Earth's Magnetic Field: The Force That's Always with YouThis story was made with Esri's Story Map Journal. Read the interactive version on the web at https://arcg.is/1ufXbi. Do you know how a compass works? Ever wonder why birds know how to fly south for the winter? How does your smartphone know where you are? The answer to all of these questions is Earth's magnetic field. It is literally in, on, and above Earth at the same time. The magnetic field is all around, and while you may not notice it, it affects many of the things you do.Since Earth’s magnetic field is so important, NOAA National Centers for Environmental Information (NCEI), in coordination with the Cooperative Institute for Research in Environmental Sciences (CIRES), develops and distributes models of the geomagnetic field and maintains archives of geomagnetic data. These data and models help today’s scientists understand geomagnetism, but humans have been aware of the force for hundreds of years.The story of Earth's magnetic field is the story of human's quest for exploration.  From the Age of Exploration to the Space Age, humans have depended on Earth's magnetic field to guide them as they discover new horizons and return back home again. In the fourth century BC, the Chinese found that certain materials would orient themselves toward north-south when allowed to rotate freely. By 1190 AD, European sailors were commonly using compasses to navigate. From there, the race was on to make better, more accurate navigational guides.

A reconstruction of an early Chinese compass which is a spoon made of lodestone, its handle pointing south, mounted on a brass plate with astrological symbols. (Wikipedia Commons)

But that was a tremendous challenge because early scientists and navigators did not understand what was causing this magnetic phenomenon. One theory was that the compasses were pointing towards stars or other heavenly bodies. Others believed that Earth had magnetic mountains capable of attracting metals over great distances. In 1600, English astronomer William Gilbert's experiments with a magnetic model of Earth led him to the conclusion that Earth itself is a large magnet.  

From the Depths of the Planet

Earth has a magnetic field just like a common magnet that you find on your refrigerator. Earth’s magnetic field begins in the very center of the planet, the core. Earth’s core is roughly 1,800 miles (2,900 km) below the surface and is made up of two layers, the inner and outer cores. The outer core is mostly composed of hot, liquid metal — iron and nickel — that are constantly churning. This conductive metal creates and sustains Earth’s magnetic field in a process called the geodynamo. The powerful magnetic field passes through the core of Earth, through the crust, and enters space.Just like a bar magnet, the magnetic field of Earth is a dipole field with two poles, located at either end. The magnetic field is strongest at each of these poles.

Magnetic field around a spherical dipole magnet like the Earth with a schematic dipole magnet drawn to its center. (Wikimedia Commons)The straight axis of the dipole is offset from the axis of Earth's rotation by approximately 11 degrees. This means that the north and south geographic poles and the north and south magnetic poles are not located in the same place. But how do we know this?  Because of that human quest to explore.      

A Guide to the North and South Poles

Magnetic Polar Wander Animation. See also: NCEI's Historical Declination ViewerThis video can be viewed in the online version of this story map

Video showing magnetic declination changes from 1590-2020 using gUFM and IGRF models. Shown are: historical isogonic lines (declination is the same value), magnetic poles (green dots), positive declination or east of north (red lines), negative declination or west of north (blue lines), and agonic, or zero declination (green line).Ocean exploration has always relied on accurate navigation. By the fifteenth century, sailors realized that “north” on a compass was not the same as Earth’s geographic north. This discrepancy between magnetic north and geographic north is called magnetic declination (also known as variation) and varies depending on location and changes over time.Early European explorers took careful notes all over the world on how declination shifted over hundreds of years, and scientists were able to piece together the locations of the magnetic poles back to 1590. Sailors and explorers were constantly adding new data to create newer, more accurate maps of the world. Over time, they realized that the locations of the North and South magnetic poles gradually shift, which results in slow changes in the magnetic declination.   

Navigational sailor's compass rose, 1607 (Wikipedia Commons)Navigators must adjust their compass readings to account for this variation. Maps from the sixteenth century included a compass rose, or two rings, one smaller and set inside the other. The outside ring displays geographic directions while the inside ring displays magnetic directions. This allows sailors to adjust for the magnetic declination.NCEI's Historical Declination Viewer is a record of declination from 1590 to the present. This tool gives users a window into the past and for the present.

