Near Earth's Equator, where solar winds press against the magnetosphere, the field lines point north.
Solar winds also carry magnetic field lines toward Earth, and those solar field lines point in different directions during the sun's 11-year cycle of activity.
Conventional thinking had suggested that north-pointing field lines would act like reinforcements to Earth's northward field, causing the planet to "raise shields" against solar winds.
The idea is based, in part, on the fact that auroras are brighter and space-weather hazards increase when solar winds carry southward-pointing field lines, Sibeck said.
"So it's reasonable to think that during periods when the sun's magnetic field lines point south, that's when the most particles get into Earth's magnetosphere."
THEMIS, however, showed that the opposite is true.
The satellite system "found the solar particle layer is much thicker when the two fields are pointing in the same direction," said Marit Øieroset, a THEMIS scientist based at the University of California, Berkeley, who first saw the effect.
In fact, 20 times more particles get through Earth's magnetic shield when the field lines are aligned than when they are opposed, she said.
To find the mechanism behind this discovery, Oieroset and Sibeck turned to computer models that could simulate the conditions observed by THEMIS.
The models showed that the likely driver is north-facing field lines connecting with Earth's magnetosphere, said Jimmy Raeder, a physicist at the University of New Hampshire in Durham who helped build the simulations.
As a field line approaches, it latches onto the poles and wraps around the planet like an octopus using a tentacle to snare its prey, he said.
The latching, known as magnetic reconnection, tears huge cracks in the magnetosphere and allows solar plasma to leak in.
"We have other observations from other satellites that this reconnection process happens over the poles at times, but we had never appreciated what it actually does," Raeder said.
A thicker layer of solar particles, however, isn't enough by itself to create geomagnetic troubles for Earth.
Right now the planet is enjoying a period of low activity called solar minimum. But particles have been building up inside the magnetosphere as the solar wind carries northward-facing field lines to Earth.
During the next solar cycle, the winds are expected to carry southward-facing field lines, which connect with the magnetosphere in such a way that they provide extra charge to any plasma inside the shield.
"You can sort of compare [the situation] to a gas stove," Raeder said.
"If you turn on the gas and you light it right away, nothing will happen—the gas stove will go on and there will be a flame.
"But if you turn on a gas stove and you don't do anything for a while and then you throw in a match, what will happen? It will say, Boom!"
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