Shock precursor type changes ion and electron acceleration

What the study found: The shock precursor in weakly magnetized, quasi-parallel transrelativistic shocks is shaped by a competition between the Bell instability and the Weibel (filamentation) instability. The dominant instability depends on magnetization, and this changes how efficiently the shock accelerates ions and electrons.
What the authors say this matters: The authors conclude that these results are applicable to a wide range of transrelativistic shocks, including termination shocks of extragalactic jets, late stages of gamma-ray burst afterglows, and shocks in fast blue optical transients.
What the researchers tested: The researchers used long-duration two-dimensional particle-in-cell simulations to study quasi-parallel transrelativistic shocks propagating in weakly magnetized plasmas. They compared cases with different magnetizations, including regimes where Bell modes or Weibel modes dominate the shock precursor.
What worked and what didn't: Bell modes dominated at relatively high magnetizations (σ ≳ 10−3), while Weibel modes prevailed at lower magnetizations (σ ≲ 10−4). In the Bell regime, shocks efficiently accelerated ions, with about εi ∼ 0.2 of the upstream flow energy converted into downstream nonthermal ion energy, and the maximum ion energy scaled as Emax ∝ t; however, the downstream nonthermal electron energy fraction was much smaller, εe ≪ 0.1. In the Weibel regime, both ions and electrons were efficiently accelerated, with εi ∼ εe ∼ 0.1, but the maximum energy grew more slowly, as Emax ∝ t1/2.
What to keep in mind: The results come from long-duration two-dimensional simulations of weakly magnetized quasi-parallel transrelativistic shocks. The abstract does not describe additional limitations beyond this scope.

Key points

• The shock precursor is shaped by competition between Bell and Weibel instabilities.
• Bell modes dominate at higher magnetization, while Weibel modes dominate at lower magnetization.
• In the Bell regime, ions gain about 20% of upstream flow energy, but electrons gain much less.
• In the Weibel regime, ions and electrons are both accelerated efficiently, at about 10% each.
• The maximum ion energy grows as Emax ∝ t in the Bell regime and as Emax ∝ t1/2 in the Weibel regime.
• The authors say the results apply to several kinds of transrelativistic astrophysical shocks.