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Atmospheric Electricity: By DR DAVID MADDISON Nature’s Spectacular fireworks! If you think about nature’s electrical fireworks, lightning and the auroras at the poles immediately come to mind. But now we can see that what happens out in space is vastly more spectacular and in fact, much larger than planet Earth. This is the story of Nature’s planet-wide spectacle which has been running ever since the Earth was formed. E arth’s atmosphere is highly electrically active, most visibly manifested by phenomena such as spectacular lightning displays and, for those living in the higher latitudes, the Aurora Australis and Aurora Borealis. Then we have the invisible ionosphere, which plays a vital role in the propagation of radio signals for long distances. Other electrical and related phenomena in the atmosphere and near space include the magnetosphere, coronal mass ejections from the Sun impacting the magnetosphere, the solar wind, the plasmosphere, the ring current, the Van Allen Belts, electrical charging of clouds and transient luminous events such as sprites, elves and blue jets. Plus there are controversial unexplained phenomena such as ball lightning. History Early experimenters noticed the similarity between sparks and discharges from electrical machines, Leyden jars and lightning, and concluded that they were the same phenomena. It was William Wall in 1708 that first noted that spark discharges resembled miniature forms of lightning. In 1750 Benjamin Franklin thought that electrical charge could be drawn from clouds with a tall enough metal aerial but he was beaten to the experiment in 1752 by Frenchman 16  Silicon Chip Thomas-François Dalibard who managed to draw sparks from a passing cloud with a 12m aerial. In 1752 Franklin is said to have performed his famous kite experiment (however, there is some dispute now as to whether Franklin ever did that experiment!). It was done by Romas and Cavallo, both of whom drew long sparks from a metal string. G. Beccaria repeated Le Monnier’s work and determined that the atmosphere had a positive charge. Other later workers found seasonal variations in the Earth’s electric field and variations with altitude. L.G. Le Monnier was another that repeated the kite experiment and discovered the electrical field within the atmosphere which came to be known as the “fair weather condition”. Vertical electric field in the atmosphere There is a high strength vertical electric field in the atmosphere. On a clear day, over flat land or the ocean at the equator, this is around 120V/m so there might be a potential difference almost 200V between your nose and the ground. We do not notice this because the body is a relatively good conductor and so the potential difference is “shorted out”, bringing the potential between your nose and the Earth close to zero. siliconchip.com.au FINE WEATHER FINE WEATHER 50km 300,000 VOLTS ELECTROSPHERE RAIN CORONA EARTH NET ELECTRIC CURRENT Atmospheric electrical circuit showing the relationship between fair weather current flow and charge transport in thunderstorm conditions. The field is in a direction that gives the surface of the Earth a negative charge under normal circumstances, the “fair weather condition”. The electric field continues to increase with height (although not in a linear fashion) until an altitude of around 50,000 metres (that’s more than three times higher than jet aircraft fly), at which point the atmosphere is sufficiently conductive, due mainly to ionisation by ultraviolet light, that no further increase in potential occurs. The potential difference between that altitude and Earth has a mean value of 280kV, varying from 150kV to 600kV. Even though the electric field gradient extends to 50km altitude, most of the charge is located within the first 10,000 metres, since the net positive charge contained between the ground and 10,000 metres is nearly equal to the net negative charge of the Earth. At 10,000 metres, the electric field gradient has reduced from around 120V/m to around 5V/m. One might ask the question, “Why not use this potential difference to generate power?” It has been asked many times before! But since air is a good insulator, the current available is very small, so little useful power could be generated. Although it is possible to use this field to drive a small electrostatic motor you can build yourself (see box), little useful work can be done. In fact, the current density of the atmosphere is around 1pA (picoamp) per square meter in built-up areas, to 2.4pA per square meter in most other areas. An often-cited figure for the total current over the whole of the Earth’s surface is around 1800A; the range is 750 to 2000A. Using typical figures of 1800A total current and 280kV mean potential, the total power for the entire Earth is 504 siliconchip.com.au Electrical equivalent circuit of atmospheric electrical flow with thunderstorms. Note that some values are slightly different to those mentioned in text as different values are used by different researchers. (After www.slac.stanford. edu/cgi-wrap/getdoc/slac-wp-020-ch11g-Kirkby.pdf). megawatts. Using the voltage and current figures in Ohm’s Law also allows us to calculate the effective resistance of the atmosphere as 155. The air of the atmosphere is generally regarded as a good insulator unless it is heavily ionised, as is the ionosphere, in which case it becomes an electrical conductor. The fact that some small current does exist from the atmosphere to the Earth does mean that the atmosphere is somewhat conductive. The main source of the lower atmosphere’s limited electrical conductivity is ions which are air molecules which have either gained or lost an electron and are no longer electrically neutral and therefore able to carry charge. How are the ions generated? In 1912 Hess, with a balloon-borne