The main auroral emission at Jupiter generally appears as a quasi-closed curtain centered around the magnetic pole. This auroral feature, which accounts for approximately half of the total power emitted by the aurorae in the ultraviolet range, is related to corotation enforcement currents in the middle magnetosphere. Early models for these currents assumed axisymmetry, but significant local time variability is obvious on any image of the Jovian aurorae. Here we use far-UV images from the Hubble Space Telescope to further characterize these variations on a statistical basis. We show that the dusk side sector is times brighter than the dawn side in the southern hemisphere and brighter in the northern hemisphere, where the magnetic anomaly complicates the interpretation of the measurements. We suggest that such an asymmetry between the dawn and the dusk sectors could be the result of a partial ring current in the nightside magnetosphere.