| Name: Saikat Das |
| Affiliation: Indian Institute of Science |
| Conference ID: ASI2026_48 |
| Title: Impact of Cosmic Ray Distribution on Growth and Saturation of Bell Instability |
| Abstract Type: Poster |
| Abstract Category: High Energy Phenomena, Fundamental Physics and Astronomy |
| Author(s) and Co-Author(s) with Affiliation: Saikat Das(Joint Astronomy Programme, Department of Physics, Indian Institute of Science, Bangalore - 560012, India), Siddhartha Gupta(Department of Astrophysical Sciences, Princeton University, 4 Ivy Ln., Princeton, NJ - 08544, USA), Prateek Sharma(Joint Astronomy Programme, Department of Physics, Indian Institute of Science, Bangalore - 560012, India) |
| Abstract: Nonthermal X-ray and radio observations of supernova remnants reveal magnetic fields of several hundred microgauss, far exceeding typical interstellar values. Such strong fields suggest significant magnetic-field amplification driven by cosmic ray (CR) streaming instabilities. The nonresonant streaming instability (NRSI), also known as the Bell instability, is particularly effective, as it can amplify magnetic fields well beyond background levels. However, most previous studies have focused on mono-energetic CR populations, which differ from the power-law momentum distributions expected in realistic astrophysical environments. In our work, we use one-dimensional kinetic simulations to investigate how mono-energetic and power-law CR distributions influence both the growth and saturation of the NRSI. We find that the linear growth rate depends only on the net CR current and is largely insensitive to the CR distribution. In contrast, the saturation mechanism depends strongly on the distribution: saturation occurs through CR isotropization, which quenches the driving current. Mono-energetic CRs efficiently amplify magnetic fields and isotropize. For power-law distributions, the lowest-energy CRs dominate current relaxation and magnetic growth, while the highest-energy CRs remain weakly scattered, limiting their contribution to saturated fields. When low-energy CRs are absent, high-energy CRs can amplify the field and isotropize. We provide a modified saturation prescription that incorporates these effects and propose a layered CR-confinement scenario upstream of astrophysical shocks, which is relevant to particle acceleration to high energies.
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