Speaker
Description
Cosmic rays (CRs) are believed to be accelerated via the first-order Fermi process, in which particles undergo scattering by magnetohydrodynamic (MHD) waves in the vicinity of a shock front. In the acceleration region of ultra-high-energy CRs, CR acceleration occurs in relativistic shocks, where the shock speed approaches the speed of light. To accelerate CRs to $10^{20}$ eV, the strong magnetic field turbulence around the relativistic shock is required (Lemoine et al., 2006; Niemiec et al., 2006). Understanding the nature of turbulence in the vicinity of the relativistic shocks is, therefore, crucial for elucidating the acceleration process.
The velocity field in the turbulence can be decomposed into solenoidal and compressive components. While solenoidal turbulence plays a key role in amplifying magnetic fields via dynamo action, both solenoidal and compressive modes contribute to the acceleration and scattering of high-energy particles. However, the relative importance of these modes in relativistic shock environments remains an open question.
In this study, we investigate turbulence generated by shock-clump interactions in relativistic shocks, where density inhomogeneities collide with the shock front. Performing high-resolution MHD simulations and the Helmholtz decomposition, we found that the compressive mode is excited to a comparable level of the solenoidal mode, which is not seen in the non-relativistic cases. In addition, we found secondary shocks which are generated by the shock-clump interactions and propagate in the downstream region. These secondary shocks introduce additional particle acceleration, potentially modifying the non-thermal energy spectrum produced at the main shock.
In this talk, we will present the results of our turbulence analysis and discuss in detail their implications for particle scattering and acceleration in relativistic shocks.
