Io may have a bright dawn terminator

Abstract

NELSON AND HAPKE1 have shown that occasional observations of post eclipse brightenings (PEB) of the planet Jupiter's satellite Io are very strongly positively correlated with the occurrence of major solar flares within 10° of the solar central meridian (as seen from Jupiter) during the 100 days before the eclipse observation. No eclipses which showed no brightening were preceded by such flares. Io is in the outer part of the jovian radiation belts. Energetic particle fluxes in the Earth's radiation belts at a roughly corresponding location are enhanced for roughly similar times (the 100 days in ref. 1) following the major geomagnetic storms caused by major flares near central meridian2,3. The 100-day time scale is also in reasonable agreement with diffusion times in recent models of the jovian magnetosphere4. Considering all this, Nelson and Hapke inferred from their flare correlation that the occurrence of a noticeable PEB effect is due to enhanced fluxes of trapped energetic particles. They suggested that there may be a process similar to thermoluminescence, which converts some fraction of the particle dose received during the eclipse, into visible light which is emitted when the surface is again sunlit, and decays with the characteristic PEB time scale of ∼10 min. Although Fanale et al.5,6 had already suggested that solid state effects in the presence of irradiation by energetic particles might play a part in the PEB phenomenon, the particular correlation with solar flares that Nelson and Hapke found points at once to an energetic particle effect because that is what would be expected to be enhanced by such flares, and at the same time apparently explains the erratic occurrence of PEB on Io and its absence on more distant satellites (because they see much lower fluxes of trapped particles, enhanced or not). In any case, Nelson and Hapke have made a novel yet plausible suggestion which can be tested by laboratory simulation experiments of the solid state process, by more eclipse observations from Earth, and by observations of jovian trapped particles by future flyby and orbiter spacecrafts, and their relation to solar flares. This note suggests another particular test of the thermoluminescence properties of Io which can be made with the unique optical observations which are programmed for the close Voyager I Io flyby in March 1979, and which does not necessarily require a previous suitable flare, as the PEB phenomenon seems to do.