A new FUV auroral feature on Jupiter

In December 2000, a series of HST/STIS FUV images of Jupiter's north auroral region displayed bright transient spots located near local midnight. In the images taken at CML Ë 220[SUP]o[/SUP] the spots (one or two) appear near the limb, poleward and equatorward of the main auroral oval, at latitude Ë 73[SUP]o[/SUP] and λ[SUB]III[/SUB] longitude Ë 145[SUP]o[/SUP]. The dimensions of each spot are very small, about 1[SUP]o[/SUP] in latitude and 5[SUP]o[/SUP] in longitude, which is about the size of the footprint of the Io satellite. However, the analysis of the position of the Galilean satellites and of known small-bodies (comets, asteroids) shows that these spots are not magnetically associated with any of these objects. The emitted power of the spots is variable and can reach several GW (more than the power emitted at the Io footprint). The lightcurves derived from multiple images are consistent with spots disappearing behind the planetary limb as the planet rotates. In addition, one short time-tagged image undoubtedly shows a bright double--spot feature pulsating with a period of 300 s. According to the VIP4 magnetic model, the auroral spots map along field lines down to the jovian magnetosphere in a small region roughly located near midnight at distances larger than 60~R_J. At these distances, a 1[SUP]o[/SUP] by 5[SUP]o[/SUP] auroral spot subtends an equatorial region smaller than 10~R_J by 10~R_J . Consequently, the auroral spots cannot be directly associated with large scale process involving the whole magnetotail but rather with localized events.