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Voyager probes left our solar system years ago, but even as they travel through interstellar space, they still detect bursts of cosmic rays from our Sun, more than 23 billion kilometers (14 billion kilometers) away. miles).
A detailed analysis of recent data from Voyager 1 and Voyager 2 has now revealed the first bursts of cosmic-ray electrons in interstellar space.
Carried to the far reaches of our solar system by shock waves from solar flares called coronal mass ejections, these energized particles seem to accelerate even beyond the boundaries 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 borders of our solar system where the solar wind is strongest.
“[But] nobody saw it with an interstellar shock wave, in a whole new virgin environment, ”he adds.
The surface of our Sun constantly emits solar wind – a flow of charged particles in the form of plasma, which generates an accompanying magnetic field. It is difficult to define the limits of our solar system, but the “bubble” created by the solar wind and the material it carries is called the heliosphere.
Finally, this solar wind, having traveled through every planet and object of our solar system, spreads in the interstellar medium. This is what largely defines the limits of our solar system.
Beyond the Sun’s magnetic field, in the cold of interstellar space where conditions are very different, it is not known what happens to the solar plasma and the cosmic rays that manage to go this far when transported by a shock wave.
Voyager probes finally give 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 starts, they say, with a massive eruption on the surface of the Sun, which sends a quasi-spherical shock wave into the solar system.
When an energy wave followed by a coronal mass ejection plasma reaches interstellar space, the shock wave propels higher energy cosmic rays to strike the tangent magnetic field generated by the wave, and another shock reflects them and accelerates them into the higher energy state, as detected by Voyager.
The plasma heats the low energy electrons which then propagate along the magnetic fields. In some cases, Voyagers’ data suggests it took up to a month for the plasma to even catch up with the rushing shock wave.
This upstream region is what scientists are now calling “the cosmic-ray clash,” and the team believe it occurs just behind the magnetic field line of interstellar space, as shown below.
The counter-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 propagating outward from solar energy events in the Sun,” explains Gurnett.
“It’s a new mechanism.”
It’s an exciting find that fits well with other recent data. Since crossing the heliosphere, Voyager probes have returned measurements that suggest that there is a stronger magnetic field beyond the heliopause than we thought – perhaps enough for the electrons to leave. before a shock wave rebound and accelerate further.
“We interpret these high-energy electron bursts as resulting from the reflection (and acceleration) of relativistic cosmic-ray electrons at the time of the shock’s first contact with the interstellar magnetic field line passing through the craft. spatial “, conclude the authors.
Understanding the physics of cosmic radiation and solar shock waves will not only help us better define the boundaries of our own solar system, but also better understand the explosion of stars and the threat of radiation in space.
After more than four decades of work, NASA’s longest space mission still teaches us a lot.
The study was published in the The astronomical journal.
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