Author: AndyB

We just published a new paper lead-authored by Lennart de Groot at Utrecht University who just recently got his PhD “Cum Laude” (quite a distinction).

The paper is online here and I’ve tried to summarise it below.

The Earth’s magnetic field as seen at Earth’s surface is about 90% explained by a “dipole” – a single north pole opposite a single south pole – the same as a bar magnet. The dipole is not stationary but moves around and fluctuates in intensity over time. Occasionally it collapses altogether and grows back with the opposite polarity – this is called a geomagnetic reversal. The last reversal happened 780,000 years ago but there have been many more recent collapses (“geomagnetic excursions”) that haven’t resulted in reversals.

Two of the authors – Lennart and Mark on the top of Mt Etna in 2008.

The dipole has been weakening steadily (nearly 10%) since the first measurements began in 1840 AD causing some to speculate that we might be heading for a new collapse which would be bad news as magnetic storms might then become more severe. To understand this recent decline, we need to put it in context of magnetic field behaviour before 1840 and for this we have to turn to “accidental” records. The best of these are preserved in baked archaeological materials (e.g. pottery and kiln linings) but these are restricted to locations where complex civilizations existed and this leaves large areas of the globe undocumented.

Records are also preserved in volcanic rocks that have cooled from magma but these are generally much noisier than the archaeological ones because the lavas have less ideal magnetic properties than do pottery. Our study has managed to use a combination of new and old methods to produce a new high quality record from Hawaiian lavas informing us about the behaviour of the field in the sparsely populated Pacific region from around 500 AD to the present.

Combining this new record with other high quality records from archaeological materials recovered from W Europe, Japan, SW USA produces an interesting picture (see figure). As already thought, the recent decline appears to be a continuation of a declining trend going on at least 1,000 year. However, as well as this decline there are also periods of sharply increased intensity in 3 of the records (Hawaii, W Europe, SW USA) but crucially these are at different times in different places (and there is no such peak in Japan). These fluctuations, though large, can’t be explained by the dominant dipole part of the magnetic field as then you would see them everywhere. They must rather be due to the non-dipole field becoming regionally very important in a way that we have not seen since we have been making deliberate records.

Records of the estimated strengh of the main “axial dipole” component of the magnetic field from different locations

The magnetic field is generated by the flow of electrically conducting liquid in the Earth’s outer core and all of these changes reflect the chaotic nature of this flow. In addition to producing records such as these, the challenge lies in producing believable models of core flow to explain the observational data. This might eventually be useful to help predict future magnetic field behaviour.

SWARM is a long awaited ESA satelite mission to measure the geomagnetic field using magnetometers deployed on three separate but identical satelites.

They have a nice blog showing the build-up to the launch and with links to the mission details:

http://blogs.esa.int/eolaunches/

It has been a long time coming…

The paper we discussed in the latest meeting of Magnetic Personalities and which motivated the piece below was Anzellini et al. (2013; Science) “Melting of Iron at Earth’s Inner Core Boundary Based on Fast X-ray Diffraction”.

They estimate the temperature of the boundary between the inner and outer core to be around 6,000 degrees Celsius based on extrapolations of the melting temperature of iron at extreme pressures and temperatures. Extrapolating from this, the top of the core would be just under 4,000°C and the heat flow to the base of the 1,000°C cooler mantle some 10 TW. For comparison, this transfer of energy is just under the 13TW present rate of energy “consumption” (energy is never consumed, only transferred and transformed) by all of human civilisation.

The Earth is hot and space is cold and the first law of thermodynamics tells us that heat must flow from the former to the latter until they are the same temperature. The Earth is far from being just a boring hot rock however and has spontaneously developed some pretty impressive ways of transferring this heat up from its depths. Earthquakes, volcanoes, tsunamis, hurricanes and the Earth’s magnetic field are all consequences of our planet trying to lose heat as fast at it can and converting (temporarily) some of this thermal energy into mechanical energy as it does so.

