EARTH’S ELECTRICAL HEARTBEAT
Earth is struck by 1.4 billion lightning bolts annually, but just as many seem to fly up from the clouds into the ionosphere. The result is a pulsing electrical heartbeat for the whole planet.
Earth is struck by 1.4 billion lightning bolts annually, and there is just as much activity above the clouds, where mysterious ultra-high-energy lightning holds the atmosphere together in a global electrical circuit that is getting out of hand.
16.7 seconds is the longest known single lightning strike, over Argentina in 2019.
Slowly count to three. Have you finished? In the course of those three seconds, around 150 lightning bolts will have struck Earth somewhere on its surface. Every year, 1.4 billion lightning bolts strike Earth’s surface. But data gathered from satellites and from the International Space Station (ISS) show that a similar number of mysterious lightning bolts are shooting high up into the atmosphere. These observations confirm a 100-year-old theory by Scottish physicist Charles Wilson, according to which opposed directions of lightning hold the world together in a huge electrical circuit. It’s like a global pulse, constantly charging and discharging the atmosphere – and it’s a process which might see the global climate of the future spin out of control.
Energetic space particles
Traditional lightning originates when water rises as clouds to heights where it freezes, then as hail and ice crystals descend into more rising water, electrons are stripped away. The result is a powerful positive charge at the top of the cloud, while lower down it becomes highly negatively charged. The negative charge attracts positive charges on Earth’s surface below the cloud, and the voltage difference begs to be neutralised.
Finally, in a split second, some 200,000 amps of current can zigzag towards the ground. The air is heated to 30,000°C and is converted into red-hot plasma in a huge spark – what we know as lightning.
However, the process is not as clear as is usually presented. In 2002, the US space agency NASA sent a plane directly through a thundercloud to measure the electric voltage inside the clouds. And it turned out that the voltage difference in thunderclouds is simply too low to trigger lightning.
When you walk across a carpet, the friction between your feet and the carpet peels electrons off the carpet, spreading through your body, making it negatively charged with ‘static electricity’. If your hand then approaches a metal door knob, an amazingly high voltage difference of three million volts per metre develops between your hand and the handle. A spark is produced; your hand gets an electric shock.
The voltage difference in thunderclouds is ‘only’ some 200,000 volts/metre – that’s 1/15th of the voltage between the hand and the handle. This has made Russian physicist Alexander Gurevich introduce a theory according to which lightning is assisted by high-energy protons from space.
The space protons also peel electrons from the air molecules, but they trigger an out-of-control chain reaction in the cloud. Eventually a tidal wave of electrons speeds down from the thundercloud through a lightning channel, neutralising the voltage in the form of a lightning strike.
Scientists’ theory about cosmic intervention is supported by data from satellites and the ISS, which have recorded gamma radiation from thunderclouds. Gamma radiation is one of the most high-energy types of radiation, and according to scientists it is emitted at the tip of a lightning channel, where electrons have been accelerated to high energies by the cosmic particles.
And lightning does not only travel between the clouds and the ground – it also shoots high up into the atmosphere to link the world in a global electrical circuit.
Lightning charges global circuit
In 1909, the wooden boat Carnegie travelled 480,000km, measuring the atmosphere’s electric charge. Every day scientists recorded a ‘pulse rate’ over the oceans, which became known as the Carnegie Curve. In 1920, the
709 km was the length of the longest lightning ever recorded between two clouds.
discovery led Scottish physicist Charles Wilson to introduce a theory according to which Earth’s surface and the bottom of the ionosphere at an altitude of 50-80km function as the negative and positive electrodes of a huge rechargeable battery. According to Wilson, the battery is charged by lightning from thunderstorms. Now American atmosphere researcher Michael Peterson and his colleagues have confirmed the theory using microwave data from satellites.
During a thunderstorm, lightning strikes supply the Earth’s surface with negative electrons, whereas rising lightning sends positive charges from the top of clouds up into the ionosphere. The reverse-directed lightning causes a voltage difference of 250,000 volts between the ionosphere and Earth’s surface, charging the battery.
In clear weather, the ionosphere will discharge. Although the air between the two poles is generally neutral, it includes a small quantity of charged atoms and molecules known as ions, sufficient to lead the positive charges from the ionosphere down towards Earth’s surface.
Recording lightning
The part of the atmospheric battery about which we know most is traditional lightning, passing from the clouds to Earth’s surface.