electrometer, established that the amount of ionisation of the air increased with height, meaning that whatever was causing it was stronger at high altitudes. This was a mystery because it was previously thought that ground-based radioactivity was causing electrons to be stripped from atoms and molecules, turning them into ions. It was eventually proved that cosmic rays from outer space were responsible for the generation of a majority of atmospheric ions, constantly replenishing them as older ions are carried to Earth in the charge transfer process. Apart from ions, charge can also be carried on dust and water droplets. Ions can also be generated by radioactivity at ground level but that is not the major contributor to ions in the atmosphere as originally thought. Interestingly, the potential gradient of 120V/m at the equator increases to 300-400V/m in industrial areas with a lot of dust. (The figure is typically 155V/m at 60° latiMay 2016  17 tude and 71V/m at the South Pole, although these figures do vary with time). How does the Earth become charged? How is this charge maintained? If it were not replenished there would soon be no charge on the Earth. It is estimated that the negative charge of the Earth would dissipate in 1060 minutes, so how the charge was replenished remained a great mystery in the early days of research into atmospheric electricity. It is now known that thunderstorms, lightning and electrified (but non-thunder) clouds are the “battery” responsible for maintaining a permanent potential difference between the upper atmosphere and the surface. Bolts of lightning do not discharge the entire atmosphere as might be thought but a majority of lightning strikes bring a large amount of negative charge to Earth’s surface. The corresponding return part of this circuit occurs elsewhere, in areas of fine weather. As mentioned above, the Earth has a net negative charge and the electric field is directed downward. Current flows from sky to Earth in areas of good weather. Beneath thunderstorms however, a positive charge builds up on the Earth’s surface. In that case the field is directed upwards and current flows from the Earth to the sky. The two areas of flow together complete an electrical circuit which keeps the charge on the Earth replenished as illustrated in the diagram. At any given time about 1% of the Earth’s surface that is beneath thunderstorms carries a positive rather than negative charge. All around the world, 24 hours a day, there are around 40 to 50 lightning flashes per second so there is certainly enough activity to keep the Earth-atmosphere system charged. What causes lightning? Lightning is an extremely complex process, even today not fully understood. Essentially what happens within a thundercloud is that due to convection currents, lots of ice particles and water droplets rub across each other, causing them to become charged. This is similar to what happens when you rub two insulating items together such as wool and a balloon. Pure ice crystals develop a positive charge while “graupel”, a water and ice mixture with a slushy consistency, becomes negatively charged. The lighter ice crystals are carried by updrafts in the cloud while the heavier graupel stays near the cloud base. This results in charge separation within the cloud, with positive charge near the top and negative charge near the bottom. The positive charge at the top of the cloud and the negative charge at the bottom of the cloud form a “battery” that is of the correct sign to drive the Earth beneath it negative. Typically the positive charges are 6 to 7km high at a temperature of around -20°C and the negative charges are 3 to 4km high with a temperature of around -10°C. This charge builds and builds – and when a sufficient charge has developed between oppositely charged areas such as between a cloud and Earth, within a cloud or between clouds, the normally insulating air breaks down and becomes conducting. The exact processes are complex and not well understood but for a cloud-to-ground strike the process is as follows: 1) A downward “leader” is formed and descends from the cloud. 2) When the leader approaches the ground an upward streamer can form which is much like a leader but in the opposite direction. 3) “Attachment” occurs where the leader and streamer connect, establishing a low-resistance electrical pathway. 4) “Discharge” or the “return stroke” occurs which is the most powerful part of the lightning strike, resulting a discharge of energy from the cloud to the ground with a typical current of 30,000A and up to several hundred mil- Global distribution of lightning strikes in units of flashes per square kilometre per year as detected by a NASA satellite carrying the Optical Transient Detector (OTD) and the Lightning Imaging Sensor (LIS). The place that receives the most lightning is near a village in the Democratic Republic of the Congo with around 160 strikes per square km per year. 18  Silicon Chip siliconchip.com.au lion volts. A number of return strokes can happen in quick succession and the return stoke neutralises the positive ground charge caused by the storm. You can see a video of these phenomena at http://youtu.be/dukk07c2eUE A conventional cloud-to-ground to discharge is called a negative stroke but there are rare positive strokes as well, which originate at the tops of clouds and are much more powerful. While aircraft are designed to withstand negative strokes, positive strokes were unknown when aircraft lightning safety standards were first established and it is unclear how well they would withstand a hit with such a lightning strike (see www.damninteresting.com/the-powerof-positive-lightning/ for a report on the first aircraft known to have been destroyed by positive lightning). There are several different types of lightning. Some variations include “superbolts” which are about one hundred times