Friday, September 20, 2019

How the Sun Gets It's Spots

For 400 years people have tracked sunspots, the dark patches that appear for weeks at a time on the sun's surface. They have observed but been unable to explain why the number of spots peaks every 11 years.

A University of Washington study published this month in the journal Physics of Plasmas proposes a model of plasma motion that would explain the 11-year sunspot cycle and several other previously mysterious properties of the sun.

Sunspots can be seen on this image of solar radiation. Each sunspot lasts a few days to a few months, and the total number peaks every 11 years. The darker spots accompany bright white blotches, called faculae, which increase overall solar radiation.

Credit: NASA/Goddard/SORCE

"Our model is completely different from a normal picture of the sun," said first author Thomas Jarboe, a UW professor of aeronautics and astronautics. "I really think we're the first people that are telling you the nature and source of solar magnetic phenomena -- how the sun works."

The authors created a model based on their previous work with fusion energy research. The model shows that a thin layer beneath the sun's surface is key to many of the features we see from Earth, like sunspots, magnetic reversals and solar flow, and is backed up by comparisons with observations of the sun.

"The observational data are key to confirming our picture of how the sun functions," Jarboe said.

In the new model, a thin layer of magnetic flux and plasma, or free-floating electrons, moves at different speeds on different parts of the sun. The difference in speed between the flows creates twists of magnetism, known as magnetic helicity, that are similar to what happens in some fusion reactor concepts.

"Every 11 years, the sun grows this layer until it's too big to be stable, and then it sloughs off," Jarboe said. Its departure exposes the lower layer of plasma moving in the opposite direction with a flipped magnetic field.

The so-called "butterfly diagram" shows that sunspot activity starts farther from the sun's equator and gradually moves toward the center. The cycle repeats every 11 years.

Credit: Hathaway 2019/

When the circuits in both hemispheres are moving at the same speed, more sunspots appear. When the circuits are different speeds, there is less sunspot activity. That mismatch, Jarboe says, may have happened during the decades of little sunspot activity known as the "Maunder Minimum."

"If the two hemispheres rotate at different speeds, then the sunspots near the equator won't match up, and the whole thing will die," Jarboe said.

"Scientists had thought that a sunspot was generated down at 30 percent of the depth of the sun, and then came up in a twisted rope of plasma that pops out," Jarboe said. Instead, his model shows that the sunspots are in the "supergranules" that form within the thin, subsurface layer of plasma that the study calculates to be roughly 100 to 300 miles (150 to 450 kilometers) thick, or a fraction of the sun's 430,000-mile radius.

"The sunspot is an amazing thing. There's nothing there, and then all of a sudden, you see it in a flash," Jarboe said.

The group's previous research has focused on fusion power reactors, which use very high temperatures similar to those inside the sun to separate hydrogen nuclei from their electrons. In both the sun and in fusion reactors the nuclei of two hydrogen atoms fuse together, releasing huge amounts of energy.

In the model presented in the new paper the red line shows the flow of electrons, or plasma, and the yellow line shows the sun's surface. The X enclosed by a circle shows magnetic field, with the electromagnetic field highest near the sun's equator. Over time the electromagnetic field wears down at the surface and the outer layer of red sloughs off into outer space, exposing the inner layer that flows in the opposite direction.

Credit: Jarboe et al./Physics of Plasmas

The type of reactor Jarboe has focused on, a spheromak, contains the electron plasma within a sphere that causes it to self-organize into certain patterns. When Jarboe began to consider the sun, he saw similarities, and created a model for what might be happening in the celestial body.

"For 100 years people have been researching this," Jarboe said. "Many of the features we're seeing are below the resolution of the models, so we can only find them in calculations."

Other properties explained by the theory, he said, include flow inside the sun, the twisting action that leads to sunspots and the total magnetic structure of the sun. The paper is likely to provoke intense discussion, Jarboe said.

"My hope is that scientists will look at their data in a new light, and the researchers who worked their whole lives to gather that data will have a new tool to understand what it all means," he said.

The research was funded by the U.S. Department of Energy. Co-authors are UW graduate students Thomas Benedett, Christopher Everson, Christopher Hansen, Derek Sutherland, James Penna, UW postdoctoral researchers Aaron Hossack and John Benjamin O'Bryan, UW affiliate faculty member Brian Nelson, and Kyle Morgan, a former UW graduate student now at CTFusion in Seattle.

