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Scientists understand that Earth's magnetic field has flipped its polarity many times over the millennia. In other words, if you were alive about 800,000 years ago, and facing what we call north with a magnetic compass in your hand, the needle would point to 'south.' This is because a magnetic compass is calibrated based on Earth's poles. The N-S markings of a compass would be 180 degrees wrong if the polarity of today's magnetic field were reversed. Many doomsday theorists have tried to take this natural geological occurrence and suggest it could lead to Earth's destruction. But would there be any dramatic effects? The answer, from the geologic and fossil records we have from hundreds of past magnetic polarity reversals, seems to be 'no.'Reversals are the rule, not the exception. Earth has settled in the last 20 million years into a pattern of a pole reversal about every 200,000 to 300,000 years, although it has been more than twice that long since the last reversal. A reversal happens over hundreds or thousands of years, and it is not exactly a clean back flip. Magnetic fields morph and push and pull at one another, with multiple poles emerging at odd latitudes throughout the process. Scientists estimate reversals have happened at least hundreds of times over the past three billion years. And while reversals have happened more frequently in "recent" years, when dinosaurs walked Earth a reversal was more likely to happen only about every one million years.Sediment cores taken from deep ocean floors can tell scientists about magnetic polarity shifts, providing a direct link between magnetic field activity and the fossil record. The Earth's magnetic field determines the magnetization of lava as it is laid down on the ocean floor on either side of the Mid-Atlantic Rift where the North American and European continental plates are spreading apart. As the lava solidifies, it creates a record of the orientation of past magnetic fields much like a tape recorder records sound. The last time that Earth's poles flipped in a major reversal was about 780,000 years ago, in what scientists call the Brunhes-Matuyama reversal. The fossil record shows no drastic changes in plant or animal life. Deep ocean sediment cores from this period also indicate no changes in glacial activity, based on the amount of oxygen isotopes in the cores. This is also proof that a polarity reversal would not affect the rotation axis of Earth, as the planet's rotation axis tilt has a significant effect on climate and glaciation and any change would be evident in the glacial record.Earth's polarity is not a constant. Unlike a classic bar magnet, or the decorative magnets on your refrigerator, the matter governing Earth's magnetic field moves around. Geophysicists are pretty sure that the reason Earth has a magnetic field is because its solid iron core is surrounded by a fluid ocean of hot, liquid metal. This process can also be modeled with supercomputers. Ours is, without hyperbole, a dynamic planet. The flow of liquid iron in Earth's core creates electric currents, which in turn create the magnetic field. So while parts of Earth's outer core are too deep for scientists to measure directly, we can infer movement in the core by observing changes in the magnetic field. The magnetic north pole has been creeping northward – by more than 600 miles (1,100 km) – since the early 19th century, when explorers first located it precisely. It is moving faster now, actually, as scientists estimate the pole is migrating northward about 40 miles per year, as opposed to about 10 miles per year in the early 20th century.Another doomsday hypothesis about a geomagnetic flip plays up fears about incoming solar activity. This suggestion mistakenly assumes that a pole reversal would momentarily leave Earth without the magnetic field that protects us from solar flares and coronal mass ejections from the sun. But, while Earth's magnetic field can indeed weaken and strengthen over time, there is no indication that it has ever disappeared completely. A weaker field would certainly lead to a small increase in solar radiation on Earth – as well as a beautiful display of aurora at lower latitudes - but nothing deadly. Moreover, even with a weakened magnetic field, Earth's thick atmosphere also offers protection against the sun's incoming particles.The science shows that magnetic pole reversal is – in terms of geologic time scales – a common occurrence that happens gradually over millennia. While the conditions that cause polarity reversals are not entirely predictable – the north pole's movement could subtly change direction, for instance – there is nothing in the millions of years of geologic record to suggest that any of the 2012 doomsday scenarios connected to a pole reversal should be taken seriously. A reversal might, however, be good business for magnetic compass manufacturers.
What do we mean by a magnetic reversal or a magnetic 'flip' of the Earth?The Earth has a magnetic field, as can be seen by using a magnetic compass. It is mainly generated in the very hot molten core of the planet and has probably existed throughout most of the Earth's lifetime. The magnetic field is largely that of a dipole, by which we mean that it has one North pole and one South pole. At these places, a compass needle will point straight down, or up, respectively. It is often described as being similar in nature to the field of a bar (e.g. fridge) magnet. However there is much small-scale variation in the Earth's field, which is quite different from that of a bar magnet. In any event, we can say that there are currently two poles observed on the surface of the Earth, one in the Northern hemisphere and one in the Southern hemisphere.By magnetic reversal, or 'flip', we mean the process by which the North pole is transformed into a South pole and the South pole becomes a North pole. Interestingly, the magnetic field may sometimes only undergo an 'excursion', rather than a reversal. Here, it suffers a large decrease in its overall strength, that is, the force that moves the compass needle. During an excursion the field does not reverse, but later regenerates itself with the same polarity, that is, North remains North and South remains South.Back to the top.How often do reversals occur?As a matter of geological record, the Earth's magnetic field has undergone numerous reversals of polarity. We can see this in the magnetic patterns found in volcanic rocks, especially those recovered from the ocean floors. In the last 10 million years, there have been, on average, 4 or 5 reversals per million years. At other times in Earth's history, for example during the Cretaceous era, there have been much longer periods when no reversals occurred. Reversals are not predictable and are certainly not periodic in nature. Hence we can only speak about the average reversal interval.Back to the top.Is the Earth's magnetic field reversing now? How do we know?Measurements have been made of the Earth's magnetic field more or less continuously since about 1840. Some measurements even go back to the 1500s, for example at Greenwich in London. If we look at the trend in the strength of the magnetic field over this time (for example the so-called 'dipole moment' shown in the graph below) we can see a downward trend. Indeed projecting this forward in time would suggest zero dipole moment in about 1500-1600 