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New Simulations Unravel Mystery Behind Aurora’s ‘String of Pearls’

Auroral beads

Credit: NASA’s Goddard Space Flight Center/Joy Ng

Fri, 08/14/2020 - 13:20
Jeremy Rehm

A newly developed computer model has captured the intense activity of Earth’s magnetic field at both small and large scales to address two unanswered questions: why the aurora sometimes looks like a string of shimmering pearls, and whether those “pearls” are harbingers of upcoming magnetic activity.

Developed by researchers at the NASA DRIVE Science Center for Geospace Storms (CGS), which is headquartered at the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, the team’s ability to find answers to these questions represents an unprecedented advancement in magnetospheric modeling.

“That we can now model geospace at this scale is very exciting because it’s a new capability that allows us to connect what people see 100 kilometers up in the aurora to what’s happening 50,000 kilometers away in space.” said Kareem Sorathia, a physicist at APL and the lead author on the study.

Their findings were published July 28 in Geophysical Research Letters.

Auroras

Auroras can take on an assortment of shapes, including these auroral beads, which resemble a miles-long string of pearls. These shapes offer a window to the happenings of Earth’s magnetosphere, but what causes them is often an enigma.

Credit: Vincent Guth on Unsplash

A comparison of a black-and-white image of auroral beads taken by NASA’s THEMIS All-Sky Imager (ASI) at Fort Yukon, Alaska, on Feb. 15, 2008
A comparison of a black-and-white image of auroral beads taken by NASA’s THEMIS All-Sky Imager (ASI) at Fort Yukon, Alaska, on Feb. 15, 2008, with a snapshot taken from the CGS team’s simulations of auroral bead formation using the magnetosphere simulation tool GAMERA.

Credit: Kareem Sorathia/Slava Merkin/American Geophysical Union

To the common spectator, auroras are mostly a ghostly dance of color in the sky. But to researchers, the aurora’s twisting shapes and sometimes turbulent activities are a figurative window into the happenings of Earth’s magnetosphere.

Particularly curious have been the relatively small, bead-like shapes the aurora takes on before bright, auroral shows at Earth’s higher latitudes. These shows, called auroral substorms, occur when Earth’s magnetic field lines, stretched behind Earth by the Sun’s solar wind like a tail, link together and slingshot back toward Earth, carrying charged particles that create the light show.

Whether auroral beads are a type of canary in the coal mine, portending an oncoming substorm, or whether they’re spectators that happen to appear for the show has been an enduring mystery.

By combining high-resolution models of Earth’s magnetosphere and upper atmosphere in the Grid Agnostic Magnetohydrodynamics for Extended Research Applications (GAMERA) tool, and using some of the world’s largest supercomputers, the authors captured the interplay between processes in the upper atmosphere and the more distant magnetotail.

Their simulations were able to recreate these auroral beads and showed they’re just along for the ride.

“In fact, we find that the configuration to create these beads can be quite common and appear at different times,” said physicist Slava Merkin, a coauthor on the study and the director of CGS.

Simulation overview, looking at Earth (small black and white circle) from an equatorial view (Sun is to the right). The simulation shows in unprecedented detail the magnetic fluctuations that occur as the Sun’s solar wind forces Earth’s magnetosphere behind the planet to create Earth’s magnetotail. At 30 minutes into this growth phase (~10 seconds into video), structures called ballooning-interchange instabilities appear. These cog-like structures in the magnetosphere in turn create so-called “entropy bubbles” deeper in Earth’s ionosphere. In the aurora, those bubbles appear as beads. The simulation continues until the magnetotail slingshots back (~30 seconds into video), which is evident when “bursty” bubbles of plasma stream back toward Earth like lava in a lava lamp.

Credit: Kareem Sorathia/Slava Merkin/American Geophysical Union

Auroral beads seem to appear when lighter plasma “bubbles,” called entropy bubbles, form from imbalances in the magnetic field of Earth’s magnetotail. Like in a fizzy soda, these lighter bubbles move by buoyancy toward Earth, appearing in the aurora as beads.

These beads can form at various times, but the conditions preceding the onset of a substorm — what Merkin dubbed the “calm before the substorm,” when the magnetosphere is stretching behind Earth — seem to be just right to make the beads brighter than usual.

“So much happens in that ‘calm before the substorm’ that it’s been hard to disentangle what happens when and what causes what,” Sorathia said, noting that that challenge is part of what has made the center’s modeling capabilities valuable. “The scale of a few tens of kilometers to millions of kilometers in the magnetosphere — nobody’s been able to do that before in a single simulation.”

CGS is part of NASA’s Diversity, Realize, Integrate, Venture, Educate (DRIVE) Science Centers initiative, which aims to foster collaborative science by establishing multi-institutional centers that can address research challenges in space and solar physics. APL joins the National Center for Atmospheric Research, Rice University, Virginia Tech, the University of New Hampshire, SRI International and others as part of CGS.

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The Applied Physics Laboratory, a not-for-profit division of The Johns Hopkins University, meets critical national challenges through the innovative application of science and technology. For more information, visit www.jhuapl.edu.

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