  We have measured the equivalent width of the H-alpha emission line for 11006
galaxies brighter than M_b=-19 (LCDM) at 0.05<z<0.1 in the 2dF Galaxy Redshift
Survey (2dF), in the fields of seventeen known galaxy clusters. The limited
redshift range ensures that our results are insensitive to aperture bias, and
to residuals from night sky emission lines. We use these measurements to trace
mustar, the star formation rate normalized to Lstar, as a function of distance
from the cluster centre, and local projected galaxy density. We find that the
distribution of mustar steadily skews toward larger values with increasing
distance from the cluster centre, converging to the field distribution at
distances greater than ~3 times the virial radius. A correlation between star
formation rate and local projected density is also found, which is independent
of cluster velocity dispersion and disappears at projected densities below ~1
galaxy (brighter than M_b=-19) per Mpc^2. This characteristic scale corresponds
approximately to the mean density at the cluster virial radius. The same
correlation holds for galaxies more than two virial radii from the cluster
centre. We conclude that environmental influences on galaxy properties are not
restricted to cluster cores, but are effective in all groups where the density
exceeds this critical value. The present day abundance of such systems, and the
strong evolution of this abundance, makes it likely that hierarchical growth of
structure plays a significant role in decreasing the global average star
formation rate. Finally, the low star formation rates well beyond the
virialised cluster rule out severe physical processes, such as ram pressure
stripping of disk gas, as being completely responsible for the variations in
galaxy properties with environment.
