  We analyze Hubble Space Telescope (HST) observations of Ganymede made with
the Space Telescope Imaging Spectrograph (STIS) between 1998 and 2017 to
generate a brightness map of Ganymede's oxygen emission at 1356 A. Our Mercator
projected map demonstrates that the brightness along Ganymede's northern and
southern auroral ovals strongly varies with longitude. To quantify this
variation around Ganymede, we investigate the brightness averaged over
36$^{\circ}$-wide longitude corridors centered around the sub-Jovian
(0$^{\circ}$ W), leading (90$^{\circ}$ W), anti-Jovian (180$^{\circ}$ W), and
trailing (270$^{\circ}$ W) central longitudes. In the northern hemisphere, the
brightness of the auroral oval is 3.7 $\pm$ 0.4 times lower in the sub-Jovian
and anti-Jovian corridors compared to the trailing and leading corridors. The
southern oval is overall brighter than the northern oval, and only 2.5 $\pm$
0.2 times fainter on the sub- and anti-Jovian corridors compared to the
trailing and leading corridors. This demonstrates that Ganymede's auroral ovals
are strongly structured in auroral crescents on the leading side (plasma
downstream side) and on the trailing side (plasma upstream side). We also find
that the brightness is not symmetric with respect to the 270$^\circ$ meridian,
but shifted by $\sim$20$^\circ$ towards the Jovian-facing hemisphere. Our map
will be useful for subsequent studies to understand the processes that generate
the aurora in Ganymede's non-rotationally driven, sub-Alfv\'{e}nic
magnetosphere.
