Collisionless shock waves are found throughout the universe. They are one of nature’s most powerful particle accelerators and play a vital role in producing cosmic rays. A new study by Northumbria University offers a detailed understanding of how these waves can accelerate particles to extreme speeds.
Using observations from NASA’s MMS (Magnetospheric Multiscale) and THEMIS/ARTEMIS missions and combined with recent theoretical advancements, scientists created a new model to explain the acceleration of electrons in collisionless shock environments.
For a long time, scientists have been trying to answer a crucial question in space physics: What processes allow electrons to be accelerated to relativistic speeds? This research addresses this long-standing puzzle in astrophysics.
Fermi acceleration or Diffuse Shock Acceleration (DSA) is the mechanism behind this. This mechanism explains electron acceleration to relativistic energies. In this, electrons must be energized to a specific threshold energy before being efficiently accelerated by DSA.
Determining how electrons achieve this initial energy is known as ”the injection problem”. The study suggests that the interaction of various processes across multiple scales accelerates electrons to high energies.
The strongest magnetic field directly measured in the universe to date
The study uses real-time data from the MMS mission and the THEMIS/ARTEMIS mission to observe a large-scale, time-dependent (i.e., transient) phenomenon upstream of Earth’s bow shock on December 17, 2017.
Electrons in Earth’s foreshock region, typically found at around 1 keV energy, surged to over 500 keV during a recent event. This leap in energy was likely due to a mix of acceleration mechanisms, including interactions with plasma waves and structures in the foreshock and Earth’s bow shock.
These mechanisms work together to accelerate electrons from around 1 keV to 500 keV, making electron acceleration particularly efficient. Refining the shock acceleration model, this study sheds new light on space plasmas and the fundamental energy transfer processes in the universe.
This research enhances our understanding of cosmic ray generation and offers insights into how solar system phenomena can help us understand astrophysical processes throughout the universe.
Dr. Raptis believes studying phenomena across different scales is crucial for understanding nature. “Most of our research focuses on either small-scale effects, like wave-particle interactions, or large-scale properties, like the influence of solar wind,” he says.
“However, as we demonstrated in this work, by combining phenomena across different scales, we could observe their interplay that ultimately energizes particles in space.”
Dr. Ahmad Lalti added: “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.”
“In this work, we use in-situ MMS and THEMIS/ARTEMIS observation to show how different fundamental plasma processes at different scales work in concert to energize electrons from low energies up to high relativistic energies.”
“Those fundamental processes are not restricted to our solar system and 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.”
Journal Reference:
- Raptis, S., Lalti, A., Lindberg, M. et al. Revealing an unexpectedly low electron injection threshold via reinforced shock acceleration. Nat Commun 16, 488 (2025). DOI: 10.1038/s41467-024-55641-9