If you look at the sky, the area of space around the Earth might look as bright as a song, but there’s a lot going on out there that we can’t see. In recent years, probes studying radiation captured by Earth’s magnetic field have found something peculiar: electrons that are carried along at nearly the speed of light.
That alone is not the odd part; near-speed of light, or relativistic, electrons are well known in the cosmos, stimulated by cosmic particle accelerators. The strange thing was that sometimes extra fast, ultrarelativistic electrons appear – but only during some solar storms and not others.
A team of scientists led by space physicist Hayley Allison from the GFZ German Center for Geosciences in Germany just figured out why. And it all has to do with invisible, particle-filled radiation belts wrapped around the Earth.
Only when plasma in a radiation belt is significantly depleted before a solar storm can electrons reach those ultra-relativistic speeds, the researchers found.
Officially known as Van Allen radiation belts, these belts are located in space almost directly around the Earth. The inner tube extends from 640 to 9,600 kilometers (400 to 6,000 miles) in altitude, and the outer belt from about 13,500 to 58,000 kilometers. What they are are regions in which Earth’s magnetic field traps charged particles from the solar wind.
Here on Earth, these areas have no noticeable impact on our daily life (although we would certainly notice if they left and the solar wind could freely pelt us with charged particles), but the area of space immediately around the planet, at an altitude from about 2,000 miles, is where we place most of our satellites. This is where it might be useful to know what kind of space weather can produce ultra-relativistic electrons.
When accelerated to such high speeds, these electrons become a hazard. Due to their high energy, even the best shielding cannot keep them out, and their charge when they enter spacecraft can destroy sensitive electronics.
So Allison and her team started analyzing data from the Van Allen probes, the twin spacecraft launched in 2012 to study the Van Allen belts (before deactivating in 2019).
During this time, the probes recorded several solar storms, intense events where an eruption from the sun buffers the Earth’s magnetosphere with solar wind and radiation.
They were looking to discover why some of these storms resulted in ultra-relativistic electrons and others not. In particular, they wanted to examine the plasma.
Plasma waves – fluctuations in the electric and magnetic fields – are known to have an accelerating effect on electrons, which can “surf” the plasma waves like a wake surfer uses water waves to accelerate.
And solar storms are known to generate plasma waves around the Earth; in fact, the Van Allen probes contributed to the discovery that the so-called “chorus” plasma waves around Earth can accelerate electrons, although the effect alone was considered insufficient to explain the observed ultra-relativistic electrons. Researchers thought there had to be some sort of two-step acceleration process.
So the team compared plasma observations from the Van Allen probes to the solar storms, both with and without ultra-relativistic electrons, in an attempt to find out what was going on.
The plasma density is difficult to measure directly, but the team was able to deduce the density from the fluctuations in the electric and magnetic fields. And the researchers found that the ultra-relativistic electrons correlated with both extreme plasma density depletion and the presence of refrain waves.
It is a result showing that a two-phase acceleration process, as previously thought responsible, is not required for ultra-relativistic electrons.
While the team focused on the most extreme electron velocities, they also found that when the plasma density was lower, the chorus waves accelerated electrons to relativistic speeds on shorter time scales than when the plasma density was higher.
“This study shows that electrons in the Earth’s radiation belt can be instantly locally accelerated to ultra-relativistic energies, if the conditions of the plasma environment – plasma waves and temporarily low plasma density – are right,” explains physicist Yuri Shprits of GFZ German Center for Geosciences. from. and the University of Potsdam in Germany.
“The particles can be thought of as surfing plasma waves. In regions of extremely low plasma density, they can simply draw a lot of energy from plasma waves. Similar mechanisms may be at work in the magnetospheres of outer planets such as Jupiter or Saturn and in other astrophysical objects. “
The research is published in Science Advances.