Supernova remnant ( SNR ) blast shells can reach the flow speed v _ { s } = 0.1 c and shocks form at its front .
Instabilities driven by shock-reflected ion beams heat the plasma in the foreshock , which may inject particles into diffusive acceleration .
The ion beams can have the speed v _ { b } \approx v _ { s } .
For v _ { b } \ll v _ { s } the Buneman or upper-hybrid instabilities dominate , while for v _ { b } \gg v _ { s } the filamentation and mixed modes grow faster .
Here the relevant waves for v _ { b } \approx v _ { s } are examined and how they interact nonlinearly with the particles .
The collision of two plasma clouds at the speed v _ { s } is modelled with particle-in-cell ( PIC ) simulations , which convect with them magnetic fields oriented perpendicular to their flow velocity vector .
One simulation models equally dense clouds and the other one uses a density ratio of 2 .
Both simulations show upper-hybrid waves that are planar over large spatial intervals and that accelerate electrons to \sim 10 keV .
The symmetric collision yields only short oscillatory wave pulses , while the asymmetric collision also produces large-scale electric fields , probably through a magnetic pressure gradient .
The large-scale fields destroy the electron phase space holes and they accelerate the ions , which facilitates the formation of a precursor shock .