Back in 2008, NASA's Interstellar Boundary Explorer (IBEX) space telescope spotted a thin ribbon of sky pelting us with particles – far more than the rest of space.
Now, a paper in Astrophysical Journal Letters claims these particles originally came shooting from the Sun, and rebounded off the outer edge of our Solar System to hit the telescope.
The study, led by Eric Zirnstein, a space scientist at the Southwest Research Institute in San Antonio, Texas, also analysed the direction of the particles, telling us about the gigantic magnetic fields pressing against our part of the galaxy.
The calculations concur with observations made by the Voyager 1 probe as it glided out of the Solar System in 2012.
Earth and everything in the Solar System are bathed in the heliosphere, a huge bubble formed by the constant stream of particles flying off the Sun called the solar wind.
The extent of the solar wind's reach marks the boundary of the Solar System. Called the heliopause, the edge is twice as far from the Sun as Pluto's orbit.
But the heliopause isn't a nice, defined line. Rather, it's a region where the solar wind interacts with the stuff outside the heliosphere – such as plasma and magnetic fields – called the interplantary medium.
The strength and direction of interstellar magnetic fields are mysteries. Only the Voyager 1 probe has ventured out of the Solar System, so astrophysicists must use indirect measures to gauge what's out there.
Bouncing particles off the inner edge of the heliopause, called the terminal shock, is one way. And what better particle source than the Sun?
The IBEX telescope, which takes images of space using particles instead of light, launched in 2008. And shortly afterwards, it detected a swathe of sky pelting us with strangely high levels of particles.
Astrophysicists think some protons from the Sun, which have a positive charge, picked up an electron as they hurtled towards the heliopause. Now neutral, they slipped through the heliopause into the interstellar medium.
But once out there, if they dropped that electron, their new-found positive charge became tangled up in the interstellar magnetic fields.
Jostled around, the proton might grab hold of another electron – and if that happened at the right time and place, the whole neutral particle would be flung back into the Solar System. Some hit IBEX.
By measuring those particles' direction of travel, astrophysicists could tell from which part of the heliopause the particles came, and the interstellar magnetic field outside.
And when they ran these IBEX data through a simulation, they saw the computed magnetic field in the interstellar medium matched that measured by the Voyager 1 probe as it drifted past the termination shock, the heliosphere, and out into interstellar space.
Belinda Smith is a science and technology journalist in Melbourne, Australia.
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