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The Earth's Dynamic Magnetic Field - January 18, 2015

Humans have recognized Earth has a magnetic field for centuries, and have used it to our advantage. Humans realized that magnets align themselves in a North-South direction, enabling compass use as early as 2000 years ago. A few hundred years after the compass was discovered, people also started to notice that magnetic North is not the same as true North, enabling more accurate compass navigation. By the year 1600, William Gilbert published De Magnete, in which he postulated that the world was magnetic. 

Despite it being over 400 years after this monumental scientific finding, we still have a tenuous grasp as to how the Earth actually produces its magnetic field. Currently, most scientists believe in the Dynamo theory; this theory states that the Earth's magnetic field is created by electric currents in the conductive material in the Earth's core, and these electric currents are created by convection currents made when heat escapes from the core, outward. This whole process is extremely complex and because it is occurring nearly 1800 miles below our feet in unbelievably hot temperatures, it is nearly impossible to observe first-hand. However, thanks to advances in technology, we can now test new theories and try to get a firmer grasp on how our Earth, and other planetary bodies, can produce a magnetic field. 

The Earth's magnetic field is thought to be self-sustaning; that is, the magnetic field created by the dynamic movement of heat in our Earth's core should remain in place as long as the super heated material continues to move and heat is lost. Just because the magnetic field is self-sustaining though, does not mean it is stable. Because the magnetic field is created through liquid movement and heat loss, the strength and direction of Earth's magnetic field can vary. Through geological and history records, scientists have found that throughout Earths history, the magnetic field has “flipped”, or reversed polarity, so that our current magnetic North would have been magnetic South, and vice-versa. 

While we currently use models to study our Earth's magnetic field, we can use rock and geological evidence to study the magnetic field in the past using paleomagnetism. Through this process, scientists study igneous rocks, which imprint the way the magnetic field is going at their time of creation, to compare our current and historic magnetic field directions and magnitude. Through our study of paleomagentism, we have learned that there have been around 170 magnetic field “flips” over the past 100 million years on Earth, and the last major “flip” was around 781,000 years ago. And, based on what we have managed to find out, the Earth's magnetic field is “flipping” directions faster than ever.

Based on geological records from around the world, we have found that magnetic pole reversal has quickened as the Earth ages. Between 2.9 and 1.5 billion years ago, the magnetic field reversed directions once every 5 million years, however between 1.5 billion and 500 million years ago, the magnetic field reversed directions once every 3.7 million years. Within the last 150 million years, the field has flipped direction once every 600,000 years and, startlingly, in the last 10-20 million years the field flips directions once every 200,000-250,000 years. This suggests that currently our magnetic field is not as stable as it once was, millions of years ago, however we are still unsure as to why. 

While we know the magnetic field can swap directions, it is not a doomsday prediction. We now know that before the magnetic pole switch there are usually periods of instability and low intensity. Before the last magnetic pole switch nearly 780,000 years ago, the magnetic field was unstable for about 6,000 years preceding the switch, with two very low points in the fields strength, each lasting about 2,000 years. While we currently do not know where in the cycle of the magnetic field “flipping” we are, we do know that it will not happen overnight. We expect to see a period of instability and low intensity before the field flips, so hopefully we can adapt and prepare for the new magnetic orientation. 

The Earth's magnetic field does more than just provide us with navigation. The magnetic field helps to buffer earth from solar wind, which would otherwise strip away the ozone layer that helps protect us from harmful ultraviolet radiation.

We also believe that the Earth's magnetic field plays a major role in migration and nesting for many animal species, like salmon and sea turtles. A new study shows that sea turtles use “imprinted” magnetic signatures to find the beach which they hatched from and will lay eggs on. Research now shows that baby sea turtles “imprint” or remember the specific magnetic signature for the beach they are born on, and after swimming thousands of miles in the open ocean, they can return to this beach to nest. Scientists believe this might happen to ensure sea turtle nest validity; sea turtles return to the beaches they were hatched to lay eggs because it had the right conditions to hatch them, so hopefully it will still provide these favorable conditions for the new generation. 

We know that the Earths magnetic field will inevitably “flip” again, however when it will occur and how long the process will take is still a mystery. The best course of action is to study the past, but prepare for the future, so that this monumental change will not catch us unaware. 

 

Harrison Jackson is the Coastal Stewards Coordinator for the Maryland Coastal Bays Program

 


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