How and Why Do Lightning Sferics have Partial Coherency?

This article extends on the mathematical model of the complete radiation patterns of bremsstrahlung process involving bremsstrahlung asymmetry and Doppler shift. Firstly, mathematical model is simplified preserving the forward - backward peaking radiation properties and involved asymmetries. This helped investigate and model the tendency of rotation in the wavefront of the emitted wave. As a result it has been found that the curl of the gradient of the radiation intensity is non-zero and the wavefront of the bremsstrahlung radiation follows a spiral wavefront in 2D and helical wavefront in 3D. Radius of rotational wavefront was found to be decreasing as the wave propagates. Spiral geometry has different magnitudes of radius with respect as it rotates. Hence, different magnitudes of spiral wavefront in 2D is the result of the involved bremsstrahlung and Doppler shift asymmetries. This is also supported diagrammatically by applying Huygen's principle on a relativistic radiation pattern. Outcomes describe why the lightning discharges display a partial temporal and spatial coherence hence why lightning sferics are not known to produce structured wavefronts. Bremsstrahlung emissions start with a rotating and irregularly shrinking radius wavefront. Therefore, spatial coherency degrades as their helical structure breaks down due to irregular shrinkage of radius. This leaves radiation from the bremsstrahlung process with partial temporal coherency.