On the origin of long time-scale pulsar scintillations - I. Cosmic ray streaming or the mirror instability?

The suggestion that cosmic rays streaming along the magnetic fields of the Galaxy produce the small scale (10⁹ m) density fluctuations in the interstellar medium (ISM) responsible for long time-scale (≳ 1 hr) pulsar scintillations is investigated and found invalid. Turbulence in the hot $$(\sim \,{10}^{6}\,\text K)$$ component of the ISM is investigated and found to cause pressure anisotropies, and these render the plasma either firehose or mirror unstable. We use a magnetohydrodynamic analysis to discuss these instabilities, and propose the mirror instability is responsible for the small scale density fluctuations.