more powerful than normal lightning and “ball lightning”, the actual existence of which is unproven but subject to intense speculation. Ball lightning is in the form of luminous free floating spherical objects up to a metre in size, is usually associated with cloud to ground discharges, lasts from a second to a minute, often moves horizontally and has been claimed to go through solid objects and has even been claimed to appear inside an aircraft cabin. St Elmo’s Fire is a coronal discharge that can occur in areas of high electric field such as might appear beneath thunderstorms. Throughout history, it has occurred as an eerie glow on the spars of sailing ships – a portent of doom according to sailors. An effect similar to St Elmo’s Fire was the glow which surrounded flight BA9 after it flew through a cloud of volcanic ash over Indonesia on 24 June 1982, nearly causing the plane to crash (https://en.wikipedia.org/wiki/British_ Airways_Flight_9) An eerie glow, visible from inside the plane, occurred when a British Airways flight passed through the ash of an erupting volcano. At the time, all four engines had failed. The global electric circuit The continuous electric current between the lower layers of the ionosphere and the Earth’s surface is known as the global electrical circuit (GEC). In recent times it has come to be understood that this system is vast, extremely complicated and variable and has many factors that influence it. The Sun has a major influence on the GEC and atmospheric electrical phenomena. Apart from bathing the world in life-giving energy, the Sun emits a constant stream of charged particles which hit the Earth’s protective magnetosphere. These eventually pass down magnetic field lines which are concentrated at the magnetic poles where they cause the Aurora Borealis in the northern hemisphere and the Aurora Australis in the southern hemisphere. The aurora has associated large electrical currents or “electrojets” flowing in the direction from the day side to the night side. Occasional explosions of huge amounts of material from The various elements of earth's Global Electrical Circuit. Image credit: University of Colorado, “Electrical Connections and Consequences Within the Earth System” (ECCWES) project. siliconchip.com.au May 2016  19 Nikola Tesla Despite the claims of many conspiracy theorists (and, we must warn, modern-day internet scams), Nikola Tesla did not invent a method for harvesting large amounts of “free energy” from the atmosphere. He did however have a lot of interest in wireless transmission of electrical energy and radio, something that is firmly based in science. In 1900, Tesla had asked J.P. Morgan to invest in his Wardenclyffe tower project to transmit a wireless radio signal across the Atlantic Ocean. Morgan agreed to this and funded the project to the extent of US$150,000. Telsa had also planned experiments in wireless power transmission but did not initially tell Morgan about this. On the 12th December 1901, Marconi managed to send radio signals across the Atlantic thus beating Tesla to this goal as Tesla’s project was still under construction. In 1903, Telsa finally revealed his intention to Morgan of wireless power transmission and Morgan declined further funding. Conspiracy theorists claim this was because Morgan saw that wireless power transmission could not be metered but the reason is that Morgan did not find the idea attractive and also he had already told Tesla he would not invest more than the originally agreed amount. In addition, Tesla’s radio system was more expensive than Marconi’s. Further, the “Rich Man’s Panic” of 1903 made it difficult for Tesla to get funding from others and also his own patents for his previous inventions expired, leading to very difficult times for Tesla. A detailed rebuttal of the conspiracy theorists’ claims can be found at http://thelibertarianrepublic.com/evil-capitalistsprevent-nikola-tesla-creating-free-energy/ See also https:// en.wikipedia.org/wiki/World_Wireless_System It turns out that short range wireless power transmission is coming into use for applications such as charging mobile phones but even medium and long range wireless power transmission is possible. Techniques involve inductive, capacitive and magnetodynamic coupling and microwaves or lasers. An artist’s 1925 image of what Tesla’s wireless power transmission system might have looked like in the “future”. The tower is transmitting electrical power to aircraft and the city in the background. 20  Silicon Chip Magnetosphere of Earth showing various structures. Image credit: NASA. the Sun occur during events known as coronal mass ejections that can impact the Earth causing disruption of communications and electrical grids and damage to satellites. These events can also lead to spectacular aurorae, often extending much further toward the equator than normal. The Sun is responsible for providing most of the radiation that ionises the ionosphere and the properties of the ionosphere vary with the 11-year sunspot cycle. The Magnetosphere The magnetosphere is a region around Earth with a magnetic field in which charged particles from the Sun (or elsewhere in space) are influenced by the field. Near the Earth, the field is shaped much like that of a bar magnet but further away it is greatly distorted by the continual flow of particles from the Sun, the solar wind. The magnetosphere protects the Earth from harmful charged particles by deflecting them around the Earth or into the polar regions. Without the magnetosphere, life would be very challenging or it may not even have evolved at all. The solar wind would also eventually strip away much of the atmosphere, as is believed to have happened on Mars (which has neither a protective magnetic field nor significant atmosphere – it’s less than 1% of Earth’s and is 95% carbon dioxide). There are a number of structures