by Lisa Grossman
Earth can raise shields to protect itself against solar storms. For the first time, satellites and ground-based detectors have watched as the planet sends out a tendril of plasma to fight off blasts of charged solar matter. The discovery confirms a long-standing theory about Earth's magnetic surroundings and offers us a way to keep track of the planet's defences.
"It's changed our thinking about how the system operates," says Joe Borovsky at the Space Science Institute in Boulder, Colorado, who was not involved in the research. "Earth doesn't just sit there and take whatever the solar wind gives it, it can actually fight back."
Earth is always surrounded by a bubble of magnetism called the magnetosphere, which protects us from the bulk of the solar wind, a stream of high-energy particles constantly flowing from the sun.
But sometimes, the sun's magnetic field lines can directly link up with Earth's in a process called magnetic reconnection, which opens up cracks in the magnetosphere. Charged particles can flow along these lines into Earth's atmosphere, leading to dazzling auroras as well as geomagnetic storms that can wreak havoc on navigation systems and power grids.
Gas in Earth's upper atmosphere is ionised by ultraviolet light from the sun, and the resulting plasma becomes trapped by magnetic fields in a doughnut-shaped ring around the planet. Previous observations of this plasmasphere showed that plumes sometimes emerge from this region.
Theory had suggested that an extra-strong electric field from the sun can rip plasma away from the plasmasphere during reconnection, triggering a plume. If this plume reaches the boundary between the earthly and solar magnetic fields, it would create a buffer zone of dense material. This would make it harder for magnetic field lines to meet up and spark further reconnection.
But while ground-based measurements can see a plume forming, their resolution isn't good enough to tell for sure whether the material reaches the magnetic boundary.
Brian Walsh at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and his colleagues have now clinched it. In January 2013, GPS sensors on the ground mapped electrons in the upper atmosphere and saw a tendril of increased electron density curling away from the north pole, indicating that a plume of plasma was veering off towards the sun.
At the same time, three of NASA's THEMIS spacecraft, which are designed to study solar storms, crossed through the magnetic boundary during the event. The craft saw a 100-fold increase in the number of electrons at the boundary, which would probably have been deposited by the plume.
"For the first time, we were able to monitor the entire cycle of this plasma stretching from the atmosphere to the boundary between Earth's magnetic field and the sun's," says Walsh. "It gets to that boundary and helps protect us, keeps these solar storms from slamming into us."
Not every solar storm generates a plasma plume, which means ground-based observations will continue to be vital for understanding the phenomenon.
"To measure things with spacecraft we have to have them in just the right place, but the ground stations can measure this stuff almost constantly," says Walsh. "We want to know, when does the Earth decide to protect us? By validating this tool, we're now able to figure that out."
Journal reference: Science, DOI: 10.1126/science.1247212
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