  A theoretical analysis for astrophysics-oriented laser-matter interaction
experiments in the presence of a strong ambient magnetic field is presented. It
is shown that the plasma collision in the ambient magnetic field implies
significant perturbations in the electron density and magnetic field
distribution. This transient stage is difficult to observe in astrophysical
phenomena, but it could be investigated in laboratory experiments. Analytic
models are presented, which are supported by particle-in-cell simulations.
