recently the data was used to show that global levels of lightning have not changed significantly, a conclusion that can only be reached by running a stable and carefully calibrated system for long periods.

My involvement with the VLF side of the Piggott is a little accidental, as I really came down here to look after computers, but one of the many good aspects to a job down here is the variety of work you have to get involved in. Over the last year, and probably into the next, the VLF has often needed a little prodding with a soldering iron or oscilloscope, which gives me a nice little problem to solve that doesn't see me staring at a monitor. The mast also needs looking after each month, which is a good excuse for a ski or a walk during work.

VLF loops in early summer

VLF recording racks

On the social side of things we're up to our usual antics. Cloudless nights and a full moon provided perfect conditions for a bit of dark-kiting. Less tricky than it sounds, as the kite is clear against the stars, the snow forms a gunmetal layer in the silvery light, and the bright moon casts sharp shadows making it possible to spot the bumpier stretches of sastrugi. We've had one or two nights with aurora, and expect more as the winter progresses. We've also dragged up the barbecue for a bit of traditional Easter fun at thirty degrees below. The burgers were, of course, still burnt, and beer cans were cold enough to freeze to lips but could be put on the flames briefly to warm back up before drinking.

BBQ at -30