  Particle models for streamer ionization fronts contain correct electron
energy distributions, runaway effects and single electron statistics.
Conventional fluid models are computationally much more efficient for large
particle numbers, but create too low ionization densities in high fields. To
combine their respective advantages, we here show how to couple both models in
space. We confirm that the discrepancies between particle and fluid fronts
arise from the steep electron density gradients in the leading edge of the
fronts. We find the optimal position for the interface between models that
minimizes computational effort and reproduces the results of a pure particle
model.