years time. This is one reason why some people believe the field may be in the early stages of a reversal. We also know from studies of the magnetisation of minerals in ancient clay pots that the Earth's magnetic field was approximately twice as strong in Roman times as it is now.Even so, the current strength of the magnetic field is not particularly low in terms of the range of values it has had over the last 50,000 years and it is nearly 800,000 years since the last reversal. Also, bearing in mind what we said about 'excursions' above, and knowing what we do about the properties of mathematical models of the magnetic field, it is far from clear we can easily extrapolate to 1500 years hence.Back to the top.How quickly do the poles 'flip'?We have no complete record of the history of any reversal, so any claims we can make are mostly on the basis of mathematical models of the field behaviour and partly on limited evidence from rocks that retain an imprint of the ancient magnetic field present when they were formed. For example, the mathematical simulations seem to suggest that a full reversal may take about one to several thousand years to complete. This is fast by geological standards but slow on a human time scale.Back to the top.What happens during a reversal? What do we see at the Earth's surface?As above, we have limited evidence from geological measurements about the patterns of change in the magnetic field during a reversal. We might expect to see, based on models of the field run on supercomputers, a far more complicated field pattern at the Earth's surface, with perhaps more than one North and South pole at any given time. We might also see the poles 'wandering' with time from their current positions towards and across the equator. The overall strength of the field, anywhere on the Earth, may be no more than a tenth of its strength now.Back to the top.Is there any danger to life?Almost certainly not. The Earth's magnetic field is contained within a region of space, known as the magnetosphere, by the action of the solar wind. The magnetosphere deflects many, but not all, of the high-energy particles that flow from the Sun in the solar wind and from other sources in the galaxy. Sometimes the Sun is particularly active, for example when there are many sunspots, and it may send clouds of high-energy particles in the direction of the Earth. During such solar 'flares' and 'coronal mass ejections', astronauts in Earth orbit may need extra shelter to avoid higher doses of radiation. Therefore we know that the Earth's magnetic field offers only some, rather than complete, resistance to particle radiation from space. Indeed high-energy particles can actually be accelerated within the magnetosphere.At the Earth's surface, the atmosphere acts as an extra blanket to stop all but the most energetic of the solar and galactic radiation. In the absence of a magnetic field, the atmosphere would still stop most of the radiation. Indeed the atmosphere shields us from high-energy radiation as effectively as a concrete layer some 13 feet thick.Human beings and their ancestors have been on the Earth for a number of million years, during which there have been many reversals, and there is no obvious correlation between human development and reversals. Similarly, reversal patterns do not match patterns in species extinction during geological history.Some animals, such as pigeons and whales, may use the Earth's magnetic field for direction finding. Assuming that a reversal takes a number of thousand years, that is, over many generations of each species, each animal may well adapt to the changing magnetic environment, or develop different methods of navigation.Back to the top.I'm interested in a more technical description. Can you tell me more?The source of the magnetic field is the iron-rich liquid outer core of the Earth. This liquid moves in complex ways as a result of the convection of the heat deep within the core and of the rotation of the planet. The motion of the core fluid is continuous and never stops, even during a reversal. It can only stop when the energy source fails. Heat is produced at least partly because of the solidification of the liquid core onto the solid inner core that sits at the centre of the Earth. This process has operated continuously over billions of years. At the top of the liquid core, some 3000 km beneath our feet and below the rocky mantle, the fluid may travel at horizontal speeds of tens of kilometres per year. The motion of this metal fluid across existing magnetic field lines of force produces electrical currents and these, in turn, generate more magnetic field. This is a process known as advection. To balance any growth of the field, and thus stabilise what we call the 'geodynamo', we need diffusion, where field 'leaks' away from the core and is destroyed. Ultimately, the core fluid flow produces a complicated magnetic field pattern at the Earth's surface with a complicated time variation.Simulations of the geodynamo on supercomputers have demonstrated the complex nature of the field and its behaviour over time. Simulations have also revealed reversals in the polarity, where the magnetic North pole is replaced by a South pole, and vice versa. In such simulations, the strength of the main dipole appears to weaken, perhaps to about 10% of its normal value (but not vanish) and the existing poles may wander across the globe and be joined by other temporary North and South magnetic poles (the 'non-dipole field').The solid iron inner core of the Earth has been shown in these simulations to be important in controlling the reversal process. Because it is a solid, the inner core can't generate magnetic field by advection, but any field that is generated in the fluid outer core can diffuse, or spread, into the inner core. The field generation process (advection) in the outer core seems to regularly attempt to reverse. But unless the field locked into the inner core first diffuses away, a true reversed field cannot become established throughout the core. Essentially the inner core resists any 'new' field diffusing in and perhaps only one in every ten such reversal attempts is successful.It is worth stressing that these results, while fascinating in themselves, are not known to be strictly true of the 'real' Earth. However, we have mathematical models of the Earth's magnetic field for the last 400 years, with early models based largely on observations made by mariners engaged in merchant and naval shipping. From these models and extrapolating down into the Earth, it is known that regions of reversed flux at the core-mantle boundary have grown over time. In these regions the compass points in the opposite direction, in or out of the core, compared to that of surrounding areas. It is the growth in area of such a reversed flux patch under the south Atlantic that is primarily responsible for the decay in the main dipolar field. This reverse patch is also responsible for the minimum in field strength called the South Atlantic Anomaly, now centred over south America. In this region energetic particles can approach Earth more closely, causing increased radiation risk to low Earth orbit satellites.There is much work yet to be done in understanding the properties of the deep Earth. This is a world where the crushing forces and core temperatures similar to that of the surface of the Sun take our scientific understanding to the limit.