DO ANIMALS USE THE FORCE?

Birds fly south for the winter and return in the spring. Monarch butterflies find their way to central Mexico each fall. Just as humans use street signs, a GPS, or a map to navigate our way around a city, animals use multiple means of navigation, including using the magnetic field as their compass. Magnetoreception is a sense that allows an organism to detect a magnetic field to perceive direction, altitude, or location. Scientists have a number of theories about how this works, but meanwhile, animals keep finding their way with their own internal compasses. Changing Directions

Due to careful declination measurements by sailors and others, it has been known for over 400 years that the locations of the magnetic North and South Poles move. Since James Clark Ross first physically located magnetic north in 1831 in Canada's Nunavut territory, the pole has primarily moved northward, traveling hundreds of miles over the last several decades. Even more surprisingly, magnetic North and South Poles can reverse or “flip”, known as geomagnetic pole reversals. 

Geomagnetic pole reversals have happened throughout Earth's history. The last one occurred 780,000 years ago. Though they sound scary, pole flips can take a long time to occur and pose no immediate threat. Scientists have determined that, in the short-term, there is no real change to Earth’s environment and no threat to life due to a pole flip. There may be impacts to infrastructure, such as powerlines and satellites, caused by space weather events, but due to the slow evolution of these pole flips, humans can mitigate these issues.It is possible to look even farther back into the history of Earth’s magnetic field by studying the rocks at the surface. As heated rocks cool, they preserve details of the magnetic field at that time. By sampling these rocks and using radiometric dating techniques, it is possible to reconstruct the history of Earth’s magnetic field for the last 160 million years. In this way, the crust acts like a continuous record of the evolution of Earth’s magnetic field history, showing the magnetic field strengthening and weakening, and the geomagnetic pole wandering and reversing (see below: The Magnetic Field of Earth’s Crust).

Satellites like GOES-16 are equipped with magnetometers, devices that measure the direction, strength, or relative change of Earth's magnetic field. (NOAA NESDIS)

The study of Earth’s magnetic field is not just the study of the past. While today’s navigation systems may be GPS and other sophisticated systems, they still depend on the magnetic field to provide orientation. Scientists measure Earth’s magnetic field using satellites and ground-based magnetometers, devices that measure the direction, strength, or relative change of the magnetic field at a particular location. Scientists even use magnetometers on satellites to measure Earth’s magnetic field in space.

Magnetic Force Field

The magnetic field, including the magnetosphere, protects Earth from solar wind.

If you could view Earth's invisible magnetic field from space, you would see that it actually stretches far beyond the planet's surface into space. Scientists also measure a part of Earth's magnetic field that those early navigators could not have dreamed existed: the magnetosphere.  Earth's magnetosphere shields our planet from solar and cosmic radiation, like a giant force field. The shape of the magnetosphere changes according to the amount of solar wind bombarding it on the side facing the sun. If the magnetosphere were not in place, Earth would likely be uninhabitable. As the shield is hit by space weather, it deforms and transfers energy from the cosmic environment to regions inside it. This plays a key role in determining the effects of space weather—including geomagnetic storms that can damage power grids, impact GPS, and create communications challenges for airlines, mobile telephones, and more—but which are also the cause of beautiful auroras or "polar lights." 

Aurora Island (Wikimedia Commons)

A large geomagnetic storm can impact modern communications and deflect compasses, which can make it difficult to measure an accurate magnetic declination—something that is vital to navigation. By constantly measuring the magnetic field, scientists can observe how the field has changed over a period of years and use these data to create models that predict how it may change in the future.

