geomagnetic field

Earth’s magnetic field ‘simpler than we thought’

Scientists have identified patterns in the Earth’s magnetic field that evolve on the order of 1,000 years, providing new insight into how the field works and adding a measure of predictability to changes in the field not previously known.

The discovery also will allow researchers to study the planet’s past with finer resolution by using this geomagnetic “fingerprint” to compare sediment cores taken from the Atlantic and Pacific oceans.

Results of the research, which was supported by the National Science Foundation, were recently published in Earth and Planetary Science Letters.

The geomagnetic field is critical to life on Earth. Without it, charged particles from the sun (the “solar wind”) would blow away the atmosphere, scientists say. The field also aids in human navigation and animal migrations in ways scientists are only beginning to understand.

Centuries of human observation, as well as the geologic record, show our field changes dramatically in its strength and structure over time.

Yet in spite of its importance, many questions remain unanswered about why and how these changes occur. The simplest form of magnetic field comes from a dipole: a pair of equally and oppositely charged poles, like a bar magnet.

“We’ve known for some time that the Earth is not a perfect dipole, and we can see these imperfections in the historical record,” said Maureen “Mo” Walczak, a post-doctoral researcher at Oregon State University and lead author on the study. “We are finding that non-dipolar structures are not evanescent, unpredictable things. They are very long-lived, recurring over 10,000 years - persistent in their location throughout the Holocene.

"This is something of a Holy Grail discovery,” she added, “though it is not perfect. It is an important first step in better understanding the magnetic field, and synchronizing sediment core data at a finer scale.”
Some 800,000 years ago, a magnetic compass’ needle would have pointed south because the Earth’s magnetic field was reversed. These reversals typically happen every several hundred thousand years.

While scientists are well aware of the pattern of reversals in the Earth’s magnetic field, a secondary pattern of geomagnetic “wobble” within periods of stable polarity, known as paleomagnetic secular variation, or PSV, may be a key to understanding why some geomagnetic changes occur.

The Earth’s magnetic field does not align perfectly with the axis of rotation, which is why “true north” differs from “magnetic north,” the researchers say. In the Northern Hemisphere this disparity in the modern field is apparently driven by regions of high geomagnetic intensity that are centered beneath North America and Asia.

“What we have not known is whether this snapshot has any longer-term meaning - and what we have found out is that it does,” said Joseph Stoner, an Oregon State University paleomagnetic specialist and co-author on the study.

When the magnetic field is stronger beneath North America, or in the “North American Mode,” it drives steep inclinations and high intensities in the North Pacific, and low intensities in Europe with westward declinations in the North Atlantic. This is more consistent with the historical record.

The alternate “European mode” is in some ways the opposite, with shallow inclination and low intensity in North Pacific, and eastward declinations in the North Atlantic and high intensities in Europe.

“As it turns out, the magnetic field is somewhat less complicated than we thought,” Stoner said. “It is a fairly simple oscillation that appears to result from geomagnetic intensity variations at just a few recurrent locations with large spatial impacts. We’re not yet sure what drives this variation, though it is likely a combination of factors including convection of the outer core that may be biased in configuration by the lowermost mantle.”

The researchers were able to identify the pattern by studying two high-resolution sediment cores from the Gulf of Alaska that allowed them to develop a 17,400-year reconstruction of the PSV in that region. They then compared those records with sediment cores from other sites in the Pacific Ocean to capture a magnetic fingerprint, which is based on the orientation of the magnetite in the sediment, which acts as a magnetic recorder of the past.

The common magnetic signal found in the cores now covers an area spanning from Alaska to Oregon, and over to Hawaii.

“Magnetic alignment of distant environmental reconstructions using reversals in the paleomagnetic record provides insights into the past on a scale of hundreds of thousands of years,” Walczak said. “Development of the coherent PSV stratigraphy will let us look at the record on a scale possibly as short as a few centuries, compare events between ocean basins, and really get down to the nitty-gritty of how climate anomalies are propagated around the planet on a scale relevant to human society.”

The magnetic field is generated within the Earth by a fluid outer core of iron, nickel and other metals that creates electric currents, which in turn produce magnetic fields. The magnetic field is strong enough to shield the Earth from solar winds and cosmic radiation. The fact that it changes is well known; the reasons why have remained a mystery.
Now this mystery may be a little closer to being solved.

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Magnetic complexity begins to untangle

After a year in orbit, the three Swarm satellites have provided a first glimpse inside Earth and started to shed new light on the dynamics of the upper atmosphere – all the way from the ionosphere about 100 km above, through to the outer reaches of our protective magnetic shield.

A series of scientific papers published recently in Geophysical Research Letters and collected in a special issue, confirms the remarkable potential of this unique mission.

