Biggest Breach of Earth’s Solar Storm Shield Discovered
“The discovery overturns a long-standing belief about how and when most of the solar particles penetrate Earth’s magnetic field, and could be used to predict when solar storms will be severe. Based on these results, we expect more severe storms during the upcoming solar cycle,” said
Earth’s magnetic field acts as a shield against the bombardment of particles continuously streaming from the sun. Because the solar particles (ions and electrons) are electrically charged, they feel magnetic forces and most are deflected by our planet’s magnetic field. However, our magnetic field is a leaky shield and the number of particles breaching this shield depends on the orientation of the sun’s magnetic field. It had been thought that when the sun’s magnetic field is aligned with that of the Earth, the door is shut and that few if any solar particles enter Earth’s magnetic shield. The door was thought to open up when the solar magnetic field direction points opposite to Earth’s field, leading to more solar particles inside the shield.
Surprisingly, recent observations by the THEMIS spacecraft fleet demonstrate that the opposite is true. “Twenty times more solar particles cross the Earth’s leaky magnetic shield when the sun’s magnetic field is aligned with that of the Earth compared to when the two magnetic fields are oppositely directed,” said Marit Oieroset of the
Researchers have long suspected that this “closed door” entry mechanism might exist, but didn’t know how important it was. “It’s as if people knew there was a crack in a levy, but they did not know how much flooding it caused,” said Oieroset.
Previous spacecraft could only sample a small part of this enormous layer of solar particles inside the Earth’s magnetic shield, but the five spacecraft in the THEMIS fleet spanned the entire rapidly-growing layer to give definitive measurements.
While the THEMIS researchers discovered the size of the leak, they didn’t know its location(s). This was discovered by
The simulation also showed how the leaks develop. As solar particles flow out from the sun, they carry solar magnetic fields past our planet. Li’s team realized that the solar magnetic field drapes against Earth’s field as it passes by. Even though the two fields point in the same direction at equatorial latitudes, they point in opposite directions at high latitudes. When compression forces the opposite fields together, they link up with each other in a process called magnetic reconnection. This process tears the two holes in Earth’s magnetic field and appends the section of the solar field between the two holes to Earth’s field, carrying the solar particles on this section into the magnetosphere, according to Li’s team. “We’ve found if the door is closed, the sun tears down a wall. The crack is huge — about four times wider than Earth and more than seven Earth diameters long,” said Li, whose paper will be published in an upcoming article of the Journal of Geophysical Research.
Solar particles by themselves don’t cause severe space weather, but they get energized when the solar magnetic field becomes oppositely-directed to Earth’s and reconnects in a different way. The energized particles then cause magnetic storms that can overload power lines with excess current, causing widespread blackouts. The particles also can cause radiation storms that present hazards to spacecraft in high orbits and astronauts passing through the storms on the way to the moon or other destinations in the solar system. “The more particles, the more severe the storm,” said
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