SOUNDS OF A SOLAR STORM

High school students listening to audio tracks of NOAA satellite data have identified the sounds of solar storms buffeting Earth’s magnetic field. The results of a United Kingdom-led citizen science project suggest that the approach of converting physical data into sound signals could help NOAA and other scientists make sense of massive amounts of data from satellites and other instruments (Learn more).Audio: Summary stereo audio from GOES-15 (Martin Archer/Queen Mary University of London) [Audio file available online.]

Predicting the Changing Magnetic Field

Scientists are constantly measuring and gathering data about Earth’s changing magnetic field. Using this information, it is possible to create a mathematical representation of Earth's main magnetic field and how it is changing.NCEI develops and distributes models of the geomagnetic field and archives geomagnetic data to further the understanding of Earth's magnetism and the Sun–Earth environment.

Interactive globe of 2019 magnetic declination lines from WMM 2015 v2. Click on the lines to view magnetic declination, and drag to rotate the globe. Shown are: historical isogonic lines (declination is the same value), magnetic poles (green dots), positive declination or east of north (red lines), negative declination or west of north (blue lines), and agonic, or zero declination (green line).

One of the key tools developed to model the change in Earth’s magnetic field is the World Magnetic Model (WMM). Developed by NCEI and the British Geological Survey, the WMM  is a representation of the planet’s magnetic field that gives compasses dependable accuracy.Smartphone and consumer electronics companies rely on the WMM to provide consumers with accurate compass apps, maps, and GPS services. The WMM is the also the standard navigation tool for the Federal Aviation Administration, U.S. Department of Defense, North Atlantic Treaty Organization, and more. Typically, a new and updated version of the WMM is released every five years, but occasionally, an out-of-cycle update to the model is needed if the poles move more quickly than anticipated.

The Magnetic Field of Earth's Crust

NCEI also provides geomagnetic products that capture more details of the magnetic field than the World Magnetic Model. They include magnetic field contributions from Earth's crust, and are suitable for applications where higher navigational accuracy is required. Like the World Magnetic Model, the Earth Magnetic Anomaly Grid and Enhanced Magnetic Model predict Earth’s magnetic field. These products have a myriad of uses across the globe, including directional drilling and submarine navigation.The Earth Magnetic Anomaly Grid (EMAG) is compiled from data collected by magnetometers on satellites, ships, and even those aboard or towed behind aircraft, like an airplane or helicopter. EMAG  maps the strength of static magnetic anomalies. A magnetic anomaly is an inconsistency in Earth's magnetic field due to chemical and magnetic changes of rocks. By mapping these anomalies, EMAG increases knowledge about geologic formations underground as well as the composition of Earth's crust. They also provide insights into the evolution of Earth’s crust and magnetic field history. They show when Earth’s magnetic field is strengthening or weakening, and geomagnetic pole reversals.

The Enhanced Magnetic Model (EMM) models Earth’s magnetic field as a combination of the relatively static crustal magnetic field, and the constantly changing core field. By combining these two sources of the magnetic field, we can more accurately determine magnetic field strength and direction. Magnetic anomaly grids are used for resource exploration and navigation where GPS is unavailable (directional oil drilling, submarine navigation, and so forth). 

As new crust is created at a spreading ridge, the cooling magma captures the intensity and direction of the magnetic field in “stripes.” A, b, and c represent the evolution of the ocean crust over time as it moves away from the spreading ridge. (Wikipedia Commons)One example of how magnetic anomalies can give insight into the evolution of Earth's crust are seafloor stripes (seen above). These formations were a foundational piece of evidence in support of the plate tectonic theory. By noting the symmetry of the stripes on either side of oceanic spreading ridges, and the disappearing of stripes below convergent boundaries, scientists were able to successfully argue that Earth’s crust is made of shifting plates. This discovery has helped researchers look backwards into time into what the continents and oceans looked like millions of years ago (e.g., Pangea). 

