When you look up at the sky, the region of space around the Earth may seem as clear as a song, but there are a lot of things going on that we can’t see. In recent years, probes studying the radiation trapped by the Earth’s magnetic field have found something strange – electrons spinning at the speed of light.
That alone is not the weird part; near-light or relativistic speed electrons are well known in the cosmos, stimulated by cosmic particle accelerators. The only thing was that from time to time it was very fast, ultrarelativistic electrons appear – but only during solar storms, not others.
A team of scientists led by space physicist Hayley Allison from the German GFZ Center for Geosciences in Germany has just found out why. And it all has to do with the bands of invisible radiation, full of particles, wrapped around the Earth.
The researchers found that only if the plasma was significantly depleted in a radiation belt before a solar storm could reach those ultra-relativistic speeds.
Officially known as Van Allen radiation belts, these belts are located 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 captures charged particles from the solar wind.
Here on Earth, these regions will not visibly affect our daily lives (although we would certainly notice if they were removed and the solar wind could cover us freely with charged particles), but the region of space immediately around the planet, at an altitude of about 2,000 kilometers, is where we placed most of our satellites. Here it could be useful to know what kind of space weather can produce ultra-relativistic electrons.
When accelerated at such high speeds, these electrons become a danger. Due to their high energies, not even the best protection can keep them out, and charging them as they enter the spacecraft can destroy sensitive electronics.
So Allison and her team began analyzing data from the Van Allen probes, the double spacecraft launched to study the Van Allen belts in 2012 (before being deactivated in 2019).
During this time, the probes recorded several solar storms, intense events in which a burst from the Sun buffers the Earth’s magnetosphere with solar wind and radiation.
They were looking to find out why some of these storms led 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” plasma waves like a wakesurfer that uses water waves to accelerate.
And solar storms are known to excite plasma waves around the Earth; in fact, the Van Allen probes contributed to the discovery that so-called “cor” mesh waves around the Earth could accelerate electrons, although the effect alone was considered insufficient to explain the observed ultra-relativistic electrons. The researchers believed that there must be some kind of two-step acceleration process.
Therefore, the team compared the plasma observations made by Van Allen probes with solar storms, both with and without ultra-relativistic electrons, in an attempt to find out what was happening.
Plasma density is difficult to measure directly, but the team was able to deduce the density from fluctuations in electric and magnetic fields. The researchers also found that ultra-relativistic electrons correlated with both extreme depletion of plasma density and the presence of chorus waves.
It is a result that shows that a two-stage acceleration process, as previously thought responsible, is not necessary for ultra-relativistic electrons.
Although the team focused on the most extreme electron velocities, they also found that when plasma density was lower, chorus waves accelerated electrons to relativistic speeds on shorter time scales than when plasma density was higher.
“This study shows that electrons in the Earth’s radiation belt can be rapidly accelerated locally to ultra-relativistic energies if the conditions of the plasma environment – plasma waves and temporarily low plasma density – are correct,” said physicist Yuri Shprits of the GFZ German Center for Geosciences. and the University of Potsdam in Germany.
“Particles can be seen as navigating plasma waves. In regions with 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 such as Jupiter or Saturn in other astrophysical objects . “
The research was published in Scientific advances.