On the mechanism of branching in negative ionization fronts
  We explain a mechanism for branching of a planar negative front. Branching occurs as the result of a balance between the destabilizing effect of impact ionization and the stabilizing effect of electron diffusion on ionization fronts. The dispersion relation for transversal perturbation is obtained analytically and reads: $s = |k|/[2 (1 + |k|)] - D |k|^2$, where $D$, which is assumed to be small, is the ratio between the electron diffusion coefficient and the intensity of the externally imposed electric field. We estimate the spacing $\lambda$ between streamers in a planar discharge and deduce a scaling law $\lambda \sim D^{1/3}$. 
M. Arrayas, M. A. Fontelos, J. L. Trueba