Collisionless shocks and the particle injection problem

5 problems

• Electron heating and electron-ion temperature ratio at shocks

  Open

  Behind fast shocks in supernova remnants, electrons end up much colder than ions, while behind slow shocks the two are similar. Which processes heat electrons inside a collisionless shock is not known.

• Electron injection into diffusive shock acceleration

  Partially resolved

  Shocks accelerate particles that bounce back and forth across them many times, but electrons are too small to do this until they are already energetic. How a fraction of thermal electrons is pre-energized is the electron injection problem.

• Saturation of cosmic-ray-driven field amplification at fast shocks

  Open

  Cosmic rays streaming ahead of a shock can amplify the magnetic field, which helps trap particles and accelerate them to higher energy. How strong the amplified field becomes, which decides whether supernova remnants can accelerate protons to about 1e15 electron volts, is unsettled.

• Ion injection at quasi-perpendicular shocks

  Open

  Simulations show ions are efficiently accelerated when the magnetic field is roughly along the shock's direction of travel, but hardly at all when it is across it. Real shocks have every orientation, so whether across-field shocks can inject ions matters for cosmic-ray yields.

• Survival of Weibel-generated magnetic fields downstream of relativistic shocks

  Open

  Shocks in nearly unmagnetized relativistic plasma, as in gamma-ray burst afterglows, create magnetic fields through an instability that bunches the plasma current into filaments. Simulations show these fields fade quickly, yet the observed emission needs them to persist far behind the shock.