Aurora: Magnetic technology from the heavens
Image credit: Gisli Jonsson, Will von Dauster, Felicity Aston
Once in danger of being consigned as a scientific curiosity chased by tourists, aurora is attracting renewed attention as the visible interface between Earth and the magnetic world. The latest research into this natural phenomenon is producing some surprising technology applications.
Even though I am well versed in the science of aurora, facts are hard to recall as I stand in the dark gazing upward at a pulsating display of green and purple. In that moment it is far easier to believe any of the many myths from folklore and superstition about the origin of the manifestation, rather than the science. The lights really do appear to be magical.
The first historical record of mankind’s fascination with the aurora appears some 20,000 years ago in primitive cave paintings of the Cro-Magnon people, but it is only in the last 150 years that a scientific understanding has evolved. We now know the phenomenon is caused by highly charged particles from the magnetosphere, exciting gases in the upper atmosphere, causing them to emit energy as light. The colour of the light is dependent on the type of gas and altitude. For example, excited nitrogen gas at an altitude of 200km will produce blue light, but below 100km, the same gas atoms will glow pink.
The fundamental mechanisms of the aurora are now well established, but there is still much we don’t know. Within the last decade Nasa’s THEMIS (Time History of Events and Macroscale Interactions during Substorms) project unravelled the process that causes aurora to brighten and move, as well as identifying for the first time the conversion of energy that powers the aurora. Other researchers are still striving to explain the formation of structures and patterns within individual events.
In fact, since the turn of the millennium, research into aurora has accelerated rather than declined. “Space weather, of which aurora forms one part, is one of the main risk factors for modern life on Earth,” says Thomas Ulich, vice director of the Sodankyla Geophysical Observatory in Finland, which has been monitoring and measuring aurora since 1914. “The reason we study near Earth space is the effect space weather has on the surface of the planet,” explains Ulich. “If you have very strong auroras, these affect the surface. We work to understand how processes like aurora connect near-Earth space and the ground.”
Aurora, once seen by many as a pretty scientific curiosity of little significance, is now recognised as the only visible portal into a geomagnetic world that will have a critical impact on our increasingly digital lives. Earth and the magnetosphere are not, it transpires, completely separate entities, but are intimately connected through complex interactions.
Professor Adam Schultz of the Institute for Energy Resources and Resilience at Oregon State University has been exploring one such interaction between aurora and the surface of Earth. His results have led to a detailed three dimensional (3D) view of the geological structure of Earth’s crust and a nascent method of earthquake prediction.
In 2015, Schultz and his team of geophysicists installed a ground network > < of temporary recording stations at 70m intervals across a west to east sweep of Alaska, centred around Poker Flat, just north of Fairbanks. Each recording station contained a precise vector magnetometer to measure direction and magnitude of the magnetic field, and a vector electrometer to simultaneously measure the electric field at Earth’s surface. The technique, known as Magnotetellurics, has been around since the 1950s, but this is the first time it had been used to survey in Alaska.
“We were measuring the voltage once every second for three months,” says Schultz. “The electric field measurements were in millivolts per kilometre and we were measuring oscillations that max out at 100 millivolts. The magnetic field measurements were even smaller. The intensity of Earth’s total magnetic field is 50,000 nanotesla. We are measuring a tiny fraction of a nanotesla.”
They may be tiny, but oscillations in the electric field measured by Schultz are caused by electric currents at Earth’s surface induced by electromagnetic activity in the ionosphere; the activity that we see as aurora. Aurora generates a strong magnetic field capable of inducing currents in the ground.
What made the study by Schultz particularly exciting is that it was able to combine the collected ground data with data collected by the Poker Flat Incoherent Scattering Radar (PFISR), an imaging system that looks upwards into the ionosphere. PFISR provided a detailed study of the aurora and the Geo-Magnetic Disturbances (GMD) in the ionosphere that drive the aurora, while the magnotetelluric recording stations simultaneously made detailed measurements of magnetic and electric fields at the surface.
“For the first time, we have knowledge of the source fields and we can see how the land responds below,” explains Schultz. “We see the source fields resulting from GMD and, at the same time, the geomagnetic currents induced at the surface.
“This is fundamental discovery. The data quality is wonderful.”
The study has already been put to some surprising uses. Schultz and his team used the magnotetelluric information they collected to create 3D images of the geological electrical conductivity of Earth’s crust and mantle from around 10km below the surface to depths of 350km. “We’re using electrical conductivity to image through hundreds of kilometres of solid rock,” clarifies Schultz. “It can tell you if there are any fluids and in what concentration. It can tell you basic issues about the structure and how the Earth’s crust might have evolved.”
An obvious application for this technology would seem to be the exploitation of natural resources, but Schultz points out that mining only concerns the first few kilometres below the surface. “It is much more likely to be used to assess geothermal potential or the risk from natural hazards,” he says. There are hopes that the technique could provide early warning of an earthquake, but Schultz is quick to underline its limitations. “Our aim is to refine the views of structure so that we can, on a statistical basis, talk about the probability of a particular area experiencing seismic activity.
