When you look at the sky, the region of space around the Earth may seem as clear as a song, but there are a lot of things we can’t see. In recent years, probes studying the radiation trapped by the Earth’s magnetic field have found something peculiar: electrons approaching the speed of light.
That’s just not the quirky part; near-light or relativistic velocity electrons are well known in the cosmos, driven by cosmic particle accelerators. The peculiar thing was that from time to time it was very fast, ultrarelativistic electrons appear, but only during some solar storms and not others.
A team of scientists led by space physicist Hayley Allison of Germany’s GFZ German Center for Geosciences has just figured out why. And it all has to do with invisible and particle-filled radiation belts that surround the Earth.
Researchers have found that only if the plasma has been significantly depleted in a radiation belt before the solar storm can reach these ultra-relativistic speeds.
Officially known as Van Allen radiation straps, these straps are in the pocket of the space that almost immediately surrounds the Earth. The inner belt stretches 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 where the Earth’s magnetic field traps particles charged by the solar wind.
Here on Earth, these regions will not significantly affect our day-to-day lives (although we would certainly realize that if they left and the solar wind could free us from charged particles), but the region of space immediately in the around the planet, at an altitude of about 2,000 kilometers, is where we place most of our satellites. Here it can be useful to know what kind of space-time ultra-relativistic electrons can produce.
When they accelerate at such high speeds, these electrons become a danger. Due to their high energies, not even the best shields can keep them out, and their charge when they penetrate ships can destroy sensitive electronics.
So Allison and her team set about analyzing data from the Van Allen probes, a twin spacecraft launched to study the Van Allen belts in 2012 (before being shut down in 2019).
During this time, the probes recorded several solar storms, intense events in which an eruption of the Sun buffered the Earth’s magnetosphere with solar wind and radiation.
They wanted to find out why some of these storms gave rise to ultra-relativistic electrons and others did not. In particular, they wanted to examine the plasma.
Plasma waves (fluctuations in electric and magnetic fields) are known to have an accelerating effect on electrons, which can “navigate” the plasma waves in the same way that a wakesurfer uses water waves to accelerate.
And solar storms are known to excite plasma waves around the Earth; in fact, Van Allen probes contributed to the discovery that plasma waves called “hearts” around the Earth can accelerate electrons, although the effect alone was considered insufficient to explain the observed ultra-relativistic electrons. The researchers thought there had to be some kind of acceleration in two steps.
Thus, the team compared the plasma observations taken by Van Allen probes with solar storms, both with and without ultra-relativistic electrons, in an attempt to find out what was going on.
Plasma density is difficult to measure directly, but the equipment was able to infer density from fluctuations in electric and magnetic fields. And the researchers found that ultra-relativistic electrons correlated with extreme depletion of plasma density and the presence of heart waves.
It is a result that shows that a two-stage acceleration process, as previously considered responsible, is not required for ultra-relativistic electrons.
Although the team focused on the most extreme electron velocities, they also found that when plasma density was lower, heart waves accelerated electrons at relativistic speeds at shorter time scales than when plasma density she was taller.
“This study shows that the electrons in the Earth’s radiation belt can accelerate rapidly locally to ultra-relativistic energies, if the conditions of the plasma environment (plasma waves and temporarily low plasma density) are correct,” he explained. physicist Yuri Shprits from the GFZ German Center for Geosciences and the University of Potsdam in Germany.
“Particles can be considered surfacing in plasma waves. In regions of extremely low plasma density they can only take a lot of energy from plasma waves. Similar mechanisms may be working in the magnetospheres of outer planets like Jupiter or Saturn and other objects. astrophysicists “.
The research has been published in Scientific advances.