Although Voyager probes left our solar system years ago, as they traveled through international space, they are still searching for cosmic rays exploding 23 billion kilometers (14 billion miles) away from our sun.
A detailed analysis of recent data from both Voyager 1 and Voyager 2 has now led to the first explosion of a cosmic ray electron in international space.
As the shock waves of solar explosions known as coronal mass ejections are carried along the shores of our solar system, these excited particles appear in motion even outside the boundaries of our sun’s powerful winds.
“The idea of accelerating shock wave particles is not new,” notes Don Garnett, an astrophysicist at the University of Iowa.
He says similar processes have been observed in the boundaries of our solar system where the solar wind is the most powerful.
“[But] “No one has seen it in a completely natural medium, with interstellar shock waves,” he adds.
The surface of our sun constantly emits solar wind – the flow of charged particles in the form of plasma, which produces a magnetic field with it. The boundaries of our solar system are difficult to determine, but the ‘bubble’ created by the solar wind and the material it carries is called the heliosphere.
Ultimately, these solar winds, after traveling past each of the planets and past objects in our solar system, scatter in the interstellar medium. This is what defines most of the boundaries of our solar system.
Outside of the sun’s magnetic field, in the cold of interstellar space where conditions are very different, it is not clear what happens to solar plasma and cosmic rays, when this arrangement is made due to the shock wave moving forward.
Voyager probes are finally giving us a chance to discover more. Astronomers are now proposing a new model for what happens to this shock wave in international space.
Large explosions occur on the surface of the sun, they say, sending semi-circular shocks into the solar system.
When a wave of energy followed by a plasma from a coronal mass ejection reaches international space, the shock wave energy high energy radiation pushes the cosmic rays to hit the tangent magnetic field produced by the wave, and the second shock reflects them and gives them more energy. Because discovered by Voyager.
Plasma heats low energy electrons which then propagate into magnetic fields. In some cases, Voyager’s data suggest that the shock wave for plasma took about a month to accelerate.
This upstream field is what scientists now call a ‘cosmic-ray forshock’, and the team thinks it occurs behind the star’s magnetic field, as shown below.
Foreshock model. (Gurnett et al., Astronomical Journal, 2020)
“We have identified by cosmic ray instruments that these are the electrons that reflected and accelerated the propagation of the sun’s icicular solar phenomena through the shocks on the star,” says Gurnett.
“It’s a new mechanism.”
It’s an exciting discovery that fits well with other recent data. After crossing the heliosphere, Voyager probes have returned parameters that indicate that there is a stronger magnetic field than heliopause that is greater than we thought – manipulating and advancing as much as possible for the electrons in front of the shock wave.
“We interpret these explosions of high-energy radiation electrons arising from the reflection (and acceleration) of a relative cosmic-ray electron at the time of the first contact of the shock with the interstellar magnetic field line passing through the spacecraft,” the authors conclude.
Understanding the physics of cosmic radiation and solar shock waves will not only help us better define the boundaries of our own solar system, it will also help us understand the dangers of exploding stars and radiation in space.
After more than four decades on the job, NASA’s longest-running space mission is still teaching us a lot.
The study was published in Astronomical Journal.
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