Listening to Lightning

Eyes closed, listening intently, I can pick apart the individual sounds.  Random, staccato clicks dominate, like an old and somewhat scratched LP recording played on grandmotherâ??s gramophone.  Behind the clicks lies the chorus.  I am transported to a dark, calm night in some distant tropical forest.  The warm still air is heavy with the sound of thousands of frogs, voices in competition then in chaotic unison, a melodious cacophony.  The mind begins to tune out the clicks and crackles; now I am convinced it is the sound of great raindrops, collected by the forest canopy above, falling onto the tense nylon of my tent.  I lie and listen to the lyrics of lifeâ?¦ but life it is not.  What I hear is not the forest of my imagination, but the stark sterility of the ionosphere filtering and reflecting one of the most violent phenomena known to man.  Iâ??m listening to lightning.


Dr Andrew Collier is a physicist from the University of KwaZulu Natal in Durban, under tenure of the Hermanus Magnetic Observatory.  Two weeks into our voyage to Antarctica, in an old t-shirt, tousled hair, and the beginnings of a russet beard, he is usually to be found in the shipâ??s library at work on a laptop.  Iâ??m hard-pressed to place this multi-faceted gentleman â?? he is at once open and friendly, with obvious enthusiasm for his science, yet his youth and current scruffiness belie the fact that he is a well-published expert in space physics, and the scientific leader of the summer expedition.  Specialising in the dynamics of our own magnetosphere, Andrew is travelling south for the fourth time to update the sensitive equipment at SANAE IV used to monitor iono-and magnetospheric phenomena.  By listening to the radio â??soundsâ?? created by the 45 to 100 lightning strikes occurring around the globe every second, he and his protégé Sherry Bremner are analysing the electromagnetic atmosphere surrounding the planet.


Lightning is both spectacularly beautiful and deadly powerful.  As clouds form in our lower atmosphere, electric charges develop due to friction between individual cloud particles.  If the cloud is large enough, these charges separate vertically, with the underside of the cloud becoming progressively more negatively charged.  The negative charge in the cloud begins to repel the negative charges in the ground, leaving it powerfully positive.  As this process progresses the electrical difference grows until it is sufficient for the charge to bridge the gap between cloud and earth.  Tendrils known as â??step leadersâ?? reach from the cloud down towards the ground and also from the ground upwards, searching for a pathway for electricity to flow.  When two tendrils meet, a massive electrical discharge occurs.  The current flows in bursts, making the air in its path hotter than the surface of the sun.  The individual atoms are energised far beyond their norm; as the burst wanes, their electrons drop back to their resting states, giving off photons in the process.  Multiple strokes can occur in split seconds along a single path, giving the light a flickering nature.  We perceive this as lightning; seconds later the sound wave generated by the rapid expansion of the heated air rolls past as thunder.  Yet even this is not all; the huge flow of current turns the lightning into a spectacularly powerful and transient dipole antenna: a powerful electromagnetic signal is transmitted at the moment of the strike, from each and every bolt of lightning.  Four million of these bursts are given off every day as storms form around the world â?? predominantly in the warm and moist tropics, where there is plenty of energy and moisture.  It is these radio waves that scientists like Andrew and Sherry are using to improve our understanding of the ionosphere â?? the charged area of our upper atmosphere â?? and the fluctuations in the Earthâ??s magnetic field.


As the electromagnetic wave from the lighting propagates away from the site of the strike, different courses become apparent.  Some of the energy is reflected by the ionosphere back towards the earth, whereupon it bounces again before repeating the process, trapped between earth and ionosphere like light in a fibre optic cable.  This is the same process used for some radio communications, but the lightningâ??s power means that the signal can travel right around the planet, especially that in the 3-30 kilohertz Very Low Frequency (VLF) range.  A suitable VLF receiver will detect these signals, and when they are converted to audio output, they are heard as clicks called spherics.  The spherics are the clicks that I imagined to be raindrops, or scratches on the record â?? the actual radio â??soundâ?? of a lightning strike occurring somewhere on the surface of the planet.


