New plasma instability sheds light on the nature of cosmic rays

Researchers at the Leibniz Institute for Astrophysics Potsdam (AIP) have identified a novel plasma instability that might fundamentally alter our knowledge of cosmic ray origin and dynamical effects on galaxies.

Victor Hess received the Nobel Prize in physics for his discovery of cosmic rays at the turn of the 20th century. He discovered that the radioactivity of the ground does not ionize the Earth's atmosphere during high-altitude balloon flights. Rather, he verified that extraterrestrials were the source of ionization. It was later discovered that cosmic "rays" are not radiation but rather charged particles from outer space traveling at almost the speed of light. Still, the term "cosmic rays" survived these discoveries.

The primary author of the new study, Dr. Mohamad Shalaby, together with his colleagues at AIP, used numerical simulations to track the paths of several cosmic ray particles and examine their interactions with the surrounding plasma, which is made up of protons and electrons. The article is available on the arXiv pre-print service.

The novel feature that the researchers observed when seeing cosmic rays traveling from one side of the simulation to the other is that it stimulates electromagnetic waves in the background plasma. The cosmic rays are subject to a push from these waves that modifies their spiraling trajectories.

Most crucially, the simplest way to understand this novel phenomena is to think of the cosmic rays as supporting an electromagnetic wave collective rather than as individual particles. Energy is transferred and the underlying waves in the background are greatly increased as a result of this wave's interaction with them.

"This realization permits us to regard cosmic rays as individual particles and not as radiation in this context, as Victor Hess first thought," says Professor Christoph Pfrommer, who leads AIP's Cosmology and High-Energy Astrophysics branch. The activity of individual water molecules coming together to produce a wave that breaks at the beach is an excellent way to describe this phenomenon.

"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," says Dr. Mohamad Shalaby.

This recently found plasma instability has several uses, one of which is providing a first explanation for the process by which electrons from the hot interstellar plasma may be pushed to high energy 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," Mohamad Shalaby writes. The greatest enigma in our knowledge of the processes that form galaxies during their cosmic history is revealed by this ground-breaking discovery, which also provides a greater understanding of the basic mechanisms behind the transport of cosmic rays in galaxies.