How satellites help explain earth’s magnetic behaviour

June 5th, 2019, Published in Articles: PositionIT

The earth’s protective magnetic field is always restless, but every now and then something weird happens – it jerks. Although scientists have known about these rapid shifts for about 40 years, the reason why they occur has remained a frustrating mystery, until now.

Since geomagnetic jerks were discovered in 1978, scientists have been trying to work out why the magnetic field suddenly and unexpectedly accelerates.

Looking back at measurement records from the worldwide network of ground-based magnetic observatories, they found that that these jerks, which appear as sharp V-shaped features in graphs of magnetic-field changes, date back as far as 1901, and that the phenomenon occurs about every three to twelve years. Also, they are not consistent across the globe. In 1949, for example, a jerk was measured in North America, but was not detected in Europe.

Since they occur relatively randomly and the mechanism that drives them has been poorly understood, these jerks have frustrated attempts to forecast changes in the magnetic field, even for a few years ahead.

Fig. 1: Tracking geomagnetic jerks.

Forecasts are important because the magnetic field protects us from solar storms, which have the potential to disrupt power supplies, communication links and navigation systems, for example.

Bearing in mind that ground-based magnetic observatories are built on land, information about these jerks has been incomplete as the ocean, of course, covers 70% of the earth’s surface. But with the European Space Agency’s (ESA) trio of Swarm satellites, which measure variations in the earth’s magnetic field from space, scientists can now study the global structure of geomagnetic jerks.

In a paper published recently in Nature Geoscience, scientists from the Paris Institute of Earth Physics and the Technical University of Denmark describe how they created a computer model for geomagnetic jerks, and offer an explanation as to why they happen.

Fig. 2: A simulation of the magnetic field in earth’s core.

The earth’s magnetic field is generated mainly by the churning of fluid within its core. Researchers know of two types of movement that cause different variations in the magnetic field: those resulting from slow convection movement, which can be measured on the scale of a century, and those resulting from rapid hydromagnetic waves, which can be detected over a few years.

They suspected that the latter type play a role in the jerks, but the interaction of these fast waves with slow convection, along with their mechanism of propagation and amplification, had yet to be revealed.

Now, the researchers have been able to document the series of events that lead to jerks which, in the simulation, arise from hydromagnetic waves emitted within the core. As molten matter rises up to reach the outer surface of the earth’s core, it produces powerful waves along the magnetic field lines near the core. The team explained that this results in sharp changes in the flow of liquid beneath the magnetic field.

The jerks originate in rising blobs of metal that form in the planet’s core 25 years before the corresponding jerk takes place. These current findings are part of a longer-term project in which scientists hope to predict the evolution of the geomagnetic field over the coming decades.

The Swarm satellites allowed the researchers to make detailed comparisons, in both space and time, with physical theories on the origin of these magnetic jerks. While the findings make fascinating science, there are some real-world benefits of understanding how our magnetic field changes. Many modern electronic devices such as smart phones, rely on the knowledge of the magnetic field for orientation information. Being able to better forecast field changes will help with such systems.

Fig. 3: The Swarm constellation.

Swarm is the fifth Earth Explorer mission approved in ESA’s Living Planet Programme, and was successfully launched on 22 November 2013. As part of the Third Party Missions programme, the e-POP instrument of the Canadian Space Agency’s CASSIOPE mission joined the constellation in February 2018. The research objectives of the Swarm mission is to provide a survey of the geomagnetic field and its temporal evolution as well as the electric field in the atmosphere using a constellation of three identical satellites carrying sophisticated magnetometers and electric field instruments.

All the three satellite are equipped with the following set of identical instruments: Absolute scalar magnetometer, vector field magnetometer, star tracker, electric field instrument, GPS receiver, laser retro-reflector and accelerometer.

Creating a 3D earth reference model

The satellites, along with ESA’s GOCE gravity mission, are also playing a critical role in the solid earth system research, which study the earth’s crust, mantle and core.