Contacts and sources:
Hannah Hickey
University of Washington

Extinction of Large Mammal Species Likely Drove Survivors Apart

hen a series of large mammal species began going extinct roughly 12,000 years ago, many surviving species began going their separate ways, says new research led by Macquarie University and the University of Nebraska–Lincoln.

Published Sept. 20 in the journal Science, the study analyzed distributions of mammal fossils across North America following the last ice age, after the retreat of massive glaciers that had encroached south to the modern-day United States. The aftermath saw the disappearance of many famously large mammal species: mammoths, mastodons, saber-toothed cats, dire wolves and ground sloths, among others.

The University of Nebraska–Lincoln's Kate Lyons stands next to a skeleton of Gigantocamelus spatulus, an extinct species of camel that weighed upward of 5,000 pounds and once roamed North America. The skeleton resides in the Nebraska State Museum at Morrill Hall.
Credit: Craig Chandler | University Communication

Surviving mammal species often responded by distancing themselves from their neighbors, the study found, potentially reducing how often they interacted as predators and prey, territorial competitors or scavengers.

The ecological repercussions of the extinctions are likely still echoing today and could preview the effects of future extinctions, said study co-author Kate Lyons.

“For 300 million years, the (cohabitation) pattern of plants and animals looked one way — and then it changed in the last 10,000 years,” said Lyons, assistant professor of biological sciences at Nebraska. “This paper addresses how that happened in mammal communities.

“If connectedness among species makes ecosystems more stable, what this suggests is that we’ve already lost a lot of those links. What this potentially tells us is that modern ecosystems are probably more vulnerable than we think they are.”

The extinction of mammoths and other large mammal species disrupted ecosystem dynamics across North America, according to new research published in the journal Science.

Credit:  Shutterstock / Scott Schrage | University Communication

Led by Macquarie’s Anikó Tóth, the team analyzed records of 93 mammal species at hundreds of fossil sites during three timespans: 21,000 to 11,700 years ago, when the extinctions began; 11,700 to 2,000 years ago; and 2,000 years ago to the present. The researchers then assessed whether, and to what extent, a given species lived among each of the other 92 at those sites.

That data allowed the team to calculate how often a random pair of species would be expected to cohabit a site, providing a baseline for whether each pair overlapped more or less often than predicted by chance — aggregating vs. segregating, respectively. The proportion of aggregating pairs generally declined following the extinctions, and the strength of associations often dropped even among species that continued to aggregate, the researchers found.

“The loss of the giant carnivores and herbivores changed how small mammals such as deer, coyotes and raccoons interacted,” Tóth said. “Our work suggests that these changes were triggered by the ecological upheaval of the extinctions.”

Tóth, Lyons and their 17 co-authors effectively ruled out climate change and geography as drivers of the growing division. Surprisingly, the team also concluded that surviving species began cohabiting less frequently even as they expanded into larger swaths of their respective geographic ranges.

A life-sized display of Archie, a Columbian mammoth, at the University of Nebraska State Museum at Morrill Hall.

Credit: Troy Fedderson | University Communication

Lyons said the specific reasons for the seeming paradox and the overall trends are unclear, though the ecological consequences of losing species such as the mammoth could explain them. Mammoths toppled trees, compacted soil and, by eating and excreting masses of vegetation, transported nutrients around ecosystems, Lyons said. Those behaviors helped sustain the so-called mammoth steppe, an ecosystem type that once covered vast areas of the Northern Hemisphere. The loss of the mammoth effectively doomed the mammoth steppe, possibly compartmentalizing the expanses of land that hosted many species.

“If you're an open-habitat species that used to occupy the mammoth steppe, and now the mammoth steppe has gone away, you might inhabit, say, open grassland areas that are surrounded by forests,” Lyons said. “But that meadow is much smaller. Instead of supporting 10 species, it now might support five. And if those patches of open habitat are spread farther apart, you might expand your geographic range and potentially your climate range, but you would co-occur with fewer species.”

Also uncertain: why common species became more common, and some rare species became even rarer, following the extinctions. Continuing to study the dynamics underlying such trends could help sharpen perspectives on current ecosystems and their possible fates, the researchers said.

“We had a complement of large mammals in North America that was probably more diverse than what we see in Africa today,” Lyons said. “Additional extinctions could have a cascading effect and huge implications for the mammal communities that we have left.”

Tóth, Lyons and their co-authors represent 18 institutions from Australia, the United States, Chile, Portugal, Finland, Canada and Denmark. All are members of the Evolution of Terrestrial Ecosystems Program at the Smithsonian Institution, which funded the team’s research.

Contacts and sources:
Kate Lyons
University of Nebraska Lincoln