High Definition Geomagnetic Model

Declination contours from NCEI's Enhanced Magnetic Model (solid lines) and World Magnetic Model (dashed lines). Red = Eastward, Green = Zero, Blue = Westward Declination

NCEI's High Definition Geomagnetic Model (HDGM) looks at Earth’s main magnetic field and the field below the surface, providing magnetic field values (total field, dip, and declination) at any point above or below Earth's crust. Although these fields generally arise from Earth’s hot circulating core, they also come about due to other events, such as space weather, which develop beyond Earth’s protective magnetosphere.The accuracy of the HDGM makes it ideal for specialized applications and navigating in environments where data are sparse or non-existent. Many types of commercial and public enterprises rely on information about geomagnetic conditions to conduct operations. The HDGM is a highly trusted magnetic reference model for the oil and gas industry. Resource exploration below Earth's surface, both onshore and offshore, plays an important role where GPS (global positioning systems) can't operate. The HDGM is used to increase safety and efficiency of directional drilling, and helps operators improve well positions, preventing danger of collision with existing wellbores, and enabling real-time steering to save rig-time and reduce drilling costs. Other uses of HDGM include drilling for geothermal exploration, earthquake research, and long-term storage of carbon dioxide.MODELING THE FIELD IN REAL-TIME

In 2017, NCEI and researchers at the Cooperative Institute for Research and Environmental Sciences (CIRES) introduced an addition to the HDGM that takes space weather effects into account.Known as the High Definition Geomagnetic Model-Real Time (HDGM-RT), this model estimates disturbances to the magnetic field from solar wind driven sources in near-real time. It is the first such model of the magnetic field contributed by both the magnetosphere and the ionosphere, the conductive part of Earth's upper atmosphere.

ONLINE CALCULATORS

NCEI provides magnetic calculators that are used over 11 million times every year, by citizens and scientists alike, to explore and correct for Earth's magnetic field across the globe. Applications include a declination calculator, magnetic field calculator, and the U.S. historic declination calculator. See the Interactive globe of the EMAG2v3 crustal magnetic anomaly grid online.Try using the EMAG2v3 crustal magnetic anomaly grid interactive globe with your mouse [available online]. The buttons below automatically move the globe to each location.

Kursk Anomaly: World's largest magnetic anomaly, in central Russia Bangui Anomaly: Large negative magnetic anomaly, centered on Bangui, Central

African Republic

 

CrowdMag: Map the force, share the dataSince the magnetic force is always with you, it makes sense to use the other thing that is always with you to help measure it — your smartphone. NCEI developed the CrowdMag app, which allows you to contribute data from the magnetometer embedded in your smartphone. This data improves NCEI's magnetic field models, like the WMM and the HDGM. When the magnetometer's data are combined with those of the accelerometer—a sensor that detects how the phone moves—the phone knows where it is in reference to magnetic north. After CrowdMag is installed, your phone will automatically send data collected by the magnetometer and accelerometer to NCEI.CrowdMag measures the strength and direction of Earth’s magnetic field at a specific location, which provides users insight into the magnetic world that surrounds them. With this app, users can perform their own magnetic surveys without having to purchase specialized equipment, and the data allow scientists to fill in the gaps from limited satellite geomagnetic data. With your help, scientists can develop magnetic models with higher accuracy than with just satellites alone.

DOWNLOAD AND INSTALL CROWDMAG

The free CrowdMag app is available for both Apple iOS devices and Android devices. To install CrowdMag on your phone or tablet, visit the Apple Store or Google Play, and search

for "Crowdmag"Your contribution will improve navigational accuracy as well as our understanding of Earth’s magnetic field. And whether you drive, cycle, fly, or float, you will be able to more easily navigate to where you are going thanks to the force that’s always with you.  Be a Geomag Citizen Scientist

Help us improve navigation!

Screenshot of CrowdMag showing user-collected magnetic data in Denver, Colorado.

Resources [Links are available online.]