within the magnetosphere. The first one is the bow shock which is the leading edge of the magnetosphere that particles first encounter, setting up a shock wave as they are moving at around 400km/s. The magnetopause is an area of balance between the planet’s magnetic field and the solar wind. The magnetosheath is a magnetically turbulent region between the bow shock and the magnetopause. The particles in this region are mainly solar wind from the bow shock. The magnetotail is the opposite structure to that of the compressed magnetic field on the side of the Earth facing the Sun. It extends far out into space and between the upper and lower structures there is a sheet of plasma. The plasmasphere is located above the ionosphere and is also known as the inner magnetosphere. It contains low energy or cool plasma. It was discovered due to the analysis of VLF Whistler data, which will be discussed later in this article. siliconchip.com.au Van Allen radiation belts Within the inner part of the magnetosphere exist the two Van Allen radiation belts, discovered in 1958. These trap energetic charged particles within the magnetic field and can cause damage to spacecraft if those are not protected. The Atmosphere Earth’s atmosphere is the medium in which an electrical gradient and associated current is maintained and also the medium in which thunderstorms develop, thus forming a vital part of the global electrical circuit. Ionosphere The ionosphere exists as a layer from about 65km altitude to about 600km. One of its most important properties is that it reflects radio waves. It can do this because it contains a significant proportion of charged particles in the form of atmospheric atoms which have had electrons removed by high energy radiation from the Sun, such as UV and Xrays as well as, to a lesser extent, cosmic rays from space. These particles are said to be “ionised”, hence the name of this layer. These particles form a plasma that is electrically conductive and hence capable of reflecting radio waves under the right circumstances and conducting electrical currents. The properties of the ionosphere vary from day to night and also according to geomagnetic activity such as solar outbursts. For further information about the ionosphere see the article on HAARP, SILICON CHIP, October 2012. Cosmic Rays Cosmic rays generate most of the ions which are the cur- rent carriers for the atmospheric electrical current. These are mostly atomic nuclei originating from somewhere out in space which strike the Earth’s atmosphere at near-light speeds. Geomagnetically induced currents Geomagnetically induced currents are currents induced into artificial electrical conductors (such as long power lines or telephone lines) on Earth, as a result of electric fields produced by rapid magnetic field variation during geomagnetic storms. These currents can cause damage or destruction of transformers and can even bring down entire electrical grids. Electrical grids are much more vulnerable when they are heavily loaded with little excess capacity. A severe geomagnetic storm, which struck the Earth on 13 March 1989, brought down Canada’s Hydro-Quebec 735kV power grid; the blackout lasted nine hours and was ended only when the utility company implemented many temporary bypasses and “fixes”. Schumann resonance The area between the Earth and the densest, most conducting part of the ionosphere forms a wave-guide that has a certain resonant frequency and conducts electromagnetic waves that are generated by lightning discharges around the planet. It was named after physicist Winfried Otto Schumann who in 1952 predicted that this cavity between the Earth and ionosphere would have a characteristic resonant frequency. The fundamental Schumann resonant frequency is 7.83Hz, with peaks at intervals of around 6.5Hz: 14.3. 20.8, 27.3 and 33.8Hz. A “Static Motor” from the 1920s The video “Free Atmospheric Electricity Powers Small Motor – Tesla Radiant Energy” (https://youtu.be/do4IO_ U3B5o) shows the operation of a small motor powered by the electric field gradient in the atmosphere. A hexacopter was used to hoist the antenna wire around 30 metres into the air. The video title implies a connection with Nikola Tesla’s ideas. Tesla never proposed harvesting “free energy” but did have ideas of wirelessly transmitting electrical energy. This screen grab from that video comes from the book “Homemade Lightning: Creative Experiments in Electricity” 3rd Edition but originally comes from the publication “Practical Electrics” from 1924. siliconchip.com.au May 2016  21 Listening to the sounds of the ionosphere. . . It is relatively easy to hear natural signals from the ionosphere and elsewhere, “Earth songs”, such as from lightning, tweeks, whistlers, the auroral chorus, hisses and other sounds, although they won’t happen all the time so patience will be required. One way to listen to to these signals is to connect via the internet to a number of VLF receivers that are on-line at all times. One site with links to a number of these receivers is at http://abelian. org/vlf/ The received signal at Florida seems particularly active. Many people find these sounds quite relaxing to listen to and they may even help you go to sleep! A selection of WR-3 receivers (row at bottom) being field tested at a radio quiet location in California before shipping to customers. The author owns one of these units. Live streaming of a VLF signal can also be heard at www.vlf.it/ cumiana/livedata.html along with a number of regularly updated spectrograms representing the outputs of different VLF station setups. Another method to listen to Earth songs is to purchase a broadband VLF receiver such as the fully assembled WR-3 receiver from www.auroralchorus.com/wr3order.htm These are made as a cottage industry by pioneering hobbyist Stephen P. McGreevy rather than a commercial organisation. Examples of auroral recordings made by Stephen McGreevy can be heard at links on www.auroralchorus.com/aucho.htm The device sells for US$170, including shipping to Australia. The WR-3 is also available without a housing or antenna and costs US$85 shipped to Australia. The user needs to supply appropriate standard headphones in both cases. Stephen McGreevy also has free plans for his BBB-4 “bare bones basic” receiver at www.auroralchorus.com/bbb4rx3.htm A different VLF receiver is available in kit form, called the Inspire VLF-3. This is part of the NASA-inspired educational project for The circuit of Stephen McGreevy’s BBB-4 VLF receiver. 