Earth raises shield against sun’s fury

SPACESHIP Earth can bring up a plasma shield to protect itself from solar storms. For the first time, satellites and ground-based detectors have watched as a tendril of plasma unfurls from a reservoir around the planet and creates a protective buffer. The discovery confirms a longstanding theory about Earth’s defences and offers us a way to “Earth doesn’t just sit  there and take whatever the solar wind gives it.  It can actually fight back” keep track of the phenomenon. “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. Earth’s magnetic field also  traps gas escaping from the upper atmosphere after it gets ionised by ultraviolet light from the sun. The resulting plasma forms a doughnut-shaped ring around the planet, called the plasmasphere (see diagram, below). Previous observations showed that plumes sometimes emerge from this region, and theory had suggested that the plumes are born when  an extra-strong electric field  from the sun rips material away.  If a plume reaches the boundary between the earthly and solar magnetic fields, the theory  goes, it can create a buffer zone  of dense material that 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, sensors on the ground mapped electrons in the upper atmosphere and saw a tendril of more densely packed particles  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 study solar storms, crossed through the magnetic boundary. The craft saw a 100-fold increase in the number of electrons at the boundary, which the team thinks were deposited there by the plume (Science, doi.org/rs3). “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, and ground-based observations will be vital for understanding when they are likely to form. “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".