World Magnetic Model Out-of-Cycle Release  Modeling Earth’s Geomagnetic Field Geomagnetism Frequently Asked Questions Introduction to Geomagnetism What is Space Weather? The Science, Beauty, and Mystery of Auroras 

Credits 

Story Map Editors / Developers Heather McCullough (NCEI), Barbara Ambrose (NGI & NCEI), Megan Cromwell (NGI & NCEI), Katie Palubicki (CIRES & NCEI), Angela Sallis (General Dynamics Information Technology & NCEI), Jesse Varner (CIRES & NCEI)

Science Contributors: Brian Meyer (CIRES & NCEI), Arnaud Chulliat (NCEI), Manoj Nair (CIRES & NCEI)NCEI partners with NOAA Cooperative Institutes, including CIRES (Cooperative Institute for Research in Environmental Sciences) and NGI (Northern Gulf Institute).https://noaa.maps.arcgis.com/apps/MapJournal/index.html?appid=3b9045c4d1aa408694d3759d1aa5ede4

Magnetic North, Geomagnetic and Magnetic Poles

World Data Center for Geomagnetism/Kyoto Univiversity

http://wdc.kugi.kyoto-u.ac.jp/index.html

Locations of geomagnetic poles and magnetic poles based on IGRF-13 from 1900 to 2015 by 5 years and at 2020 (red) and 2025 (prediction).

http://wdc.kugi.kyoto-u.ac.jp/poles/polesexp.html

The Geomagnetic poles (dipole poles) are the intersections of the Earth's surface and the axis of a bar magnet hypothetically placed at the center the Earth by which we approximate the geomagnetic field. There is such a pole in each hemisphere, and the poles are called as "the geomagnetic north pole" and "the geomagnetic south pole", respectively. On the other hand, the magnetic poles are the points at which magnetic needles become vertical.As a rule, the N-pole dips downward by an angle called "inclination" in the northern hemisphere.

http://wdc.kugi.kyoto-u.ac.jp/poles/polesexp.html

The Geomagnetic poles (dipole poles) are the intersections of the Earth's surface and the axis of a bar magnet hypothetically placed at the center the Earth by which we approximate the geomagnetic field. There is such a pole in each hemisphere, and the poles are called as "the geomagnetic north pole" and "the geomagnetic south pole", respectively. On the other hand, the magnetic poles are the points at which magnetic needles become vertical.As a rule, the N-pole dips downward by an angle called "inclination" in the northern hemisphere.

Molten iron river discovered

speeding beneath Russia and Canada

New Scientist, EARTH, 19 December 2016. By Andy Coghaln https://www.newscientist.com/article/2116536-molten-iron-river-discovered-speeding-

beneath-russia-and-canada/

Deep below our planet’s surface a molten jet of iron nearly as hot as the surface of the sun is picking up speed.This stream of liquid has been discovered for the first time by telltale magnetic field readings 3000 kilometres below North America and Russia taken from space.

The vast jet stream some 420 kilometres wide has trebled in speed since 2000, and is now circulating westwards at between 40 and 45 kilometres per year deep under Siberia and heading towards beneath Europe (see diagram, below). That is three times faster than typical speeds of liquid in the outer core.No one knows yet why the jet has got faster, but the team that discovered the accelerating jet thinks it is a natural phenomenon that dates back as much as a billion years, and can help us understand the formation of Earth’s magnetic fields that keeps us safe from solar winds.Straight to the core

“It’s a remarkable discovery,” says Phil Livermore of the University of Leeds, UK, who led the team. “We’ve known that the liquid core is moving around, but our observations haven’t been sufficient until now to see this significant jet.”“We know more about the Sun than the Earth’s core,” says another team member, Chris Finlay, from the Technical University of Denmark in Kongens Lyngby. “The discovery of this jet is an exciting step in learning more about our planet’s inner workings.”What made the discovery possible was the combined monitoring power of the European Space Agency’s trio of satellites, called Swarm, which were launched in 2013. From orbit, they can measure magnetic field variations as deep down as 3000 kilometres below Earth’s surface, where the molten core meets the solid mantle.“Having all three meant we could strip away magnetic fields from elsewhere such as the ionosphere and the crust, providing our sharpest ever image of the fluctuations at the core-mantle boundary alone,” says Livermore.Plugging the new data into models also allowed the team to figure out how the fluctuations changed over time.Invisible river