A thorough understanding of the solid earth’ system is essential for deciphering the links between processes occurring deep inside earth and those occurring nearer the surface that lead to seismic activity such as earthquakes and volcanic eruptions, the rise of mountains and the location of underground natural resources.

Because these parts of the world are completely hidden from view, understanding what is going on deep below the surface can only be done by using indirect measurements.

Fig. 4: Density variations in the crust and upper mantle.

New results, based on a paper published recently in Geophysical Journal International and presented at the Living Planet Symposium in May 2019, show how scientists are using a range of different measurements including satellite data along with seismological models to start producing a global 3D Earth reference model.

The model will improve analyses of earth’s lithosphere (the rigid outer shell), and the underlying mantle to understand the link between earth’s structure and the dynamic processes within. While this is just a first step, 3D Earth offers insights into the deep structure of our world. For example, the new models of the thickness of the crust and the lithosphere are important for unexplored continents like Antarctica.

More work on the model is still underway, but the researchers plan to release the 3D Earth models in 2020.

Tracking magnetic north

One of the many other areas of research using information from Swarm focuses on explaining the increased pace of the magnetic north pole shift. Unlike the geographic North Pole, which is in a fixed location, magnetic north wanders. This has been known since it was first measured in 1831, and subsequently mapped drifting slowly from the Canadian Arctic towards Siberia.

Between 1990 and 2005 magnetic north accelerated from its historic speed of 0 to 15 km a year, to its present speed of 50 to 60 km a year. In late October 2017, it crossed the international date line, passing within 390 km of the geographic pole, and is now heading south. Scientists believe that this sprint is being caused by tussling magnetic blobs deep below our feet.

Fig. 5: Magnetic north on the move.

This has practical implications. One of the practical consequences of this is that the World Magnetic Model has to be updated periodically with the pole’s current location. The model is vital for many navigation systems used by ships, Google maps and smartphones, for example. In fact, recently, the World Magnetic Model had to be updated urgently because of the speed at which the pole is moving.

Scientists are also using ESA’s Swarm mission data to measure and untangle the different magnetic fields that stem from earth’s core, mantle, crust, oceans, ionosphere and magnetosphere. The earth’s magnetic field exists because of an ocean of superheated, swirling liquid iron that makes up the outer core. Like a spinning conductor in a bicycle dynamo, this moving iron creates electrical currents, which in turn generate our continuously changing magnetic field.

Tracking changes in the magnetic field can, therefore, tell researchers how the iron in the core moves. Phil Livermore, from the University of Leeds in the UK, said, “several theories have been proposed to explain this behaviour but, since they rely upon changes in the small-scale magnetic field, they cannot explain the recent trajectory of the pole.

“Using data collected over two decades by satellites, including ESA’s Swarm trio, we can see that the position of the north magnetic pole is determined largely by a balance, or tug-of-war, between two large lobes of negative magnetic flux at the boundary between earth’s core and mantle under Canada and Siberia.”

Research is showing that changes in the pattern of core flow between 1970 and 1999 elongated the Canadian lobe, significantly weakening its signature on earth’s surface, causing the pole to accelerate towards Siberia.

Simple models taking account of this process and describing future geomagnetic change predict that over the next decade the north magnetic pole will continue on its current trajectory and will travel a further 390 to 660 km towards Siberia.

Understanding earth’s intricacies

Understanding the intricacies of how Earth works as a system and the impact that human activity is having on natural processes are huge environmental challenges. Satellites are vital for taking the pulse of the planet, delivering the information needed to understand and monitor our world, and for making decisions to safeguard our future. Earth observation data is also key to a myriad of practical applications to improve everyday life and to boost economies.

Acknowledgement

This article is a compilation of three European Space Agency (ESA) articles, republished with ESA’s kind permission, from the Living Planet Symposium 2019. It is one of the world’s largest conferences on earth observation, and brings together thousands of scientists and data users to discuss the latest results and the future of earth observation. The original articles can be read here, here and here.

Contact ESA, www.esa.int

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