Rune Floberghagen, ESA’s Swarm Mission Manager, said, “These results show that all the meticulous effort that went into making Swarm the best-ever spaceborne magnetometry mission is certainly paying off.”

Swarm is tasked with measuring and untangling the different magnetic signals that stem from Earth’s core, mantle, crust, oceans, ionosphere and magnetosphere – an undertaking that will take at least four years to complete.

In doing so, the mission will provide insight into many natural processes, from those occurring deep inside the planet through to weather in space caused by solar activity. In turn, this information will yield a better understanding of why the magnetic field is weakening.

The three satellites may be identical, but to optimise sampling in space and time their orbits are different and change over the course of the mission’s life – a key aspect of the mission.

Swarm is the first mission to take advantage of ‘magnetic gradiometry’, which is achieved by two of the satellites orbiting side-by-side at a distance of about 100 km. This is used to unravel the details of the magnetic field produced by magnetised rocks in Earth’s crust.

Nils Olsen from DTU Space in Denmark said, “We are extremely satisfied with these preliminary results.

“Not only do they validate the gradiometry concept, but they also confirm the remarkable accuracy of the satellites’ absolute magnetic measurements.

The Swarm constellation also makes it much easier to monitor the changes that occur in the main field produced in the Earth’s core, which protects us from harmful charged cosmic particles.

One of the three lead proposers of the Swarm mission, Gauthier Hulot from IPG Paris, added, “Our magnetic field is largely generated by Earth’s outer core. The constellation provides detail on the way the field is changing and thereby weakening our protective shield.

“This is what will ultimately make it possible to predict the way this field will evolve over the next decades.”

Very early on in the mission, when the three satellites were orbiting much closer together, first like pearls-on-a-string and later side-by-side, with one satellite progressively reaching a slightly higher orbit and separating from the lower pair, much advantage could be taken of the GPS receivers, thermal-ion imagers and Langmuir probes, which complement the magnetometers on each satellite.

Consequently, most of the early results focus on signals produced by the very dynamic electric currents near Earth.


This sheds new light onto the behaviour of the many currents flowing within the ionosphere and along the connections to the magnetosphere.

It is also leading to a better understanding of the dynamics of small-scale structures of ionospheric plasma, the very type of phenomena related to space weather that limit GPS positioning and radio transmissions.

All this, however, is only the first in what is likely to be a long list of results.

Roger Haagmans, ESA’s Swarm Mission Scientist, said, “The Swarm satellites will be in orbit for another three years at least. While the data accumulate, scientists will no doubt find other novel ways of making the best of the mission.

“New science has already emerged, for example, a joint analysis of data from Swarm and ESA’s Space Science Cluster mission has been published.”

“For the time being, the priority is to make sure that the science community can take advantage of the first results of the mission,” remarks Nils Olsen, who leads the Swarm Satellite Constellation Application and Research Facility, a consortium of European, US and Canadian institutes in charge of producing advanced models of the various sources of the geomagnetic field from Swarm data.

Cherik: One Lazy Saturday Morning

So @aurelia-which-means-sunrise asked for Cherik, a lazy Saturday morning in bed.

In my head, Erik has difficulty doing lazy, but for Charles he tries his best.

———
Charles, although still asleep, registered the loss of warmth that had just minutes ago been blanketing his backside. He half-stirred and rolled towards Erik’s side of the bed, already beginning to cool. He sighed and curled into the mattress, chasing that cocoon-like feeling. He reached out with his mind, tracing Erik’s path out of the mansion.

Erik, you promised.

I know. Go back to sleep, Charles.

Charles followed along with his consciousness, Erik allowing him to see through his eyes. The sun hadn’t yet risen, not even dawn, and the fall air was crisp, and cold. Erik’s breath froze in the misty morning. Charles shivered in the bed and pulled his blanket up under his chin. He sighed again, while the steady rhythm of Erik’s footfalls hitting the gravel path lulled him back to sleep.

A short time later, Charles stirred for the second time. He could feel Erik’s lips, cold from his frosty morning run, pressed against his temple. Charles smiled against his pillow, and with eyes still closed, reached out with greedy hands trying to find and pull Erik close.

“Come back to bed,” he whined with a trace of petulance.

Erik laughed and deftly unwound Charles’ fingers from where they were tangled in the hem of his sweatshirt, tugging at him halfheartedly. “Soon. I will.”

Charles nodded wordlessly as Erik slipped away. A minute later he heard the shower running, and he suddenly became aware of a delicious aroma wafting his way. He opened one eye to see his favorite mug on the night table, steaming, hot, Earl Grey by the scent. Erik had brought him tea. Erik was a lovely person—even if he was incapable of sleeping in.

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