Today, lightning strikes are recorded by a global network of measuring stations known as the Global Lightning Dataset. Lightning discharges emit low-frequency radio noise which can be recorded by the measuring stations’ sensors at a distance of up to 10,000km. These sensors detect lightning strikes in real time, and they are hooked up with the global GPS system, so the measuring stations can indicate the location of lightning strikes to within 2-3km. The network is more concentrated within the US, and lightning locations there can be determined within a radius of 200 metres.
In order to gain more knowledge about lightning, NASA launched the GOES-16 satellite in 2018, which monitors most of North and South America, able to detect lightning hundreds of kilometres long travelling between clouds. While the Earth-based network of measuring stations indicates accurate positions of lightning strikes, the satellite provides scientists with a more general impression of the extent and motions of thunderstorms.
The combination of these two types of data has improved our ability to predict where a thunderstorm is heading. American meteorologists are now able to issue more accurate lightning warnings to those it can adversely affect, such as power companies and airports.
The data can also be used to improve wildfire warnings. Studies have shown that the fires are lit in particular by lightning that does not fully neutralise the voltage difference between the cloud and Earth’s surface in one single fast flash. This ‘slow’ lightning can be 10-100 times longer, with weaker electrical currents discharged through the soil.
The GOES-16 satellite can spot this type of lightning, and combine data from the satellite with the more accurate local indications in order to provide firefighting services with a better chance of reaching and fighting fires before they become severe.
Lightning threatens the climate
Unfortunately it looks as if firefighters will be kept busy. As global warming intensifies, more water will evaporate from the oceans, increasing the quantity of water in the atmosphere, so causing more thunderstorms and more lightning strikes. A prognosis for the US indicates that the number of lightning bolts will increase by 12% for every degree by which global temperatures rise. A 2016 report by Australia’s Climate Council predicted the frequency of potential severe thunderstorm days is likely to rise by 30% for Sydney, 22% for Melbourne and 14% for Brisbane by the end of the century.
The same is true in the Arctic, where the cold climate otherwise makes lightning rare. But global temperature rises have already had a huge impact north of the Arctic Circle, where temperatures have risen by 2-4 degrees in some regions since 1960. According to the World Wide Lightning Location Network, the number of annual lightning strikes north of the Arctic Circle has risen from 35,000 in 2010 to 250,000 in 2020.
Lightning strikes in the tundra are now igniting increasing numbers of wildfires in Arctic peat bogs. It’s a worrying development, given that the tundra hosts 14% of all carbon stored in the ground globally. Burning peat bogs could emit vast quantities of CO2 to the atmosphere. Even without that, more lightning hastens global warming because the intense electric charges produce nitrogen oxides, NOx, powerful greenhouse gases as produced by diesel exhausts.
According to calculations, the lightning strikes create 8.6 millon tonnes of NOx a year, with a similar quantity from lightning travelling between clouds. The World Meteorological Organization has recently added lightning to the list of climate variables to be continuously monitored to determine how they will affect the climate of the future.
Mapping upward lightning
Whether the more mysterious rising lightning from clouds to the ionosphere emits NOx is still unknown. Red sprites and blue jets (see opposite) cannot be observed from Earth’s surface, and so they are the least explored and understood part of the global electrical circuit.
But scientists have gained new insights since ASIM (the Atmosphere-Space Interactions Monitor) arrived at the International Space Station in 2018. The instrument, which was developed by DTU in Denmark, observes space above the top of the clouds in an array of wavelengths spanning from visible light to X-rays and gamma radiation. ASIM collects 100,000 measurements per second, allowing detailed observations of the ultra-brief rising lightning bolts. Red sprites speed to an altitude of 90km in 100 milliseconds. Blue jets complete 40-50km in 400 milliseconds.
In 2021, scientists published observations of a huge blue jet from a thunderstorm above the Pacific island of Nauru. The lightning began with five explosion-like blue flashes at the top of the cloud, which only lasted 10 microseconds before one of the flashes lit the powerful blue jet that reached an altitude of 52km.
ASIM’s observations in the years to come will provide more detailed data about the mechanisms behind both blue jets and red sprites, and will teach us more about how these charge the positive electrode in the sky, powering Earth’s electric heartbeat.
300 people were killed in 1807 when lightning struck a gunpowder factory in Luxembourg.