The forward boundary of the upstream wave region ahead of the earth's bow shock is located statistically by using combined field and plasma data from a single spacecraft, Heos 1. Interpreting the wave as arising from reflected protons traveling upstream with effective velocity V∥=pV along the interplanetary field, where V is the solar wind bulk speed, we find that the overall average boundary corresponds to a velocity V∥?2V for protons reflected from the daylight sector of the bow shock. More refined partitions of the data give p?1.6 around the subpolar point and p>2 along the daylight flanks of the shock.