We have measured the equivalent width of the H \alpha emission line for 11006 galaxies brighter than M _ { b } = -19 ( \Omega _ { \Lambda } = 0.7 , \Omega _ { m } = 0.3 , H _ { 0 } = 70 { km s ^ { -1 } Mpc ^ { -1 } } ) at 0.05 < z < 0.1 in the 2dF Galaxy Redshift Survey ( 2dFGRS ) , 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 \mu ^ { \ast } , the star formation rate normalized to L ^ { \ast } , as a function of distance from the cluster centre , and local projected galaxy density .
We find that the distribution of \mu ^ { \ast } steadily skews toward larger values with increasing distance from the cluster centre , converging to the field distribution at distances greater than \sim 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 \sim 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 .