22  Silicon Chip natural VLF signals. An Inspire VLF-3 kit costs US$155, including shipping to Australia. The order page is at http://theinspireproject.org/default.asp?contentID=27 Note that unlike the WR-3 above, this is a kit and does require assembly. The theory of operation is described at http://theinspireproject.org/downloads/pdf/inspire%20Theory_of_Operations. pdf Assembly instructions are at http://theinspireproject.org/ downloads/pdf/VLF_Instructions_Apr2011.pdf It is suggested that you look at those first before deciding if you are comfortable building this kit. A simple telescopic antenna is also needed, or a 2m length of wire. If you want to try to build your own device to receive broadband VLF signals, SILICON CHIP described a simple VLF preamplifier, designed to plug in to your PC sound card input, in the April 2011 issue. The circuit is shown below (this was part of our feature on detecting earthquakes via VLF radio but is just as valid for listening to VLF from space!). (See www.siliconchip.com.au/Issue/2011/April/Can+Earthquakes+Be+Predicted+By+VLF+Radio+Signals%3F). +9V SUPPLY LONG WIRE OR VERTICAL WHIP ANTENNA (Id = 4mA) 1k 10k 100nF G K A A D S ZD1 3.3V 100nF Q1 MPF102 SHIELDED CABLE TO PC SOUND CARD MIC INPUT (PINK) 3.5mm STEREO PLUG MPF102 10M 220 ZD2 3.3V S G D K SC  2011 VLF PREAMP ZD1, ZD2 A K A method for listening to Schumann Resonances is described at www.vlf.it/poggi1/schumann.html Also see www.backyardastronomy.net/schumann_resonance.html Spectrum Lab (www.qsl.net/dl4yhf/spectra1.html) is a popular free spectrum analyser software program with which natural sounds from the ionosphere can be analysed (as well as it having many other uses). Using this program along with a PC sound card and an active antenna as a front end is described at www.qsl. net/dl4yhf/speclab/natradio.htm The author of that article also notes that even a long wire can be connected to a sound card as a temporary antenna but surge protection is very important or your computer could be destroyed. Note that receiving the Earth’s natural radio signals requires broadband receivers (as opposed to narrow band receivers that are required for receiving many man-made signals). Such receivers have to be used as far away as possible from power lines and so A WR-3 receiver it is best not to use them in cities as the signal will be dominated without housing by 50Hz or 60Hz hum from power lines (frequency depends on or antenna – a what country you are in). A minimum distance from power lines cheaper way to of 500m is suggested but possibly far more is needed for high purchase this voltage lines. unit but you will VLF falls within the audio frequencies and the receivers are eshave to provide sentially low noise, high gain amplifiers that have their output fed the housing directly into an audio amplifier. No RF conversion is necessary and antenna as in a normal radio receiver. The antenna can be a simple long yourself. length of wire or magnetic loop antenna. siliconchip.com.au Schumann resonance around the earth between the surface and ionosphere (not to scale) showing the fundamental mode (a single standing mode wave around the entire planet) and the second and third order harmonics. Image credit: Neotesla at japansk. The fundamental resonance corresponds to a wavelength of around 38,300km (using the speed of light in a vacuum) which is approximately the circumference of the Earth, explaining why this cavity resonates at that frequency, with a single standing wave around the entire planet. The wavelength is a little less than the actual circumference of the Earth (around 40,075km at the equator) due to considerations of spherical geometry and due to other effects such as the slight conductivity of air due to the presence of ions. The frequencies vary a little with the changing state of the ionosphere due to day and night variations and the impact of solar storms on the ionosphere. When a lightning strike occurs, the discharge acts as a huge transmitting antenna and emits electromagnetic radiation over a range of frequencies. This energy is discharged into the wave-guide where components of that energy are reinforced at the resonant frequencies. There is a constant supply of this energy from lightning as there are, at any given time, 40-50 lightning strikes per second worldwide. The measurement of Schumann resonances has various applications in lightning detection and analysis, monitoring of the ionosphere and space weather and monitoring of climate via global thunderstorm activity. (The phenomenon of Schumann resonances has been hijacked by “alternative” medicine practitioners and New Age people who attribute all manner of healing properties to these electromagnetic waves which simply cannot interact with the human body because of their extraordinarily long wavelengths. Unfortunately, a vast majority of on-line material to do with Schumann resonance is of a non-scientific nature). Natural VLF radio signals from lightning and elsewhere Lightning discharges generate a rich variety of electromagnetic radiation as a broadband pulse, including light, radio waves, X-rays and gamma rays, although much of the energy of the emissions from lightning is in the VLF frequency range (3kHz-30kHz) with some