Podcast: Unravelling the intricacies of Earth's magnetic field

Diagram of Swarm mission

Diagram of the European Space Agency's Swarm mission, a constellation of three satellites that will improve our understanding of the Earth's magnetic field

7 January 2014 by Richard Hollingham

This week in the Planet Earth podcast, Ciaran Beggan and Tom Shanahan of NERC's British Geological Survey (BGS) explain why accurately measuring the Earth's magnetic field is so important.

To assist those who find text-based content more accessible than audio, a transcript of this recording is available below.

Richard Hollingham: I'm Richard Hollingham. Welcome to the Planet Earth podcast.

The launch in November last year - the European Space Agency's Swarm satellites. These three spacecraft designed to make the most accurate measurements ever of the earth's magnetic field. Not only are the structures for these unusual looking satellites made in the UK but several UK science teams are involved in analysing the data they produce and that's why I'm at the British Geological Survey in Edinburgh with Kieran Beggan. Kieran, you're working on the data from these satellites but before we talk about the satellites themselves and what they will be doing. Why is this so important and why do we need to know about the earth's magnetic field?

Kieran Beggan: If you don't have a magnetic field the atmosphere of the earth would be eroded away by the solar winder the billions of years since the planet was formed, so it's very important in the long term for life. It's also got plenty of economic applications so, for example, everyone is familiar with a compass and the way you can use a compass and a map to navigate. So that's one of the main reasons that we study the earth's magnetic field at the BGS.

Richard Hollingham: People still use compasses? Compasses are still important?

Kieran Beggan: Yeah, people think GPS is the bee all and end all of navigation and of course it is very important but there are places where you cannot use GPS, for example, underground and a lot of the oil wells drilled in the North Sea, for example, are guided by having a compass essentially on the end of the drill bit and they are trying to guide the wells towards smaller and smaller targets over the years so they need to know very accurately the magnetic field of the earth in the North Sea to guide the underground drill bit.

Richard Hollingham: Let's talk about the satellite then or the three satellites that are in orbit. You've got a model here which is rather nice. This one is just over a metre long. A curious looking spacecraft. It looks almost like an insect with a long arm on the end and the arm on the end that has the magnetometer, that's doing the measurements?

Kieran Beggan: That's correct. It's got this large body where the computer is and the batteries and things are located and coming out the back of it trailing behind as it flies along it's got a long arm and at the end there's what's called a scalar magnetometer and that measures the absolute strength of the magnetic field and then about half way back along the boom it's got this curious... I would call it a phallic, if that's allowed, looking bit where you've got some star cameras, these are cameras that look at the sky, and connected to that you've got the instrument that measures the direction of the magnetic field. The star cameras allow the orientation of the satellite in space to be determined and that gives us the direction of the field and then the absolute measurement gives us strength of field.

Richard Hollingham: And what will these be able to tell you that you don't already know?

Kieran Beggan: Well there are about 150 observatories on the earth where we measure the magnetic field and thousands of spots where we know it quite precisely every year but a lot of the earth, as you know, is ocean, 75% so we can't measure the magnetic field in these regions very accurately so the satellite will cover all those regions, the earth, like the oceans, like the poles where we can't really get to to make magnetic field measurements.

Richard Hollingham: With me here also is Tom Shanahan, one of the engineers to the British Geological Survey and he has set up here in the corridor one of the instruments you use. What is this and what are we looking at? It looks like a camera, tripod with a theodolite on top.

Tom Shanahan: That's pretty much correct, the difference is that the theodolite on top is specific for making magnetic measurements in the sense that it has had all magnetic material removed from it and the tripod itself is also totally non-magnetic, so you can see here it is made from wood and aluminium-

Richard Hollingham: Let's just tap that to prove that! So a wood tripod, aluminium top and this is just to make sure that the thing that you're measuring is the earth's magnetic field and not the magnetic field of the instrument you are measuring with?

Tom Shanahan: That's right. Anything that has got any ferrous content will contaminate our measurements so we go to great lengths to make sure that the measurements are clean and any of the instruments we use don't have any ferrous content.

