Observations of the Jovian UV aurora by Voyager

Abstract

Voyager ultraviolet spectrometer (UVS) observations of the Jovian aurora during 1979 are analyzed. From the relationship between observing geometry variations and corresponding fluctuations of detected intensity are derived constraints on variations in source intensity and on mean optical depth over the auroral arcs. The UVS observations are fit most accurately when the northern auroral oval is assumed to lie near the computed Io torus footprint or the locus found earlier by the UVS, with the latter position yielding better fits. Moreover, the derived intensity maxima consistently occur at the positions of the maximum value of −dB/dλIII (B is magnetic field strength, and λIII is System III longitude) rather than at minimum B (“windshield wiper” effect). These positions are also consistent with observations made by the International Ultraviolet Explorer satellite. If maximum auroral excitation occurs at longitudes where the mirror point altitude is decreasing with time as the precipitating particles drift in longitude (i.e., (dB/dλIII)(dλIII/dt) < 0), then these intensity maximum positions suggest that at least the particles exciting the spatially variable portion of the aurora are drifting west (dλIII/dt > 0). This implies either that these particles are electrons or that they are positive ions drifting east more slowly than the corotation lag of the Io torus region carries them to the west. The former possibility is difficult to justify on grounds of energetics, while the latter case requires ion energies less than or equal to 1 MeV (for a 5% corotation lag and a purely dipolar field). The latter case seems most consistent with high‐energy charged particle measurements, and the presence of the windshield wiper effect suggests that pitch angle scattering of the dominant aurora‐exciting ions is slower than the strong limit. The possibility of a local time variation in the auroral intensity profile was investigated, and no evidence for a local time dependence was found. This absence is expected on theoretical grounds but is opposite to the situation at earth. The model fits indicate a total UV emission (at depth) in H Ly α and the (λ < 1216 Å) H2 bands of about 3 to 8 × 1012 W during the Voyager 1 flyby and about 3.8 × 1012 W for the northern aurora alone during the departing leg of the Voyager 2 flyby.