The ionosphere begins at 80-100km above the surface of the earth and gradually merges into the much greater magnetosphere.  Although immense in scale, the magnetosphere is still subject to the solar wind:  on the sunward side of the planet, it is compressed to only about 10 earth radii, but in our lee it streams out to a point unknown, perhaps hundreds of times the earthâ??s radius away.  It forms somewhat of a shield, deflecting much of the radiation bombarding us from our star, but also trapping energy along the magnetic field lines.  Some of this energy reaches earth, particularly where the field lines converge at the poles.  Other energised particles and waves spiral endlessly within the magnetosphere, bouncing back and forth.  Wave-particle interactions occur continuously in this background radiation.  Where a harmonic is created between waves and particles, it can detract or contribute to the waveâ??s energy.  The VLF waves which gain sufficient energy penetrate the ionosphere down towards the earth.  If converted to an audio signal, we perceive these waves as slightly more drawn out soundsâ?¦ remarkably akin to the chirruping of frogs or birds.  Known as chorus, it forms the counterpoint to the spherics caused by lightning strikes.  Not only can I listen to lightning; Iâ??m eavesdropping on the magnetosphere.



Click play to listen to a short VLF recording taken  at SANAE IV on January 14, 2008



The raindrops and chorus of frogs continues unabated, patterns emerging and receding into chaos.  Entropy reignsâ?¦ and then without warning, Order strikes.  An unearthly sound prevails, clear, unwavering.  It shimmers steadily down the spectrum, from an elevated harmonic to a low breathy whisper, and then is gone.  While the spherics and chorus so closely resemble the sounds of life, this is ethereal; a sirenâ??s song from another planet.  Yet once again, Iâ??ve been fooled: this is a whistler, a third and beautiful radio signal, also caused by the distant lightning.


Part of the electromagnetic signal generated by the lightning is not reflected by the ionosphere â?? it passes through and follows the magnetic field line into nearby space, before curving back down along the line and re-entering at the magnetic conjugate point.  In the process, it interacts with energised particles in much the same way that the waves which form the chorus do.  While the chorus is the product of low-amplitude (low energy) waves occurring over time, the wave from the lightning has large amplitude but much more transient nature.  The longer wavelength waves – with lowest frequency – are slowed more than the shorter wavelength (high frequency) waves by these particle interactions.  Like colours separating on blotting paper, the signal is â??stretchedâ?? over time during its passage.  When it returns to earth, the same lightning strike that formed the earlier spheric has become a whistler.  The high frequencies arrive first, and then the sound shimmers down through the spectrum as the delayed lower frequencies arrive.  Unlike the spheric, the whistler requires more specialised conditions to occur.  There may be only one whistler for thousands of observable spherics, and the observer needs to be near the conjugate point, listening on the VLF spectrum, to hear the whistler.  Like so many rarities, its beauty is undeniable. 


Whistlers have been observed over the years by people listening at the VLF frequency range, having first been detected as interference in radio signals in the First World War.  Only linked to lightning in the 1930â??s, they were initially monitored manually by recording the sounds to tape and then playing them back to an observer.  This was understandably time-consuming, and thus automated whistler detection systems were developed.  Andrew Collier was instrumental in writing computer software to analyse the signals for whistlers.  The data thus gained can then be compared to that from lightning detectors worldwide, and the origin of the whistler determined.  By comparing the structure of whistlers from different places and times, scientists are able to understand more about the nature of the magnetosphere and ionosphere.  The data also has implications for monitoring global climate change – a change in atmospheric temperature of one degree causes a doubling in the rate of lightning strikes.   Despite the value of this data, there are still only a few automatic VLF whistler detectors in existence.  One is to be found in Tihany in Hungary and another at Duneedin in New Zealand.  South Africa contributes the other two: one at the South African Astronomical Observatory in Sutherland, and the last in Antarctica, at SANAE IV.  The SANAE detector is particularly valuable; the traverse through the magnetosphere is much longer at between conjugate points in the high latitudes.


The raindrops have become spherics; the frogs are alive no more; no beautiful alien is singing to me across space.  Still, the sense of wonder remains.  I listen to lightning crackling across the skin of the planet, its song echoing in nearby space, and hear the hubbub of solar wind in the magnetosphere.  Eyes closed, listening intently, the sounds blend into perfect oblivion.


Sherry Bremner poses next to the SANAE IV VLF Antenna

3 Responses to “Listening to Lightning”

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