Earth’s magnetic field is generated by the movement of molten iron in the outer core, so examining the magnetic field can reveal details of the behaviour of the core that underpins it.The discovery of the jet involved tracking two massive but unusually strong lobes of magnetic flux originating from the core-mantle boundary, situated beneath Canada and Siberia respectively, but moving with the flow of the molten iron. Because their motion could originate only from the physical movement of molten iron, the lobes served as markers, allowing the researchers to track the flow of iron.Livermore likens it to being able to track the course of a river at night by watching candles floating on the surface. “As the iron moves, it drags the magnetic field with it,” he says. “We can’t see the flow of iron itself, only the motion of the flux lobes.”He says there may be a southern counterpart to the jet, but because there are no trackable flux lobes in the south, any magma stream there can’t be picked up geomagnetically.

Rotating cylinders

Livermore and his colleagues say the jet is created by movement of molten iron around the inner, solid iron core.Next to the inner core, there are parallel cylinders of swirling molten iron in the outer core running from north to south. Where these swirling cylinders meet the solid core, and squash

against it, they act like a pair of rollers, squeezing out additional molten iron sideways to create the jet stream.This produces and moves the two lobe-like magnetic fields, which is what the satellites detected and tracked.Why the jet is getting faster is more of a mystery. It may be related to the rotation of the inner core, which was found in 2005 to rotate a bit faster than Earth’s crust, says Xiaodong Song of the University of Illinois in Champaign, Illinois.Xiaodong was a member of the team that used seismological data to make that 2005 discovery. “If these seismological and geomagnetic observations can be tied together to a common process in the fluid core, it would be really exciting,” he says.Livermore thinks the acceleration of the jet is down to push-back from magnetic fields. The flow of iron generates the magnetic field, but, he says, the magnetic field may then be affecting the flow of the iron.Studying the jet should enable geophysicists to better understand how the planet’s core behaves, and the factors that influence Earth’s magnetic field strength.Flip the polarity

“If we can understand how the field is generated, we understand how it changes over time and whether and when it will weaken and reverse,” says Livermore.Other geophysicists agree. “The more we understand the core’s behaviour at various time and spatial scales, the more we can hope to understand the beginnings, renewal and future of our magnetic field,” says William Brown of the British Geological Survey’s geomagnetism team.Earth’s magnetic field seems to have been weakening, especially since around 1840, at about 5 per cent per century. The magma stream should help geophysicists predict more accurately if and when the magnetic field of the planet’s core will flip, and the magnetic north and south poles trade places, which happens every few hundred thousand years.And thanks to the satellite monitoring system, says Xiaodong, we have now opened a new window to view in “real time” the activity of molten iron deep in Earth’s core.Journal reference: Nature Geoscience, DOI: 10.1038/NGEO2859Article amended on 10 January 2017Correction:This article has been updated to correct the frequency with which the north and south poles of Earth’s magnetic field swap places.

Read more: https://www.newscientist.com/article/2116536-molten-iron-river-discovered-speeding-beneath-russia-and-canada/#ixzz6G7pQzsQv

Dogs defecate in accordance with earth's magnetic field, research finds

A scientific study has revealed that dogs adapt their excremental habits to align with the planet's geomagnetic field. Here's how they worked it out.Mona Chalabi  @MonaChalabiWed 8 Jan 2014 20.29 GMT Last modified on Wed 22 Feb 2017 18.41 GMT

https://www.theguardian.com/news/datablog/2014/jan/08/dogs-defecate-earths-magnetic-field-research-findsFor years, scientists have known that several species spontaneously align their bodies with the earth’s magnetic field when engaging in certain behaviours.Now, a team of 12 scientists from universities in Germany and the Czech Republic have come together in a unique study that observed 37 breeds of dog over a two-year period.Exactly 1,893 defecations and 5,582 urinations later, the team reach one incredible finding: "dogs preferred to excrete with the body being aligned along the north–south axis".Before you grab your schnauzer and a clipboard, here's how the scientists work it out.Method

In open fields, away from manmade structures and off the leash, the alignment of dogs’ spines was recorded using a hand-held compass.

