2:00 PM - 2:15 PM
[PEM17-07] Simulation of Weibel instability in weakly magnetized plasmas
Keywords:Cosmic ray, Supernova remnant, Collisionless shocks
The standard model for high energy cosmic ray acceleration is the Fermi acceleration at collisionless shocks.
Cosmic ray particles repeatedly cross astrophysical shocks such as supernova remnant (SNR) shocks to reach their high energy.
However, there are some problems with this standard model.
One of them is called the injection problem.
A sufficient amount of plasma particles need to be pre-accelerated to high enough energy to initiate an efficient Fermi acceleration.
Electrons are more susceptible to injection problems.
To understand how the electrons are pre-accelerated, we study the micro-physics in the shock transition region.
Plasma particles incoming from the upstream of the shock and particles reflected in the downstream coexist in the transition region.
The reflected particles excite various plasma instabilities.
It is known that Weibel instability, an electromagnetic instability driven by the reflected ions, becomes the dominant instability in non-relativistic, high-Mach number shocks.
Previous studies of Weibel-dominated shocks suggest that the strength of the background magnetic field can drastically change the linear and nonlinear dynamics of the shock.
It is also known that spontaneous magnetic reconnection is triggered in some parameters.
However, the detailed role of the background magnetic field and precise conditions for magnetic reconnection were unknown.
We show that the background magnetic field which is weak but enough to magnetize the electrons changes the linear and nonlinear growth of the Weibel instability and results in magnetic reconnection.
This condition corresponds to the Mach number of 100s which is the case for typical young SNRs.
We have performed 2D and 3D particle-in-cell (PIC) simulations to confirm the theory and investigate the nonlinear evolution in detail.
Cosmic ray particles repeatedly cross astrophysical shocks such as supernova remnant (SNR) shocks to reach their high energy.
However, there are some problems with this standard model.
One of them is called the injection problem.
A sufficient amount of plasma particles need to be pre-accelerated to high enough energy to initiate an efficient Fermi acceleration.
Electrons are more susceptible to injection problems.
To understand how the electrons are pre-accelerated, we study the micro-physics in the shock transition region.
Plasma particles incoming from the upstream of the shock and particles reflected in the downstream coexist in the transition region.
The reflected particles excite various plasma instabilities.
It is known that Weibel instability, an electromagnetic instability driven by the reflected ions, becomes the dominant instability in non-relativistic, high-Mach number shocks.
Previous studies of Weibel-dominated shocks suggest that the strength of the background magnetic field can drastically change the linear and nonlinear dynamics of the shock.
It is also known that spontaneous magnetic reconnection is triggered in some parameters.
However, the detailed role of the background magnetic field and precise conditions for magnetic reconnection were unknown.
We show that the background magnetic field which is weak but enough to magnetize the electrons changes the linear and nonlinear growth of the Weibel instability and results in magnetic reconnection.
This condition corresponds to the Mach number of 100s which is the case for typical young SNRs.
We have performed 2D and 3D particle-in-cell (PIC) simulations to confirm the theory and investigate the nonlinear evolution in detail.