Richard Hollingham: And that's true of the Swarm satellite as well isn't it that it is made of carbon fibre?

Tom Shanahan: That's true particularly of the sensor end and they will have to be very careful not to introduce any contamination like that.

Richard Hollingham: Back on the ground then you use this to do measurements across the country and this is a nearly continuous process in the UK.

Tom Shanahan: We usually visit about ten different sites across the UK every year to make these measurements but this instrument is actually also used at the observatories and we have to make manual measurements at the observatories to provide a correction to our continuous measurements because they can only measure the variations in the field. What this instrument will do is provide us with a calibration, effectively, to give us the full absolute level of the magnetic field at the observatories.

Richard Hollingham: And these observatories you've got three in the UK and they're making, what, continuous measurements of the earth's magnetic field?

Tom Shanahan: That's right. They make a measurement every second at the observatories in the UK and several overseas as well and this instrument would typically make a measurement with this once every two weeks to provide this calibration to correct the continuous measurements to an absolute level.

Richard Hollingham: Kieran, these measurements are going on across the UK. There are continuous measurements and also you're doing these surveys and that's because it's changing, the field is changing all the time.

Kieran Beggan: Yeah, that's correct. The magnetic field changes continuously and there are several different reasons for that. The main magnetic field which comes from the centre of the earth that changes very slowly over the course of decades to thousands of years and then you've got changes in the magnetic field that vary every day and they come from [unintelligible] for example. The atmosphere overhead is slightly magnetic so that changes and then you get what are called 'space weather' events and those are where the earth's magnetic field and the sun's magnetic field interact and those take place over the course of anything from several seconds up to maybe a couple of days.

Richard Hollingham: So you can't just take one measurement a year and change the maps, you have to take these measurements on a daily basis or even shorter than that?

Kieran Beggan: Yeah, we're measuring the map every one second, so we make the measurement at three places in the UK, one in Lerwick, one in the Scottish borders and one down in Devon and we make those measurements simultaneously once every second of the year and we have those records digitally going back to the 1980s and then before that it was recorded on paper and we had those records back to the 1840s.

Richard Hollingham: So you've got these measurements back to the 1840s and you're still doing those of what's going on on earth. You've now got the Swarm satellites in orbit and the challenge over the next few months is to bring those two together?

Kieran Beggan: That's correct. When you make a measurement on a satellite in space and there is what is known as an ambiguity so the problem is that the satellite moves at seven kilometres a second, so when you make a measurement it is quite difficult to say well what did I actually measure there, did I measure the magnetic field due to the earth or did I measure magnetic field due to some change because of 'space weather' so you need instruments on the ground to calibrate the satellite data so that you can try and understand what the satellites are actually measuring.

Richard Hollingham: What is the process going to be then over the next months? Getting the data down and saying is that right, is that not right?

Kieran Beggan: Once the European Space Agency release the data onto their website we will start downloading it and then we will be comparing it with our observatory data that we've been measuring and there will be a load of complicated processes to go through but hopefully we will see that the measurements we make on earth are similar to the measurements the satellite has made in space and then we will be able to calibrate them. It's an ongoing process though and as we get more and more data from the satellites we will go back and correct the calibration and our understanding of the satellite data, so so it's an initiative process and we will always be continually updating our understanding.

Richard Hollingham: Are you quite excited about this? It's been a long delayed mission but to finally start getting data from space?

Kieran Beggan: Absolutely, yeah. This is a once in a lifetime or once in a career satellite mission and you would be hard pushed not to be excited.

Richard Hollingham: Kieran Beggan and Tom Shanahan here at the British Geological Survey in Edinburgh. Thank you both and there's more about Swarm including some pictures on the Planet Earth online website. We will also post some photos on Facebook and Twitter of the recording here today including pictures of that wooden tripod. And that's the Planet Earth podcast from the Natural Environment Research Council. I'm Richard Hollingham, thanks for listening.