Researchers Saikat Das, Siddhartha Gupta, and Prateek Sharma from the University of Delhi have recently published a study in the Astrophysical Journal that explores the impact of cosmic ray distribution on the growth and saturation of a phenomenon known as the Bell instability. This research delves into the behavior of cosmic rays (CRs) in weakly magnetized plasmas and its implications for magnetic field amplification and particle acceleration.
Cosmic rays are high-energy particles that permeate the universe. When these particles stream through a plasma, they can drive large-amplitude magnetic fluctuations via a process called nonresonant streaming instability, or Bell instability. The researchers used one-dimensional kinetic simulations to investigate how different cosmic ray momentum distributions influence the growth and saturation of this instability.
The study found that the linear growth of the Bell instability is primarily governed by the cosmic ray current and is largely insensitive to the specific distribution of cosmic ray energies. However, the saturation of the instability depends strongly on the cosmic ray distribution. Saturation occurs when the cosmic rays become isotropized, which effectively quenches the driving current.
For mono-energetic cosmic rays, which have a single energy, the magnetic field is amplified and the cosmic rays become isotropized. In the case of power-law distributions, where the number of cosmic rays decreases with increasing energy, the lowest-energy cosmic rays dominate the current relaxation and magnetic growth. The highest-energy cosmic rays, however, remain weakly scattered and contribute less to the saturation process. Interestingly, in the absence of low-energy cosmic rays, high-energy particles can effectively amplify magnetic fields and become isotropized.
The researchers propose a modified saturation prescription that accounts for these effects. They also suggest a layered cosmic ray-confinement scenario upstream of astrophysical shocks, which could be relevant for understanding particle acceleration to high energies. This research provides valuable insights into the behavior of cosmic rays in plasmas and has potential applications in the energy sector, particularly in the development of advanced propulsion systems and the understanding of space weather phenomena.
The research was published in the Astrophysical Journal, a peer-reviewed scientific journal that focuses on the dissemination of research in the field of astronomy and astrophysics.
This article is based on research available at arXiv.