in the ELF range (3Hz-3kHz, the atmospheric science definition of ELF being different to the conventional ITU one). VLF and ELF waves can propagate over long distances by travelling in the natural ELF and VLF waveguide formed between the Earth and ionosphere. In addition to propagating beneath the ionosphere, some ELF and VLF siliconchip.com.au signals can exit the ionosphere where they will follow the magnetic field lines of the magnetosphere. They can reach 10,000km or more above the Earth before re-entering at a different location. There are several different types of emissions possible, which are characterised as static, tweeks, whistlers, the chorus and hiss. Note that tweeks, whistlers and the chorus start out as static but by the time they are received they have changed because of the complex path they have taken to the receiver. Lightning strike “static” Lightning strike static, sometimes incorrectly called sferics (derived from “atmospherics”) are the signals from Spectrogram of static lightning that most people signals. Audio of this signal will be familiar with, as they can be heard at www. are the same sounds as re- spaceweather.com/audio/ ceived on an AM broadcast inspire/1lowdenssfer.mp3 band radio during an electrical storm. They sound like constant crackling and popping, somewhat like the noises made when someone eats potato chips. The static signals are from lightning strikes within about 1,000km. They are characterised on a spectrogram (frequency on vertical axis, time on horizontal axis) as vertical lines indicating that all frequency components in the signal arrived at the same time. NOTE: The term sferic should probably apply to all electromagnetic signals that come from lightning, not just the “static” described here; however this static is often incorrectly called a sferic. While that is true, there are other types of sferics as well. Tweeks Tweeks are lightning radio emissions that have travelled from around 2,000km or more distant, within the wave-guide formed between Spectrogram of tweek the Earth and the ionosphere. signals. Audio of this signal The ionosphere varies in its can be heard at www. properties through its thick- spaceweather.com/audio/ ness so some frequency com- inspire/3tweeks.mp3 ponents will travel faster than others. This is akin to how light travels slower in water or glass than in free space and explains how the colours of a rainbow are generated with a glass prism – some frequencies emerge sooner than others. Tweeks have a vertical line at high frequencies but if you follow the line down you will see it curves off to the right, indicating that the lower frequencies have been delayed in their arrival compared with higher frequencies. This results in a somewhat musical quality to the signal. Spectrogram of whistler signals. Audio of this signal Like tweeks, whistlers can be heard at www. have a musical quality due spaceweather.com/audio/ to the propagation of dif- inspire/7purewhist.mp3 Whistlers May 2016  23 ferent frequency components of the signal at different velocities leading to different frequency components of the signal becoming offset in time. It is the interaction of the signal with the plasma environments of the ionosphere and magnetosphere that cause the time delay for the different frequency components. Whistler signals travel along the magnetic field lines of the Earth and can go to the opposite side of Earth and return. Whistlers were discovered in 1886 when a previously unknown noise was heard on phone lines. Chorus Two types of “choruses” can be occasionally be listened to on VLF radio, the dawn chorus and the auroral chorus. As the name implies, the dawn chorus is best listened to at dawn and can resemble the dawn chorus of birds, although it has also been described as having a sound like dogs barking or squawks from flocks of birds. It consists of a wide variety of overlapping sounds. Its presence is dependent upon geomagnetic activity such as the emission of a solar flare from the Sun. The auroral chorus is generated within the aurora and can be heard in areas close to where the aurora occurs and of course is also strongest during geomagnetic activity. A recording of a “VLF auroral chorus” can be heard at https://youtu.be/ FQdrcDyYRiQ Hiss Hissing sounds are typically emitted via the aurora and are high-pitched when converted to audio. Hiss can also originate in the magnetosphere, including the bow shock region where the incoming solar wind impacts with the magnetosphere. Above thunderstorm electrical phenomena There is currently intense interest in the relatively recently-discovered electrical phenomena that occur above thunderstorms, usually at altitudes of 50 to 100km. These electrical discharges are triggered by conventional lightning and have a variety of types that have come to be known as sprites, elves, blue jets, halos, trolls and several other types. Collectively they are known as transient luminous events. 1973 over the South China Sea but when he reported it he was not believed as “everyone knew lightning went down not up”. Other sightings were later made and reported by other pilots. There is also an anecdotal report from 1730 by Johann Georg Estor of optical activity above a thunderstorm which is interpreted as a sighting of a sprite. It was not until 1989 that Jack Winkler of the University of Minnesota recorded such a discharge by accident when he was looking for something else. The phenomenon became known as a sprite after the elusive mythical creature with magical powers. Only very powerful lightning causes them and they are much weaker electrically and of shorter duration than conventional lightning. The first images of sprites from space were taken incidentally as part of thunderstorm videos taken from the Space shuttle during the period 1989-91. Israeli astronaut Ilan Ramon was on the ill-fated Columbia space