  Several recent studies have reported the detection of an anomalous color
spread along the red giant branch (RGB) of some globular clusters (GC) that
appears only when color indices including a near ultraviolet band (such as
Johnson U or Stromgren u) are considered. This anomalous spread in color
indexes such as U-B or c_{y} has been shown to correlate with variations in the
abundances of light elements such as C, N, O, Na, etc., which, in turn, are
generally believed to be associated with subsequent star formation episodes
that occurred in the earliest few 10^{8} yr of the cluster's life. Here we use
publicly available u, g, r Sloan Digital Sky Survey photometry to search for
anomalous u-g spreads in the RGBs of nine Galactic GCs. In seven of them (M 2,
M 3, M 5, M 13, M 15, M 92 and M 53), we find evidence of a statistically
significant spread in the u-g color, not seen in g-r and not accounted for by
observational effects. In the case of M 5, we demonstrate that the observed u-g
color spread correlates with the observed abundances of Na, the redder stars
being richer in Na than the bluer ones. In all the seven clusters displaying a
significant u-g color spread, we find that the stars on the red and blue sides
of the RGB, in (g, u-g) color magnitude diagrams, have significantly different
radial distributions. In particular, the red stars (generally identified with
the second generation of cluster stars, in the current scenario) are always
more centrally concentrated than blue stars (generally identified with the
first generation) over the range sampled by the data (0.5r_{h} < r < 5r_{h}),
in qualitative agreement with the predictions of some recent models of the
formation and chemical evolution of GCs. Our results suggest that the
difference in the radial distribution between first and second generation stars
may be a general characteristic of GCs.
