  A streamer coupling theory is developed to describe the formation of
homogenous emission, and the high moving speed of emission patterns in high
pressure discharges. By considering the effects of both electron diffusion and
electronic drift in the streamer head, the minimum required preionization level
$n_{\rm min}$ for the formation of streamer coupling is found to depend on
electric field strength, gas pressure and electron temperature. The homogeneity
and moving speed of the emission pattern in streamer coupling head increase
with preionization level $n_0$, when $n_0 > n_{\rm min}$. The predicted results
for atmospheric helium plasma indicate $n_{\rm min} \sim 10^5~{\rm cm^{-3}}$
and moving speed of $10^4 - 10^6$ m/s, in agreement with experiments.