An image taken from the published study Photograph: Hart et alThe breed of the dog, its sex, age, body mass and condition were recorded as well as the date, time and location.We’ve included a sample of the male dogs' behaviour here. It’s impossible to say what was wrong with M07. [Graphics may be viewed online.]The researchers then monitored the proportion of those urinations and defecations that were aligned with the earth’s north-south axis.

If you don't understand the graphic, it may be that you need to get yourself a PhD. Photograph: /Hart et al

Results

Dogs join cattle, roe deer, red deer, hunting red foxes, red foxes, coyotes and grey wolves as yet another mammal to have a mechanism of "magnetoreception".Although their altered behaviour was only evident under calm conditions, it’s still a breakthrough in demonstrating measurable, predictable changes in dog behaviour in response to the earth’s magnetic field.And before you go pointing any fingers, this isn’t made up (although it’s not clear how George Arnett found it online) and you can see some of their findings for yourself here [link available online].For the original article, see:

Dogs are sensitive to small variations of the Earth’s magnetic field (Hart et al., 2013) https://frontiersinzoology.biomedcentral.com/track/pdf/10.1186/1742-9994-10-80

Cleaning up after yourself. It’s job number two.Earth's ancient magnetic field just got a lot older

Early magnetism could have helped create conditions to support life.Nature, Alexandra Witze, 30 July 2015

Portrait of a young planetThe Jack Hills zircons show that a magnetic field existed as early as 4 billion years ago, fluctuating in strength from a value similar to today's — around 25 microteslas — to about 12% of that. The two most-ancient zircons in the study suggest that the field could be up to 4.2 billion years old. But these rocks are difficult to analyse because they were re-heated around 2.6 billion years ago, which left a record of magnetic activity then that partially overlaid older evidence.

Whenever it arose, an ancient magnetic field would have been a good, although not perfect, shield against the solar wind. An occasional solar storm might still have been able to blast through the magnetic field and strip Earth's atmosphere of water and volatile compounds that are necessary for life, Tarduno's team says.

But David Grinspoon, a researcher at the Planetary Science Institute in Tucson, Arizona, is not convinced that a planet must have an active magnetic field in order to be habitable. Many experts question the idea that Mars lost its atmosphere simply because its magnetic field switched off, he says. Powerful stellar winds may actually provide energy for yet-unknown life on planets that lack a protective magnetic field.

The presence of an ancient magnetic field on Earth also suggests that plate tectonics could have been operating more than 4 billion years ago, Tarduno adds.

But Tarduno’s team might not be able to look much further into the past. The oldest Australian zircons, which date to 4.4 billion years ago, contain a limited and confusing magnetic record. Instead, researchers may focus on broadening their knowledge of conditions on the ancient Earth, rather than trying to determine whether its magnetic field existed more than 4.2 billion years ago.

Nature  doi:10.1038/nature.2015.18072https://www.nature.com/news/earth-s-ancient-magnetic-field-just-got-a-lot-older-1.18072

Dynamics of the Earth's Inner Magnetosphere (Zheng, NASA) https://ccmc.gsfc.nasa.gov/RoR_WWW/support/UAH_2013/Zheng_innermag_UAH.pdf

Planetary Magnetospheres: Van Allen Belts of Solar System ... http://www-pw.physics.uiowa.edu/van90/Krimigis_lectureOct2004.pdf

The Sun (Chapter 16) http://www.as.utexas.edu/astronomy/education/fall09/scalo/secure/301F09.Ch16.Sun.Slides.pdf

The future of Geomagnetic Earth Observations (Kathy Whaler)http://indico.ictp.it/event/7723/session/2/contribution/15/material/slides/0.pdf

GEM Tutorials – gem https://gem.epss.ucla.edu/mediawiki/index.php/GEM_Tutorials

Space Weather Prediction Center (NOAA) https://www.swpc.noaa.gov/