Friday, 11 January 2019

Updating the World Magnetic Model: From the centre of the Earth, straight to your pocket


What links the centre of the Earth, billions of smartphones, and BGS scientists? The answer is: the recently updated World Magnetic Model (WMM) 

Will Brown of the BGS Geomagnetism Team explains.

The WMM describes the primary component of the geomagnetic field, and is normally produced every five years. It also predicts the Earth’s field for the next five years. Sometimes however, the Earth’s core behaves in an unexpected manner, and so we’ve recently updated the current WMM2015 with a release of WMM2015v2.

At BGS we monitor and map the Earth’s magnetic field using a global network of surface observatories, including our own nine, and satellites in low-Earth orbit such as ESA’s Swarm mission. We use these measurements to build models of the magnetic field that allow us to interpolate between our measurements and estimate the strength and direction of the field at any location.

A model can be thought of like a map of the “topography” of Earth’s magnetic field, but what many people don’t realise is that Earth’s magnetic field isn’t a single fixed feature: it’s a combination of many effects and it changes through time. 

Map of magnetic variation through time from 1900 to 2015 – the magnetic poles are where the strong red and blue contours converge, and the north pole moves very quickly in recent years.

The magnetic poles drift, the field strengthens and weakens, and the immense magnetic field of the Sun, carried by the solar wind, constantly batters at it from the outside. The effect of all these changes vary depending on when and where you are on, under, or above the Earth’s surface. Our models dissect the field we measure into its different parts, or sources, and provide a map of each that varies in time, even predicting the future of some parts.

So where does your phone come in? The WMM is the standard magnetic model used for navigation by organisations such as NATO, the Ministry of Defence, and the US’ Department of Defense, and also by smartphone operating systems such as Android and iOS. When you open your smartphone’s map app, you may see an arrow pointing which way you’re facing, and there’s something quite clever going on underneath. Your phone contains a magnetometer that is measuring the Earth’s magnetic field. In order to make sense of this information a reference model like the WMM is needed to correct the measurements of magnetic north made by your phone to True North. You go through the same procedure if you use a map and compass when out hiking: set your compass for the map’s magnetic variation adjustment (we provide these too for Ordnance Survey maps!), and then convert your compass reading of magnetic North, to give a direction relative to the map’s grid North.


Magnetic variation in degrees – you really need to set your compass correctly when hiking!
 
The rate-of-change of magnetic variation in degrees per year – the changes are quickest near the north pole.

The WMM is a joint effort from BGS and the US’ NOAA NCEI, on behalf of the UK’s Defence Geographic Centre and the US’ National Geospatial-Intelligence Agency. The WMM is a model of the primary component of the geomagnetic field: that of Earth’s core. The core field, which gives us our familiar magnetic poles and allows us to use a compass, is generated by dynamo action in the swirling iron-rich fluid of the outer core, roughly 3,500 km below out feet. The ever-changing flow of the outer core leads to an ever-changing magnetic field. This is a complex process that we don’t fully understand the physics of yet, and so we have to update our model regularly.

Since late 2014 the core field has varied in an unpredicted, and currently unpredictable, manner. This led to the WMM becoming less accurate, particularly at high northern latitudes, much faster than normal, and so we released an update ahead of the next regularly scheduled WMM release in late 2019. We can map the field changes that have occurred since 2015, and show that they seem to be related to two phenomena, an abrupt unpredictable change called a “geomagnetic jerk” in 2014/2015, and an acceleration of flow in the core in the northern hemisphere. This update to the WMM will be used until the next release in December 2019, when we’ll make our best estimate of the likely change in the core field until 2025.
The change in the vertical component of magnetic field at the core-mantle boundary between 2015 and 2018 – the three intense patches in the northern hemisphere are related to changes like the Livermore et al (2017) “core jet” model 

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