A study published in the journal Science Advances reports new measurements from NASA's Van Allen Probes spacecraft. The measurements show electrons that are capable of achieving ultra-relativistic energies. This phenomenon was observed only under specific conditions devoid of plasma, report scientists Hayley Allison and Yuri Shprits from the German Research Centre for Geosciences.
Ultra-relativistic energies refer to particles that almost reach velocities equivalent to the speed of light. When energies have such high velocities, they can easily destroy sensitive electronics such as satellites and other infrastructure. Because of this, understanding the conditions under which they occur is key to protecting against unnecessary damage. Watch the video below to learn more about how ultra-relativistic energies work.
This new discovery explains why high electron energies do not occur in all solar storms, as such energies require an essentially plasma-less environment. "This study shows that electrons in the Earth's radiation belt can be promptly accelerated locally to ultra-relativistic energies if the conditions of the plasma environment - plasma waves and temporarily low plasma density - are right,” explains Shprits, who is also a professor at the University of Potsdam.
“The particles can be regarded as surfing on plasma waves. In regions of extremely low plasma density, they can just take a lot of energy from plasma waves. Similar mechanisms may be at work in the magnetospheres of the outer planets such as Jupiter or Saturn and in other astrophysical objects"
The measurements that the team took came from data from the Van Allen Probes. Using machine learning, they showed the occurrence of solar storms that both did and didn’t produce ultra-relativistic electrons. From there the team determined that plasma density was the main influence in whether electrons reached high acceleration or not. Only storms with five to ten times lower plasma density produced electrons accelerated up to over seven million electron volts.
"Thus, to reach such extreme energies, a two-stage acceleration process is not needed, as long assumed - first from the outer region of the magnetosphere into the belt and then inside. This also supports our research results from last year," concludes Hayley Allison.