shuttle crash in 2003 and specifically sought to capture sprite images from space. He managed to transmit several images back to Earth. Remarkably, his camera was found among the crash debris and more images were recovered. Japanese astronaut Satoshi Furukawa was moved to continue Ramon’s legacy and attempted to capture more sprite images from the International Space Station in 2011. He captured images of six sprites and Ramon’s work now continues. Only one in 10,000 lightning events leads to a sprite. They are believed to be caused when the intense electrical field created by stronger positive lightning bolts causes a sympathetic electrical breakdown of the upper atmosphere. Sprites can reach all the way to the top of the ionosphere but start at around 50km altitude and last around 17 milliseconds. A halo occasionally precedes a sprite (see diagram) and lasts around one millisecond. Elves Discovered in 1994, ELVES is an acronym standing for Emissions of Light and Very low frequency perturbations due to Electromagnetic pulse Sources. Triggered by lightning, they are a flat, disc-like discharge around 400km in diameter and last for about a millisecond. Sprites Jets An electrical discharge above a thunderstorm was first observed by a US Air Force pilot named Ronald Williams in A variety of “jet” phenomena have been observed such as blue jets and gigantic jets. They were discovered in 1994. An elve over the South Pacific and two sprites over Australia, captured by astronaut Ramon in 2003 from the Columbia which sadly later crashed killing all seven crew. A montage of a variety of forms of sprites as seen from jet aircraft on specialised missions to photograph sprites. Image source: NHK. 24  Silicon Chip siliconchip.com.au Lightning detection and tracking Every time there is a lightning strike, an enormous amount of RF energy is produced. The frequency ranges from the proverbial “DC to Daylight” but the low-frequency energy is attenuated less than high frequencies, therefore is easier to detect. You’ll hear this yourself as crackles and crashes on any AM radio station that’s not tuned to a local station – in fact, even a strong, local station can be all but blanked out by a local thunderstorm with lots of lightning. Having said that, the interference from a thunderstorm can be many hundreds of kilometres away if the lightning strikes are big enough! SILICON CHIP published a build-it-yourself hand-held lightning detector in the July 2011 issue which relied on detecting this RF energy (siliconchip.com.au/Issue/2011/July/ A+Portable+Lightning+Detector). All parts are readily available and the PCB can be obtained from the SILICON CHIP on-line shop. Looking for that burst of “static” is exactly the same process which commercial lightning tracking services use, although they use multiple detectors around the world and measure the exact time the lightning strike is detected. By triangulation, they can pinpoint the spot where the lightning struck to within a few hundred metres. We published an article in the November 1996 describing the (then) new LPATS lightning detection system, which used this exact approach. We understand that the company behind this system has now been incorporated into the Weatherzone group (www.weatherzone.com.au) but the theory of operation is much the same. Weatherzone is a commercial operation (although it has a lot of free weather data, including radar and lightning). On the other hand, Blitzortung.org is a community of volunteer lighting detector station operators, software developers and other system support personnel who run a website which plots lightning strikes around the world in real time. A related site is LightningMaps.org who take the data from Blitzortung.org and visualise it in various ways. Some links for some third party Apps for mobile devices can be seen at www. System Blue lightning detector kit for use with the Blitzortung.org project. This hardware is not intended for stand-alone use; it is designed to be connected to the Blitzortung.org servers via the Internet. Detailed information on this model is not currently available on the web site but extensive documentation on the previous System Red model is available for perusal at www.blitzortung.org/Documents/TOA_Blitzortung_RED.pdf siliconchip.com.au Lightning strikes during a storm in south east Australia on 29 January 2016. The lighter colours are the most recent strikes (less than 20 minutes old) and the darker colours represent strikes that occurred in the past (100 to 120 minutes ago). Map from www.blitzortung.org/en/ page_0/index.php You do not need to own the Blitzortung hardware to view a lightning strike map. lightningmaps.org/apps; a real-time map can also be displayed in a browser on a mobile device without any app, www.lightningmaps.org/realtime Currently Blitzortung.org has 500 lightning detector stations around the world which are connected via the Internet and there are also numerous servers to process the received data. Each receiver records the arrival of a lightning strike with microsecond precision and the connected VLF receivers locate the position of the strike based on algorithms which use time of arrival and time of group arrival methods. Anyone can join Blitzortung.org and set up their own lightning detector station which must be made from the kit they supply. The kit of hardware, which must be assembled, can be purchased for under 300 Euros. “System Blue” is the current hardware model of the detector. Here are some links to some other DIY lightning detector circuits: www.lissproductions.org/wuhu_manual/2011/11/29/diylightning-detectors/ and www.techlib.com/electronics/lightning.html This circuit is said to be very easy to build (see picture): www.techlib.com/electronics/lightningnew.htm Another DIY lightning detector. See a video of operation at www.techlib.com/files/ld1.avi May 2016  25 Various