2009ApJ...703L..59O

Hα emission from supernova remnants (SNRs) implies the existence of neutral hydrogen in the circumstellar medium. Some of the neutral particles penetrating the shock are ionized by the charge-exchange process and make a cold ion beam in the shock downstream region. We perform linear analyses of collisionless plasma instabilities between the cold beam and the hot downstream plasma. We find that, under typical SNR conditions, either the resonant instability or the Weibel instability is the most unstable. This mechanism may amplify the magnetic field to more than 100 µG and changes the shock structure. As a result, the radio spectrum and the large magnetic field can be explained, apart from the widely discussed Bell's mechanism.