At the heart of thermodynamics lies the concept of the heat engine: a system that converts thermal energy into useful work (mechanical energy). The idea was formulated from considerations of man-made steam engines increasingly brought into service during the industrial revolution. Nevertheless, the pioneers of thermodynamics were also doing a great service to the Earth Sciences because heat engines are actually at work all over and inside of our planet.

The two biggest engines in the Earth’s interior are in the mantle and outer core. Both regions are undergoing vigorous convection – that is they are transferring heat by mass movement (due to buoyancy) of hot and cold bits – though at very different rates. The creeping of the continents and oceans at rates of centimetres per year, and the resulting earthquakes that ensue, are down to convection in the mantle. Simply, the crust and top of the mantle loses heat to the adjacent oceans and atmosphere, becomes dense, and sinks at subduction zones. This pulls open a gap at the surface allowing the hotter mantle down below to rise at volcanic mid-ocean ridges. This process of mantle convection via plate tectonics is an extraordinarily effective means of transferring the Earth’s heat that is both much faster and far more interesting than simple conduction. It is not our planet’s only great and beautiful internal heat engine however.

The Earth’s liquid core is a lot hotter than the solid mantle above it and so, once more, heat must flow upwards and outwards. This core-mantle heat flow is the ultimate driver of mechanical flow in the liquid core and so we have another heat engine. In detail however, things in the core are a little more complicated and uncertain than in the mantle. The core fluid (an iron alloy) is a very good thermal conductor which gets in the way of thermal convection as is happening in the mantle.

Nevertheless, the core is vigorously convecting. As the core is losing heat, it is freezing from the centre (where the pressure is greatest) outwards. This means that the solid inner core is progressively growing at the expense of the liquid outer core above. As well as changing the size of the outer core, this freezing process is also affecting its composition. Only the densest, iron part of the fluid freezes onto the inner core so the outer part is left with the lighter, residual part of the alloy. Light means buoyant, buoyant means flow (upwards), flow means convection. But this time: bottom-up compositionally driven convection rather than the top-down thermally driven convection exhibited by the mantle.

In the core, the fact that the liquid is electrically highly conductive means that things get even more interesting. The mechanical energy generated by the heat engine can undergo a further conversion before it finally gets turned back into thermal energy: this time into electromagnetic energy. As you drive your car, part of your engine’s motion is converted into electricity and used to charge your car’s battery. This dynamo process – converting mechanical energy into electrical current (with an associated magnetic field) – is also happening, quite naturally, in the core. We call it the geodynamo and it’s what generates our planet’s magnetic field.

So, the Earth’s mantle and core both contain great heat engines producing work. Going back to the steam engines, the work they produced was put to good use – moving railway trains and the like. How does the Earth make use of all the work it produces? Well, if it is the aim of the Earth to lose its heat as fast as possible to space then this work is put to very good use indeed. I have already written of the massive efficiency gain in heat transfer that plate-style convection confers the mantle over simply waiting to cool down by conduction. It might surprise you to learn that the core goes to all the trouble of generating a huge magnetic field for much the same reason – to lose heat as fast as it can.

Relative motions between a conductor and a magnetic field generate electric currents in the conductor which, in turn, generate heat there. The core’s trick is to use its thermal energy to generate a moving magnetic field that, in turn, generates heat in distant conductors. Most of this magnetic-thermal energy conversion takes place within the core, but it also goes on in the mantle, the crust, the oceans and even out to space, rapidly allowing heat to migrate outwards. This is, again, a very good way of transferring heat long distances quickly.

I find it extraordinary that the creation of something as grand, complex and useful as the geomagnetic field can be neatly ascribed to the simple transfer of heat.

