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Earth is surrounded by a protective magnetic environment, the magnetosphere, which deflects a supersonic stream of charged particles from the Sun, known as the solar wind. As the particles flow around the Earth’s magnetosphere, it forms a highly turbulent boundary layer called the magnetosheath. In a new discovery, reported in the May 9, 2018 issue of the journal Nature, researchers on NASA’s Magnetospheric Multiscale (MMS) mission have uncovered a new type of magnetic event — electron magnetic reconnection — in the magnetosheath region.
Our planet is surrounded by the magnetosphere, shown here in blue. As charged particles from the Sun flow around the Earth’s magnetosphere, it forms the magnetosheath, shown in yellow. Image credit: Mary Pat Hrybyk-Keith / NASA’s Goddard Space Flight Center.
The MMS was launched from the Cape Canaveral Air Force Station Space Launch Complex on March 12, 2015.
MMS consists of four identical spacecraft that orbit around Earth through the dynamic magnetic system surrounding our planet to study magnetic reconnection, a common event throughout the Universe that occurs when magnetic fields change by connecting and then breaking apart.
Now, for the first time, the MMS spacecraft have observed magnetic reconnection in a turbulent region of Earth’s outer atmosphere known as the magnetosheath.
There, they found a new breed of magnetic reconnection — electron magnetic reconnection — that is much different than the kind that happens in the much less turbulent magnetosphere closer to Earth.
“Turbulence occurs everywhere in space — on the Sun, in the solar wind, the interstellar medium, dynamos, around stars, in active galactic nuclei jets, supernova remnant shocks and more,” said team member Professor Michael Shay, from the University of Delaware.
“The turbulence in the magnetosheath contains a lot of magnetic energy,” added team leader Dr. Tai Phan, senior fellow in the Space Sciences Laboratory at the University of California at Berkeley.
“People have been debating how this energy is dissipated and magnetic reconnection is one of the possible processes.”
The energy comes directly from the Sun’s corona, a blazing hot environment that shoots particles out in all directions at speeds around 1 million miles per hour. This is the forceful solar wind. When its power hits the magnetosheath, waves of plasma chaos roll through it.
Scientists don’t know yet how all of that turbulent energy is dissipated, but the discovery of electron magnetic reconnection may help them learn more.
Dr. Phan, Professor Shay and their colleagues got what they hoped to get — evidence that magnetic reconnection was happening even in that chaotic turbulence.
But in the process, they discovered magnetic reconnection here works much differently than the kind observed elsewhere.
“Instead of huge jets of ionized hydrogen atoms, triggered by many collisions of magnetic fields, this form of magnetic reconnection shoots off much tinier electron jets with very few collisions occurring,” Professor Shay said.
This has never been recognized before, partly because no instruments could capture the process.
“The relative difference in size between the electrons and the ions is similar to the difference between ball bearings and basketballs. The electrons are harder to spot. And they are moving much faster — 40 times faster,” Professor Shay said.
“I had simulated this possible kind of reconnection. But no one had ever observed it happening in space.”
“The smoking gun evidence is to measure oppositely directed electron jets at the same time, and the four MMS spacecraft were lucky to corner the reconnection site and detect both jets,” said team member Dr. Jonathan Eastwood, of Imperial College London, UK.
The magnetosheath reconnection was too fast and too tiny for the MMS instruments to capture in the usual way, but researchers developed a new way of using one of the instruments — the Fast Plasma Investigation — that gave them the perspective they needed to see what was going on.
“The key event happens in 45 milliseconds. This would be one data point with the regular data, but instead we can get six to seven data points in that region with this method, allowing us to understand what is happening,” said team member Amy Rager, a graduate student at Catholic University of America.
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T.D. Phan et al. 2018. Electron magnetic reconnection without ion coupling in Earth’s turbulent magnetosheath. Nature 557: 202-206; doi: 10.1038/s41586-018-0091-5
