Temporal monitoring of Jupiter's auroral activity with IUE during the Galileo mission. Implications for magnetospheric processes

Abstract

In this study, we analyze FUV auroral spectra of Jupiter acquired on a quasi-continuous basis between August 17 and September 25, 1996 as part of the last IUE Key Project. This campaign was coordinated with Galileo measurements. We show that it is possible to define an “auroral activity index” which quantifies the variability of the flux radiated in the H 2 bands (1560–1620 Å). The activity indices in both hemispheres are generally similar and indicate a strong interhemispheric conjugacy, suggestive of particle precipitation on closed field lines. We identify variability on three different scales: small variations of 10–20% are observed on short time scales (∼a few hours), variations by a factor of 2–4 occur on scales of 5–10 days, and a long-term trend is observed on a scale which exceeds the 6 weeks of observations. The energy output in this spectral range is a direct measurement of the energy losses of the magnetosphere along auroral magnetic field lines. We establish here that its intermediate time-scale variations, the best documented ones in this study, are amazingly well correlated with the state (quiet or disturbed) of the magnetic field measured by the magnetometer on board Galileo, although distances from ∼15 to ∼115 R J and all local times were sampled during that time. This indicates temporal, rather than spatial, variations of the magnetic field, and suggests that some dynamical process, yet to be identified, affects the magnetosphere as a whole and triggers recurrent energy releases along auroral field lines. The recurrence time derived from this study is larger than the 3-day periodicity previously assigned to variabilities in the magnetotail on the basis of Galileo measurements during other time periods. A crude preliminary comparison also indicates correlations with the intensity of the auroral kilometer radio waves and anticorrelations with the local plasma density, both derived from the Galileo PWS experiment. Comparison with other particle and field instruments is underway.