IUE observations of the aurora on Jupiter have been performed with high spectral resolution in a search for Doppler shifted H Ly &agr; emission produced through charge exchange by fast precipitating protons, as observed in the Earth's aurora. No emission has been observed corresponding to proton energies greater than 200 eV, placing a strict upper limit on the contribution of KeV--MeV protons to the production of Jupiter's aurora. However, a large fraction of the H Ly &agr; emission has appeared Doppler-shifted mainly toward the blue by roughly 50 km/sec, corresponding to a kinetic energy of 10--20 eV for a fast proton or H atom, and there are higher velocity wings on the line extending out to equivalent energies of 150--200 eV. The blue shift indicates motion up out of the atmosphere, and we suggest that the emission results from the in situ acceleration of ionsopheric protons in Jupiter's auroral ionosphere by analogy to the ionospheric potentials observed in the Earth's auroral zones. These observations demonstrate that the acceleration of ionospheric plasma in an H2 atmosphere can lead to bright Ly &agr; emission, with implications for the production of the outer planet airglow emissions. The observed Doppler shift represents a significant fraction of the escape velocity, and the upflowing protons and H atoms may contribute substantially to the low energy plasma may contribute substantially to the low energy plasma in Jupiter's magnetosphere if the plasma motions are at high altitudes. ¿ American Geophysical Union 1989