Analysis of international ultraviolet explorer (IUE) and Voyager ultraviolet spectrometer (UVS) spectra of the Jovian auroral emission indicates that the Jovian auroral brightness is modulated in longitude (brighter near 180¿ in the north and 20¿ in the south) and that there is a color ratio asymmetry associated with this brightening. The purpose of this study is to investigate the origin of this apparent asymmetry. To that end, we use a series of six typical images of the north auroral region taken in the H2 Lyman bands with the faint object camera (FOC) aboard the Hubble space telescope (HST) and which cover a full Jovian rotation. Although the images do not display any strong brightening near 180¿, once we have simulated the signal IUE would see through its aperture, we find the characteristic longitudinal modulation. We attribute most of this modulation to a combination of viewing geometry effects near the east and west ansae of the auroral oval (already taken into account in previous studies) and of the spatial degradation of the source by the IUE instrument function (never considered so far), and we suggest qualitatively that these effects may also affect the color ratio asymmetry. Nevertheless, part of the asymmetry seems to be due to an intrinsic modulation associated with a bright feature crossing the polar cap along the 160¿ meridian (transpolar emission) and present in most of the images. We then use a series of FOC images taken during an atypically strong auroral event, and we show that the same effects can again account for the anomalous brightness variations observed simultaneously with IUE.¿ 1997 American Geophysical Union