Images of the Jovian FUV aurora show several morphologically and dynamically different regions. The main oval is usually fairly steady while the high-latitude emission inside the oval can vary over timescales of <1 min. We describe time and spatially resolved spectral observations of the Jovian aurora made with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. Time-tagged spectra have been used to determine variations of the amount of hydrocarbon absorption above the auroral source layer. Rapid brightenings are observed in the high-latitude emission with typical lengths of 50--100 s. They generally are not correlated with enhancements of the main oval in the same longitude sector, indicating decoupled source regions in the magnetosphere. The location of the high-latitude spots and their variability suggest that, unlike the main auroral oval, the polar cap brightenings are connected to short-term variations of the solar wind parameters. Results from a two-stream electron transport model are used to convert observed spectral color ratios into mean electron energies ⟨E⟩. Electron energies during these brightenings typically range from 40 to 120 keV, close to the values found in the main oval. The time evolution of ⟨E⟩ generally shows little correlation or anticorrelation with the energy flux precipitated during these transient events. This feature suggests that the mechanism responsible for rapid transient brightenings does not increase the energy of the precipitated electrons, but it enhances their number flux. Pitch angle scattering into the loss cone by magnetic pumping of energy perpendicular to the field lines following magnetic field compression is a possible process.