4:00 PM - 4:15 PM
[PEM17-09] Particle acceleration with precursor wave driven wakefield in relativistic shocks
Keywords:cosmic rays, shock waves, acceleration of particles
The acceleration mechanism for generating ultra-high-energy cosmic rays (UHECRs) with energies above 1018eV is one of the most important unsolved problems in astrophysics. Relativistic shocks in extragalactic astrophysical objects such as jets from active galactic nuclei (AGNs) and gamma-ray bursts (GRBs) are considered as efficient acceleration sites and commonly proposed sources for UHECRs. Observations of anisotropy in the arrival direction of the UHECRs favor the extragalactic origin.
In relativistic shocks, large-amplitude electromagnetic waves are excited by synchrotron maser instability (SMI) in the shock transition and propagate upstream (Hoshino & Arons 1991). The precursor wave emission via the SMI is widely studied by means of 1D particle-in-cell (PIC) simulations. Hoshino (2008) demonstrated that the pump electromagnetic wave decays into a Langmuir wave via parametric decay instability and found that nonthermal particles are generated in the manner analogous to wakefield acceleration (WFA) during the nonlinear process of the Langmuir wave collapse. The WFA is first proposed in laboratory plasmas (Tajima & Dawson 1979) and recently applied for UHECR acceleration (e.g., Chen et al. 2002). The WFA in the context of relativistic shocks is considered a promising candidate for UHECR acceleration.
It was believed that in multidimensional shocks, the intense wave emission via the SMI would cease soon due to inhomogeneity along the shock surface. Recently, however, our high-resolution 2D PIC simulations in pair plasmas showed that the wave emission continues (Iwamoto et al. 2017, 2018). Although our studies give positive results for the WFA in relativistic shocks, we could not directly demonstrate it because finite mass difference of two opposite charges is essential for exciting the wakefield.
In this presentation, by performing 2D PIC simulation of ion-electron shocks, we will demonstrate that the wakefield is indeed induced by the large-amplitude precursor wave. Especially for relatively high magnetization, the precursor wave amplitude is significantly amplified and exceeds that in pair plasmas due to a positive feedback process associated with ion-electron coupling through the wakefield. Nonthermal electrons and ions are generated during the nonlinear process of the wakefield collapse and the particle energy spectrum shows a power-law distribution. We will discuss the particle acceleration mechanism in details.
In relativistic shocks, large-amplitude electromagnetic waves are excited by synchrotron maser instability (SMI) in the shock transition and propagate upstream (Hoshino & Arons 1991). The precursor wave emission via the SMI is widely studied by means of 1D particle-in-cell (PIC) simulations. Hoshino (2008) demonstrated that the pump electromagnetic wave decays into a Langmuir wave via parametric decay instability and found that nonthermal particles are generated in the manner analogous to wakefield acceleration (WFA) during the nonlinear process of the Langmuir wave collapse. The WFA is first proposed in laboratory plasmas (Tajima & Dawson 1979) and recently applied for UHECR acceleration (e.g., Chen et al. 2002). The WFA in the context of relativistic shocks is considered a promising candidate for UHECR acceleration.
It was believed that in multidimensional shocks, the intense wave emission via the SMI would cease soon due to inhomogeneity along the shock surface. Recently, however, our high-resolution 2D PIC simulations in pair plasmas showed that the wave emission continues (Iwamoto et al. 2017, 2018). Although our studies give positive results for the WFA in relativistic shocks, we could not directly demonstrate it because finite mass difference of two opposite charges is essential for exciting the wakefield.
In this presentation, by performing 2D PIC simulation of ion-electron shocks, we will demonstrate that the wakefield is indeed induced by the large-amplitude precursor wave. Especially for relatively high magnetization, the precursor wave amplitude is significantly amplified and exceeds that in pair plasmas due to a positive feedback process associated with ion-electron coupling through the wakefield. Nonthermal electrons and ions are generated during the nonlinear process of the wakefield collapse and the particle energy spectrum shows a power-law distribution. We will discuss the particle acceleration mechanism in details.