above-thunderstorm electrical phenomena. As a rough guide of horizontal scale, this diagram could represent around 150km or even more in extent. From http://la.climatologie.free.fr/orage/tle-english.jpg These start at the cloud tops and rise to about 40 to 50km altitude. Jets seem to be associated with hail activity rather than lightning. You can see one over Darwin, NT in one of the videos referenced below. Others Other transient luminous event phenomena include trolls, gnomes and pixies. Videos of transient luminous events The video at http://esamultimedia.esa.int/images/ ISS/2005-09-27_Lighting_story/ISS-lyn.avi is an animation of a variety of transient luminous events. “Rare Lightning: Giant Red Sprites, Elves and Sprite Halo’s Real Time Video” https://youtu.be/D7mqs6fng7o “Extreme Rate Lightning Phenomena - Sprite Documentary - World History” https://youtu.be/Vzz0QoCkAlU “Lightning Sprites phenomena” https://youtu.be/FGMU89tcqeA “Rare Lightning: Giant Red Sprites, Elves and Sprite Halo’s Real Time Video” https://youtu.be/D7mqs6fng7o “Blue Jet (Transient Luminous Event) - Thunderstorm Darwin 8th Dec 2015” https://youtu.be/zLYPKuoxH1c “Upper Atmospheric Lightning: Gigantic Jet” https:// youtu.be/gk9ju2WUY5Q Conclusion There is a wide variety of interesting electrical phenomena in the Earth’s environment and these can be responsi26  Silicon Chip ble for both spectacular visual displays and can also affect radio listening. Among these are the electric field of the Earth, lightning, the aurora and VLF radio. The amateur can make many interesting observations themselves with simple equipment. NOTES: SILICON CHIP has no commercial association with the manufacturers of any of the devices mentioned in this article. Also, take all appropriate safety precautions when undertaking any possible observations described herein. Circuits referenced that were not designed by us have not been tested by SILICON CHIP. Auroral photography from central Victoria taken at Little Desert National Park by Mark Sansom. It was taken with a Sigma 17-70 lens set to 17mm, f/2.8 and ISO3200 with a 30 second exposure on 13th October 2012. siliconchip.com.au Viewing the Aurora Australis from southern Australia Kp lines for a part of the Southern Hemisphere showing the likelihood of visibility of the aurora for a certain Kp index. Note that the Kp is just an indicator, and just because a certain Kp index is achieved it does not mean that the aurora will be visible. Conversely, the aurora may be visible at a Kp of less than what appears on the map for a certain location. While either of the polar auroras are best viewed close to the not indicate that the aurora will definitely be visible. Rather, Kp inpoles, it is still possible to view them from southern Australia dicates that the aurora is likely to be visible in a given region for a under certain circumstances with a camera and the Aurora Auscertain index based on past experience. Zero represents little or no tralis can sometimes even be viewed with the naked eye. The auroral activity while nine represents a major geomagnetic storm. viewing can be surprisingly good, even if not as good as if you One website written by a Melbourne-based person to assist were further south (eg, Antartica!). mainly Southern Hemisphere auroral viewers is https://auroIt is important to choose a night time viewing location which ras.live It consolidates the main numbers of interest into a user is far from light pollution sources, such as cities and towns and friendly format and the web site is also mobile friendly so can be with a clear view of the southern horizon (make sure you really used on a smart phone in the field. are looking south – check with a compass or smart phone App). There are many other websites and Apps covering data related For personal safety it is best to have someone with you and also to the aurora and “space weather” in general. One App which you tell others where you are going and when you are expected back. might want to look at is “Aurora Forecast” which is free for iOS The best time for viewing can be determined by a number of and Android but there are many others. Just search the App Store smart phone Apps and web resources. There are certain “space or Google Play with the term “aurora”. weather” numbers to look for that should give the best viewSome popular websites for forecasting are www.spaceweather. ing. Numbers to be considered are as follows: com/, www.aurora-service.eu/, www.auroBz, the strength of the interplanetary magra-service.net (Southern Hemisphere) and netic field in the direction of the rotational axis www.sws.bom.gov.au/ (Australia’s Bureau of the Earth. It is created by waves and other of Meterology). Maps of predicted aurora disturbances in the solar wind when it interacts locations for both hemispheres can be seen with the Earth’s magnetosphere. The more negat www.swpc.noaa.gov/products/auroraative the number the better the aurora will be. 30-minute-forecast Speed and Density of the solar wind. In Some videos of auroras from southern both cases the higher the number the better, Australia and New Zealand are as follows: as a greater speed and density of the solar “Aurora Australis (G4) on 17-18 March wind will see a stronger interaction with the 2015 at Geelong, VIC” https://youtu.be/ magnetosphere. esAUdlIUnFM, “Aurora Australis (Southern Kp is the planetary index and is represented Lights) Point Addis, AUSTRALIA 21 Dec by a scale from 0 to 9 and is a measure of the Predicted location of auroras 2015 ” https://youtu.be/_4PGJwqC_90, level of geomagnetic activity. The higher the updated every 30 minutes at “Aurora Australis : Camera VS Naked Eyes” Kp index the further from the poles the aurora www.swpc.noaa.gov/products/ https://youtu.be/oq6GO-i7t4Y SC should, in principle, be visible however it does aurora-30-minute-forecast siliconchip.com.au May 2016  27