The vertical character of discharges and the excitation of the cavity

The fact that the SR field is vertically polarized is no accident — it follows from the geometry of the discharges. In a storm cloud the charge separates vertically: positive charge accumulates in the higher, colder parts and negative charge lower down (governed, among other things, by the gravitational sorting of ice particles). The cloud-to-ground discharge therefore flows predominantly in the vertical direction Bozoki 2023.

The vertical current channel radiates a vertical electric field. In the Earth-ionosphere cavity, bounded by two approximately horizontal conductive layers (the ground and the ionosphere), the most efficiently excited modes are precisely those whose electric field is vertical and magnetic field horizontal. Each individual discharge thus contributes to the vertically polarized resonance field of the SR Bozoki 2023.

This property has a practical impact: the vertical electric component (E_z) can be measured with an antenna perpendicular to the ground and the horizontal magnetic components with coils, and from their combination the direction and distance of the source storms can be inferred.