CORRELATED VARIATIONS OF UV AND RADIO EMISSIONS DURING AN OUTSTANDINGJOVIAN AURORAL EVENT

An exceptional Jovian aurora was detected in the FUV on December 21, 1 990, by means of Vilspa and Goddard Space Flight Center (GFSC) Interna tional Ultraviolet Explorer (IUE) observations. This event included in tensification by a factor of three between December 20 and 21, leading to the brightest aurora identified in the IUE data analyzed, and, in the north, to a shift of the emission peak towards larger longitudes ( these variations are even more dramatic once the actual source brightn ess distribution is retrieved from the raw data). The Jovian radio emi ssion simultaneously recorded at decameter wavelengths in Nancay also exhibits significant changes, from a weak and short-duration emission on December 20 to a very intense one, lasting several hours, on Decemb er 21. Confirmation of this intense radio event is also found in the o bservations at the University of Florida on December 21. The emissions are identified as right-handed Io-independent ''A'' (or ''non Io-A'') components from the northern hemisphere. The radio source region dedu ced from the Nancay observations lies, for both days, close to the UV peak emission, exhibiting in particular a similar shift of the source region toward larger longitudes from one day to the next. A significan t broadening of the radio source was also observed and it is shown tha t on both days, the extent of the radio source closely followed the lo ngitude range for which the UV brightness exceeds a given threshold (a pproximately 3 kW m-1). The correlated variations, both in intensity a nd longitude, strongly suggest that a common cause triggered the varia tion of the UV and radio emissions during this exceptional event. On o ne hand, the variation of the UV aurora could possibly be interpreted according to the Prange and Elkhamsi (1991) model of diffuse multicomp onent auroral precipitation (electron and ion): it would arise from an increase in the precipitation rate of ions together with an inward sh ift of their precipitation locus from L almost-equal-to 10 to L almost -equal-to 6. On the other hand, the analysis of Ulysses observations i n the upstream solar wind suggests that a significant disturbance in t he solar wind, involving the generation of an interplanetary shock and the presence of a CME have interacted with the Jovian magnetosphere a t about the time of the auroral event. Both arguments suggest that we may have observed for the first time a magnetic storm-type interaction in an outer planet magnetosphere, affecting simultaneously several au roral processes. Conversely, the observed relationship between the lev el of UV auroral activity and the detection of decameter emission (DAM ), if it were a typical feature, might argue in favour of a more direc t and permanent association between the auroral processes leading to U V and radio aurorae. possibly related to ''discrete-arc''-like activit y and electron precipitation.