According to research from Northumbria University, several factors, including astrophysical shock waves, are the likeliest cause of cosmic ‘particle accelerators. ‘ These have long stumped scientists due to their ability to speed up individual electrons to extreme, near-relativistic velocities.
Although several mechanisms have been proposed for creating these high-speed particles, more commonly known as cosmic rays, previous studies have focused on individual candidates. This latest research is the first to examine and ultimately identify a collection of processes working at the macroscopic and microscopic levels that collectively function like cosmic particle accelerators to launch individual electrons into interstellar space at close to the speed of light.
“Most of our research focuses on either small-scale effects like wave-particle interactions or large-scale properties like the influence of solar wind,” explained the study’s lead author, Dr Savvas Raptis of The Johns Hopkins University Applied Physics Laboratory. “However, as we demonstrated in this work, by combining phenomena across different scales, we were able to observe their interplay that ultimately energize particles in space.”
Cosmic Particle Accelerators and the Injection Problem
Scientists have known for decades that cosmic rays are composed of electrons racing through space at incredibly high speeds. However, they still don’t understand the underlying processes behind their creation.
Fermi acceleration, or Diffuse Shock Acceleration (DSA), is a generally accepted component of creating cosmic rays. This mechanism involves the addition of non-thermal energies to otherwise low-energy particles, which results in their ejection at high speeds. However, for DSA to work, the particles must attain significant initial energy before blasting into space.
According to the Northumbria researchers, understanding how a particle with an energy level of approximately one keV is charged up to the high energy 500 keV observed in cosmic rays is challenging. The mystery behind how this occurs is known as “the injection problem.”
In their published study, the team zeroed in on several processes occurring at various levels that work together to charge the particles to the desired energy levels. Once that happens, the researchers determined collisionless shock waves blast the particles into interstellar space like cosmic particle accelerators.
Researchers Observe Phenomenon Upstream of Earth’s Bow Shock
The research team began its investigation by collecting real-time data from NASA’s Magnetospheric Multiscale (MMS) mission. Launched in 2015, the MMS measures the interaction between the solar wind and the Earth’s magnetosphere, the region of space around the planet dominated by its magnetic field.
They found the answer when comparing the MMS magnetosphere data with information collected by NASA’s THEMIS/ARTEMIS mission, which was designed to study the upstream plasma environment close to the moon. Specifically, they discovered what they described as a “large scale, time-dependent (i.e., transient) phenomenon, upstream of Earth’s bow shock” that occurred on December 17, 2017.
A closer look at the data comparison revealed that electrons residing in an area of space known as the “foreshock” region where the solar wind is “predisturbed” by its contact with the bow shock were charged to levels reaching and even surpassing 500 keV.
The researchers determined that this process involved “the complex interplay of multiple acceleration mechanisms,” including plasma waves, electron interactions, transient structures in the foreshock region, and the planet’s bow shock. They also note that the discovery was only recognizable due to the ability of NASA’s top-line instruments to measure several different effects at varying scales occurring in real-time within this region of space.
“One of the most effective ways to deepen our understanding of the universe we live in is by using our near-Earth plasma environment as a natural laboratory,” said study co-author Dr. Ahmad Lalti. “In this work, we use in-situ observation from MMS and THEMIS/ARTEMIS to show how different fundamental plasma processes at different scales work in concert to energize electrons from low energies up to high relativistic energies.”
Phenomenon Expected to be Common “Across the Universe”
In the study’s conclusion, the authors conclude that their success in using local phenomena to extrapolate what mechanisms may be at work in more distant regions of space could help guide other researchers “trying to understand astrophysical processes throughout the universe.”
According to Dr. Lalti, the multiscale processes they have uncovered that cause several phenomena to work together, like cosmic particle accelerators, are unlikely to be restricted to the local solar system. Instead, the researcher believes these phenomena “are expected to occur across the universe.”
“This makes our proposed framework relevant for better understanding electron acceleration up to cosmic-ray energies at astrophysical structures light-years away from our solar system, such as at other stellar systems, supernovae remnants, and active galactic nuclei,” Dr. Lalti explained.
The study “Revealing an Unexpectedly Low Electron Injection Threshold via Reinforced Shock Acceleration” was published in Nature Communications.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.