Riding the Cosmic Wave: How Plasma Instability Is Changing Our View of the Universe

Cosmic Rays Plasma Art Concept

Scientists have discovered a new plasma instability, revolutionizing our understanding of cosmic rays. This breakthrough reveals that cosmic rays generate electromagnetic waves in plasma, influencing their paths. This collective behavior of cosmic rays, akin to waves formed by water molecules, challenges previous theories and promises insights into cosmic ray transport in galaxies and their role in galactic evolution. Credit: SciTechDaily.com

Scientists from the Leibniz Institute for Astrophysics Potsdam (AIP) have discovered a new plasma instability that promises to revolutionize our understanding of the origin of cosmic rays and their dynamic impact on galaxies.

At the beginning of the last century, Victor Hess discovered a new phenomenon called cosmic rays that later on earned him the Nobel prize. He conducted high-altitude balloon flights to find that the Earth’s atmosphere is not ionized by the radioactivity of the ground. Instead, he confirmed that the origin of ionization was extra-terrestrial. Subsequently, it was determined that cosmic “rays” consist of charged particles from outer space flying close to the speed of light rather than radiation. However, the name “cosmic rays” outlasted these findings.

Recent Advances in Cosmic Ray Research

In the new study, Dr. Mohamad Shalaby, scientists at AIP and the main author of this study, and his collaborators have performed numerical simulations to follow the trajectories of many cosmic ray particles and study how these interact with the surrounding plasma consisting of electrons and protons.

Simulation of Cosmic Rays Counter-Streaming Against a Background Plasma and Exciting a Plasma Instability.

Simulation of cosmic rays counter-streaming against a background plasma and exciting a plasma instability. Shown is the distribution of background particles responding to the streaming cosmic rays in phase space, which is spanned by particle position (horizontal axis) and velocity (vertical axis). The colours visualize the number density and the phase space holes are manifestations of the highly dynamical nature of the instability that dissipates ordered into random motions. Credit: Shalaby/AIP

When the researchers studied cosmic rays flying from one side of the simulation to the other, they discovered a new phenomenon that excites electromagnetic waves in the background plasma. These waves exert a force on the cosmic rays, which changes their winding paths.

Understanding Cosmic Rays as Collective Phenomena

Most importantly, this new phenomenon can be best understood if we consider the cosmic rays not to act as individual particles but instead to support a collective electromagnetic wave. As this wave interacts with the fundamental waves in the background, these are strongly amplified and a transfer of energy takes place.

“This insight allows us to consider cosmic rays as behaving like radiation and not individual particles in this context, just as it has been originally believed by Victor Hess,” remarks Professor Christoph Pfrommer, head of the Cosmology and High-Energy Astrophysics section at AIP.

Distribution of Momenta of Protons and Electrons

Distribution of momenta of protons (dashed lines) and electrons (solid lines). Shown is the emergence of the high-energy tail of electrons at a slower moving shock. This is the result of interactions with electromagnetic waves exerted by the newly discovered plasma instability (red), which are absent for a faster shock (black). Because only high-energy electrons produce observable radio emission, this shows the importance of understanding the physics of the acceleration process. Credit: Shalaby/AIP

A good analogy for this behavior is individual water molecules collectively forming a wave that breaks at the shore. “This progress only came about by considering smaller scales that have previously been overlooked and that question the use of effective hydrodynamic theories when studying plasma processes,” explains Dr. Mohamad Shalaby.

Implications and Applications

There are many applications of this newly discovered plasma instability, including a first explanation of how electrons from the thermal interstellar plasma can be accelerated to high energies at supernova remnants.

“This newly found plasma instability represents a significant leap in our understanding of the acceleration process and finally explains why these supernova remnants shine in the radio and gamma rays,” reports Mohamad Shalaby.

Moreover, this groundbreaking discovery opens the door to a deeper understanding of the fundamental processes of the transport of cosmic rays in galaxies, which represents the greatest mystery in our understanding of the processes that shape galaxies during their cosmic evolution.


“Deciphering the physical basis of the intermediate-scale instability” by Mohamad Shalaby, Timon Thomas, Christoph Pfrommer, Rouven Lemmerz and Virginia Bresci, 12 December 2023, Journal of Plasma Physics.
DOI: 10.1017/S0022377823001289

“The mechanism of efficient electron acceleration at parallel non-relativistic shocks” by Mohamad Shalaby, Rouven Lemmerz, Timon Thomas, Christoph Pfrommer, 4 May 2022, Astrophysics > High Energy Astrophysical Phenomena.

“A New Cosmic-Ray-driven Instability” by Mohamad Shalaby, Timon Thomas and Christoph Pfrommer, 24 February 2021, The Astrophysical Journal.
DOI: 10.3847/1538-4357/abd02d

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