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Voyager probes left our Solar System years ago, but even as they travel through interstellar space, they are still detecting bursts of cosmic rays from our Sun, more than 23 billion kilometers (14 billion miles) away.
A detailed analysis of recent data from both Voyager 1 and Voyager 2 has now revealed the first electron bursts of cosmic rays in interstellar space.
Carried to the fringes of our Solar System by the shock waves of solar flares known as coronal mass ejections, these energized particles appear to accelerate even beyond the confines of our Sun’s powerful winds.
“The idea that shock waves accelerate particles is not new,” notes University of Iowa astrophysicist Don Gurnett.
He says similar processes have been observed within the confines of our Solar System, where the solar wind is most powerful.
“[But] no one has seen it with an interstellar shock wave, in a completely new and pristine medium, “he adds.
The surface of our Sun continuously emits solar wind, a stream of charged particles in the form of plasma, which generates an accompanying magnetic field. It is difficult to define the boundaries of our Solar System, but the “bubble” created by the solar wind and the material it carries is called the heliosphere.
Eventually, this solar wind, having traveled beyond every planet and object in our Solar System, spreads out into the interstellar medium. This is what largely defines the boundaries of our solar system.
Beyond the Sun’s magnetic field, in the cold of interstellar space where conditions are very different, it is not clear what happens to the solar plasma and cosmic rays that manage to travel so far when carried on a shock wave.
The Voyager probes are finally giving us the opportunity to learn more. Astronomers are now proposing a new model for what happens to these shock waves in interstellar space.
It all begins, they say, with a massive eruption on the Sun’s surface, sending an almost spherical shock wave into the Solar System.
When an energy wave followed by plasma from a coronal mass ejection reaches interstellar space, the shock wave pushes cosmic rays of higher energy to hit the tangent magnetic field generated by the wave, and another shock reflects them. and accelerates them into the higher energy state, as noted by Voyager.
The plasma heats low-energy electrons which then propagate along the magnetic fields. In some cases, the Voyager data suggests it took up to a month for the plasma to even reach the advancing shock wave.
This upstream region is what scientists now call the “cosmic-ray preliminary shock,” and the team thinks it occurs just behind the magnetic field line of interstellar space, as shown below.
The preliminary shock model. (Gurnett et al., The Astronomical Journal, 2020)
“We have identified through cosmic ray instruments these are electrons that have been reflected and accelerated by interstellar shocks that propagate outward from solar energy events to the Sun,” says Gurnett.
“This is a new mechanism.”
It’s an exciting finding that fits well with other recent data. Since crossing the heliosphere, Voyager probes have sent measurements suggesting that there is a stronger magnetic field beyond the heliopause than we thought, perhaps enough for electrons at the front of a shock wave to bounce and accelerate further.
“We interpret these high-energy electron bursts as resulting from the reflection (and acceleration) of relativistic cosmic ray electrons at the moment of the shock’s first contact with the interstellar magnetic field line passing through the spacecraft,” the authors conclude.
Understanding the physics of cosmic radiation and solar shockwaves will not only help us better define the boundaries of our Solar System, but will also help us better understand exploding stars and the threat of radiation in space.
After more than four decades of work, NASA’s longest-running space mission still teaches us a lot.
The study was published in The Astronomical Journal.
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