Sorry about the lack of posts in recent times. Here at least is what I’ve been busy with…

https://news.liv.ac.uk/2012/07/30/scientists-probe-links-between-magnetic-polarity-reversals-and-mantle-processes/

 

Recently, I was casually asked by someone at a gathering whether any of the research I did was of use in the “real world”. The questioner was of course alluding to differences they perceive between “pure” and “applied” science and, in particular, to the issue of whether my research was of any direct practical benefit to human society. It was a fair question. The UK taxpayer funds my entire salary as well as the costs associated with my research (from overheads charged by the university for use of their buildings to flights to and from conferences and field areas). What’s in it for them?

Certain applications of palaeomagnetism are of direct practical use to society. For example, in dating past volcanic eruptions and measuring the emplacement temperature of lavas and pyroclastic flows, they can help in assessing volcanic risk. Palaeomagnetism can also help determine the tectonic stability of whole regions, assessing their suitability as depositories for high-level nuclear waste for example.

Nonetheless, most of the research projects I am involved with and indeed, most palaeomagnetic studies undertaken around the world, are purely “academic”. That is to say: they are not specifically aiming to achieve anything other than an increase in knowledge for its own sake. A common misconception is to think that this is the same as saying that they are of no practical benefit whatsoever. That is definitely not the case.

The main reason why practically all first-world nations invest in  “blue skies” research is that they realise it pays dividends. Pure science, done well, contributes to a body of knowledge that, down the road, is the fuel for the innovations and improvements that we generally consider to be  increasing civilisation.

By providing quantitative evidence for continental drift and sea-floor spreading, palaeomagnetic studies done back in the 1950s and 1960s played a crucial role in the acceptance of the theory of plate tectonics, first amongst Earth scientists and then by the public at large. This unifying theory now provides the underlying framework for, more or less, the entire subject of geology. How does the taxpayer benefit from funding this research more than 50 years ago? Materially, by the provision of cheap hydrocarbon and mineral resources that are found using models based on the plate tectonic paradigm. Bodily, by the much improved understanding of seismic and volcanic risk provided by the theory. Intellectually, by the awareness of this simple and beautiful aspect of our planet’s behaviour, now and for the last few decades, taught to every child in school (under the guise of “geography”, tragically).

So, palaeomagnetism had its moment of glory, but how about since then? Well, it continues to contribute to a body of knowledge about our planet and its history which is increasingly important to human society as it struggles to deal with global environmental change and resource depletion. To cite a recent example, Torsvik et al. (2010) used a plate motion reference frame based on palaeomagnetic data to show that most of the Earth’s diamond deposits occurred above the margins of large seismic structures visible at a depth of nearly

2900 km. As well as potentially being of fundamental scientific importance in linking surface geology to whole mantle convection processes, the study will also serve to guide prospectors in the search for future sources of diamonds (which don’t just make pretty jewellery).

Understanding the Earth’s magnetic field is also a smart thing for us to be investing in. The geomagnetic field performs a valuable service to humans in shielding technology on the Earth’s surface and in orbit around it from much of the torrent of charged particles sent

our way by the sun. From studying palaeomagnetic records, we now have evidence suggesting that the field has been weakening for around the last 2,500 years. Will it continue to do so requiring the makers of satellites and power-grids to invest in better protection from the solar wind in the future? Only by better documenting and understanding past geomagnetic variations will we be able to answer this question with any confidence.

Personally, I am convinced that the Earth sciences are poised for another big step forward in the coming decade, perhaps the biggest since the plate tectonic revolution. Studies of seismology, mantle and core convection, geodynamo behaviour, and plate tectonic history are on the brink of coming together into an interpretable whole. The result will be an overarching theory of planetary evolution that palaeomagnetism will play a crucial and fundamental role in formulating.

For me, you can’t get much more “real world” than that.

This is a brand new blog created by Andy Biggin at the University of Liverpool that is intended to communicate exciting and educational information about geomagnetic research to the public.

I don’t have much experience of blogging so I am not sure where this will go. I will start by providing links to some of the great stuff that is already out there and then write about some of the specific projects that I am / have been involved in personally. At the same time, I’ll encourage my colleagues to write posts on their research and answer any specific queries that individual readers might have. Comments / feedback very welcome!