Magnetic Field Amplification and Particle Acceleration in Weakly Magnetized Trans-relativistic Electron-ion Shocks

Magnetic Field Amplification and Particle Acceleration in Weakly Magnetized Trans-relativistic Electron-ion Shocks
Taiki Jikei, Daniel Groselj, Lorenzo Sironi
arXiv:2512.03169v1 Announce Type: new
Abstract: We investigate the physics of quasi-parallel trans-relativistic shocks propagating in weakly magnetized plasmas by means of long-duration two-dimensional particle-in-cell simulations. The structure of the shock precursor is shaped by a competition between the Bell instability and the Weibel instability. The Bell instability is dominant at relatively high magnetizations $(sigmagtrsim10^{-3})$, whereas the Weibel instability prevails at lower magnetizations $(sigmalesssim10^{-4})$. Bell-dominated shocks efficiently accelerate ions, converting a fraction $varepsilon_{mathrm{i}}sim0.2$ of the upstream flow energy into downstream nonthermal ion energy. The maximum energy of nonthermal ions exhibits a Bohm scaling in time, as $E_{max}propto t$. A much smaller fraction $varepsilon_{mathrm{e}}ll0.1$ of the upstream flow energy goes into downstream nonthermal electrons in the Bell-dominated regime. On the other hand, Weibel-dominated shocks efficiently generate both nonthermal ions and electrons with $varepsilon_{mathrm{i}}simvarepsilon_{mathrm{e}}sim0.1$, albeit with a slower scaling for the maximum energy, $E_{mathrm{max}}propto t^{1/2}$. Our results are applicable to a wide range of trans-relativistic shocks, including the termination shocks of extragalactic jets, the late stages of gamma-ray burst afterglows, and shocks in fast blue optical transients.arXiv:2512.03169v1 Announce Type: new
Abstract: We investigate the physics of quasi-parallel trans-relativistic shocks propagating in weakly magnetized plasmas by means of long-duration two-dimensional particle-in-cell simulations. The structure of the shock precursor is shaped by a competition between the Bell instability and the Weibel instability. The Bell instability is dominant at relatively high magnetizations $(sigmagtrsim10^{-3})$, whereas the Weibel instability prevails at lower magnetizations $(sigmalesssim10^{-4})$. Bell-dominated shocks efficiently accelerate ions, converting a fraction $varepsilon_{mathrm{i}}sim0.2$ of the upstream flow energy into downstream nonthermal ion energy. The maximum energy of nonthermal ions exhibits a Bohm scaling in time, as $E_{max}propto t$. A much smaller fraction $varepsilon_{mathrm{e}}ll0.1$ of the upstream flow energy goes into downstream nonthermal electrons in the Bell-dominated regime. On the other hand, Weibel-dominated shocks efficiently generate both nonthermal ions and electrons with $varepsilon_{mathrm{i}}simvarepsilon_{mathrm{e}}sim0.1$, albeit with a slower scaling for the maximum energy, $E_{mathrm{max}}propto t^{1/2}$. Our results are applicable to a wide range of trans-relativistic shocks, including the termination shocks of extragalactic jets, the late stages of gamma-ray burst afterglows, and shocks in fast blue optical transients.