NASA’s THEMIS sees Auroras move to the rhythm of Earth’s magnetic field

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NASA/CSA/University of California, Berkeley/University of Calgary/NSF

An artist's rendering (not to scale) of a cross-section of the magnetosphere, with the solar wind on the left in yellow magnetic field lines emanating from the Earth in blue. The five THEMIS probes were well-positioned to directly observe one particular magnetic field line as it oscillated back forth roughly every six minutes. In this unstable environment, electrons in near-Earth space, depicted as white dots, stream rapidly down magnetic field lines towards Earth's poles. There, they interact with oxygen nitrogen particles in the upper atmosphere, releasing photons brightening a specific region of the aurora.

Emmanuel Masongsong/UCLA EPSS/NASA

The majestic auroras have captivated humans for thousands of years, but their nature — the fact that the lights are electromagnetic respond to solar activity — was only realized in the last 150 years. Thanks to coordinated multi-satellite observations a worldwide network of magnetic sensors cameras, close study of auroras has become possible over recent decades. Yet, auroras continue to mystify, dancing far above the ground to some, thus far, undetected rhythm. Using data from NASA’s Time History of Events Macroscale Interactions during Substorms, or THEMIS, scientists have observed Earth’s vibrating magnetic field in relation to the northern lights dancing in the night sky over Canada. THEMIS is a five-spacecraft mission dedicated to understanding the processes behind auroras, which erupt across the sky in response to changes in Earth’s magnetic environment, called the magnetosphere.

These new observations allowed scientists to directly link specific intense disturbances in the magnetosphere to the magnetic response on the ground. A paper on these findings was published in Nature Physics on Sept. 12, 2016.

“We’ve made similar observations before, but only in one place at a time – on the ground or in space,” said David Sibeck, THEMIS project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who did not participate in the study. “When you have the measurements in both places, you can relate the two things together.”

Understanding how why auroras occur helps us learn more about the complex space environment around our planet. Radiation energy in near-Earth space can have a variety of effects on our satellites – from disrupting their electronics to increasing frictional drag interrupting communication or navigation signals. As our dependence on GPS grows space exploration expands, accurate space weather forecasting becomes ever more important.

The space environment of our entire solar system, both near Earth far beyond Pluto, is determined by the sun’s activity, which cycles fluctuates through time. The solar system is filled with solar wind, the constant flow of charged particles from the sun. Most of the solar wind is deflected from Earth by our planet’s protective magnetosphere.

However, under the right conditions, some solar particles energy can penetrate the magnetosphere, disturbing Earth’s magnetic field in what’s known as a substorm. When the solar wind’s magnetic field turns southward, the dayside, or sun-facing side, of the magnetosphere contracts inward. The back end, called the magnetotail, stretches out like a rubber band. When the stretched magnetotail finally snaps back, it starts to vibrate, much like a spring moving back forth. Bright auroras can occur during this stage of the substorm.

In this unstable environment, electrons in near-Earth space stream rapidly down magnetic field lines towards Earth’s poles. There, they interact with oxygen nitrogen particles in the upper atmosphere, releasing photons to create swaths of light that snake across the sky.

To map the auroras’ electric dance, the scientists imaged the brightening dimming aurora over Canada with all-sky cameras. They simultaneously used ground-based magnetic sensors across Canada Greenlto measure electrical currents during the geomagnetic substorm. Further out in space, the five THEMIS probes were well-positioned to collect data on the motion of the disrupted field lines.

The scientists found the aurora moved in harmony with the vibrating field line. Magnetic field lines oscillated in a roughly six-minute cycle, or period, the aurora brightened dimmed at the same pace.

“We were delighted to see such a strong match,” said Evgeny Panov, lead author researcher at the Space Research Institute of the Austrian Academy of Sciences in Graz. “These observations reveal the missing link in the conversion of magnetic energy to particle energy that powers the aurora.”

The brightening dimming of the aurora corresponds to the motion of the electrons magnetic field lines.

“During the course of this event, the electrons are flinging themselves Earthwards, then bouncing back off the magnetosphere, then flinging themselves back,” Sibeck said.

When waves crash on the beach, they splash froth, then recede. The wave of electrons adopt a similar motion. The aurora brightens when the wave of electrons slams into the upper atmosphere, dims when it ricochets off.

Before this study, scientists hypothesized that oscillating magnetic field lines guide the aurora. But the effect had not yet been observed because it requires the THEMIS probes to be located in just the right place over the ground-based sensors, to properly coordinate the data. In this study, scientists collected THEMIS data at a time when the probes were fortuitously positioned to observe the substorm.

“Even after nearly 10 years, the probes are still in great health, the growing network of magnetometers all-sky cameras continue to generate high quality data,” said Vassilis Angelopoulos, co-author THEMIS principal investigator at University of California, Los Angeles.

THEMIS is a mission of NASA’s Explorer program, which is managed by Goddard. University of California, Berkeley’s Space Sciences Laboratory oversees mission operations. The all-sky imagers magnetometers are jointly operated by UC Berkeley, UCLA, University of Calgary University of Alberta in Canada.

“The intention with THEMIS has always been that we would put these measurements together make these observations,” Sibeck said. “This is an extremely satisfying study a pleasure to see the right use of this mission data.”

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