Observation of an exceptionally bright (peaking at ~1.8 MR) Jovian auroral morning arc was obtained with the Space Telescope Imaging Spectrograph (STIS) on 21 September 1999, both in the imaging and spectral modes. The images of the HST orbit are used to describe the variation of the position of the bright arc, while the time-tagged spectra are examined to derive the properties of the precipitating auroral electrons, such as their mean energy and the electron current density at the top of the atmosphere. The first and the last images of the HST orbit, separated by 37 min, show that the bright morning emission is situated on the reference oval, with a "leading" edge fixed in λIII longitudes (i.e., rotating with the planet), and a "trailing" edge that extends into the nightside. The auroral arc is divided in two branches, as was also observed in some previous analyses. An isolated bright spot is also observed at λIII ~184¿. Its brightness reaches 500 kR and it also approximately corotates with Jupiter. Four regions of the auroral morning arc captured by the STIS aperture were extracted from the spectral observation. The four associated low-resolution spectra (~4.8 ¿) show very different characteristics. In particular, two spectra reveal unusually high color ratios (18.5 and 45.5), with corresponding mean electron energies of ~280 and ~460 keV, respectively. The current densities associated with three of the spectra lie in the range 0.09--0.2 ¿A m-2, consistent with previous estimates, while the fourth spectrum is characterized by a mean current density of 0.54 ¿A m-2, outside the range ~0.04--0.4 ¿A m-2 obtained in a previous study of G140L spectra of the Jovian main oval. Assuming that main oval aurorae are caused by field-aligned electric fields, the relationship between the energy flux and the current density derived from the spectra has been compared to the Knight's theory of field-aligned currents. Because of the very high acceleration potential derived from two of the extracted spectra, a relativistic treatment of the Knight theory was used. Assuming an electron temperature Te = 2.5 keV, it is seen that the two regions corresponding to earlier local times (higher λIII longitudes) reveal an electron source density lower than the values observed in the equatorial plane during the Voyager flybys. On the other hand, the equatorward region (lowest latitude) exhibits an electron source density in the upper range of usual values. Analysis of time-tag spectra reveals that the variations of the energy flux and the color ratios are large but continuous and generally covary.