Conditions for electric breakdown in the lower ionosphere due to a lightning discharge

Conditions are theoretically studied, under which a breakdown is realized in the lower ionosphere above a thunderstorm after a positive cloud-to-ground (+CG) lightning discharge. Two altitudes are seen as candidates for a breakdown initiation: (i) at which the relaxation time equals the discharge time, and (ii) at the "knee" of the conductivity profile. According to this, a criterion of a breakdown occurrence is formulated. The conductivity is the key factor for a breakdown onset (i.e. more important than the charge moment change), due to its large variability at these heights.