The ornamental design for copper rain chain, as shown and described.
FIG. 1 is a perspective view of one segment of a copper rain chain of the invention;
FIG. 2 is a front elevational view of the one segment of the copper rain chain of the invention, the rear elevational view being a mirror image thereof;
FIG. 3 is a left-side elevational view of the one segment of the copper rain chain of the invention, the right-side elevational view being a mirror image thereof;