Chiral Symmetry Breaking in the weakly coupled QED in a Magnetic Field

The catalysis of chiral symmetry breaking in the massless weakly coupled QED in a magnetic field is studied. It is shown that the effect is due to the dimensional reduction $D\to D-2$ in the dynamics of fermion pairing in a magnetic field. The dynamical mass of fermions (energy gap in the fermion spectrum) is determined. The temperature of the symmetry restoration is estimated analytically.