“Monitoring electric and magnetic fields can give precursory information, but predicting an individual earthquake? No way.”
Geomagnetically Induced Currents (GICs) were first detected in the 1800s when telegraph lines across Canada and the northern United States were regularly disrupted by the appearance of aurora. At times, wires were completely unworkable during displays but at others, operators were able to continue sending messages even though no batteries were attached. The wires were instead powered by currents induced solely by the geomagnetic activity of aurora.
Any long conductor on the Earth’s surface like power lines, railway tracks or seafloor cables will be affected by GICs. This has caused a particular problem in oil and gas pipelines which can run for hundreds of kilometres across northern regions such as Alaska, Canada, Scandinavia and Siberia. These regions lie directly beneath a zone of intense auroral activity known as the auroral arc.
During aurora, GICs as high as 1,000 amperes have been detected in pipelines. Where these currents flow from the pipe into the earth, an electrochemical reaction between the pipeline steel and soil, water or moist air can cause corrosion. The affect is most pronounced where there are changes in the pipeline such as bends, branches, variation in pipe size or changes in surrounding soil conductivity. According to reports by French National Space Agency, CNES, pipelines designed to last 50 years can suffer 10 per cent wall erosion in just 15 years due to this problem.
The most common solution has been to maintain a negative charge on the pipeline to produce a weak counter current. A negative output of a DC power supply is applied to keep the pipeline potential with respect to the earth in the region of -0.85 to -1.35 volts. This technique is known as Cathodic Protection. However, although Cathodic Protection inhibits the electrochemical reactions that cause corrosion, it doesn’t provide an infallible defence.
Currents driven by aurora and geomagnetic storms are highly variable and create voltage swings that make it difficult to maintain a desired pipe-to-soil potential. Auroral currents frequently and rapidly change polarity which can, in some cases, render Cathodic Protection useless. Therefore, better monitoring of susceptible pipelines is seen as the best protection and this is only effective if accurate and reliable forecasts of the aurora and space weather that causes GICs can be provided.
The oil and gas industry is not the only business demanding better aurora forecasts. There is increasing demand from aviation too. More than 7,500 flights use trans-polar routes every year. These routes cut across the Arctic, traversing regions such as Greenland and Northern Canada. At such high latitudes, satellite communications become unreliable and aircraft instead rely on high-frequency (HF) radio for communication.
During aurora the ionosphere is altered by currents which cause atmospheric heating and an increase in energetic particles, which lead to a greater density of ionized gas. This affects the propagation of radio waves and can result in radio blackouts that last for days, which force aircraft to divert to lower latitudes where satellite communications can be used. Some airlines even warn pregnant women not to fly trans-polar routes due to risk of exposure to increased radiation during aurora and extreme space weather events.
It would be of clear benefit to the aviation and oil and gas industry, as well as utilities companies that operate power grids, to have advance warning of aurora. In answer to this need, America’s Space Weather Prediction Centre, part of the National Oceanic and Atmospheric Administration (NOAA), launched the Deep Space Climate Observatory (DSCOVR) in February 2015.
Suspended in a permanent position one million miles away, directly between Earth and the Sun, DSCOVR is the first operational satellite dedicated to space weather. It provides advanced solar measurements that enable early warning of space weather events that cause aurora and magnetic substorms. DSCOVR is capable of measuring solar wind speeds up to 1,300km per second and can measure magnetic fields with a strength of thousands of nanotesla, despite that even the most extreme storms are never more than 100 nanotesla. The high-quality data improves ability of forecasters to monitor and warn of severe space weather using a new computer model. The model took more than a year to develop and went operational in 2016.
“We would forecast the whole world with one number,” says Doug Biesecker, chief programme scientist for DSCOVR. “That was always inadequate because higher latitudes are more impacted than lower latitudes. The new Geospace Model allows us to create regional forecasts. This is the first time we’ve been able to provide something beyond a single number for the whole globe.”
DSCOVR provides a minimum of 12 minutes notice between an extreme event being detected by the satellite and the impact of that event reaching Earth. This is more than adequate considering that utilities industries say they only need five minutes warning to be able to react and prevent serious harm to infrastructure. The Geospace Model can be used to predict with high accuracy how strong a space weather event might be and when it will hit Earth, but Biesecker is keen to go further. “We are not at the stage yet where we can offer a tailored forecasting service and we’re still a few years away from something that predicts GICs,” he admits. What is missing is a regional geoelectric field model to estimate the geomagnetic field that is inducing specific currents at the surface. “We could predict the strength of GICs, but we need magnotetelluric surveys in order to tell what the impact might be,” says Biesecker.
Fortunately, teams like that led by Professor Schultz are aiming to provide exactly what is needed. They plan to replicate the magnotetelluric survey run in Alaska in other areas of the US, beginning with New England and the Great Plains. In Australia, the government is also replicating the study. Meanwhile Biesecker is already working on the next DSCOVR. “We need to become ever more aware of the hazards of space weather as our world evolves,” he concludes.