On September 28, 1998, again observing with the MIRLIN camera at the IRTF, Glenn Orton and Brendan Fisher obtained 10.3 micron movies of the occultation disappearance of Io behind Jupiter and its reappearance 2 hours later. (Here is a discussion of the earlier July 21 observations and a discussion of the technique.) Because these data, unlike the July 21 ones, are obtained after Jupiter opposition, the disappearance is in sunlight, and the reappearance is in eclipse. Both clearly show the steps which correspond to the disappearance and reappearance of the Loki volcanic hot spot near the ends of the events. The Loki brightening is continuing, 4 months after it began in late May. That makes it one of the longer eruptions seen thus far. The 10 micron observations, distributed over the duration of the brightening, will allow us to test models of the eruption process and constrain the volume of material erupted and the total heat flow.
In the above disappearance light curve the smooth decline corresponds to the occultation of the Io disk, emitting thermally because it is warmed by sunlight. The drop near 360 seconds is the disappearance of the Loki hot spot. The shape of that hot spot disappearance curve is set by the refractive properties of the Jupiter atmosphere. Loki emits a flux roughly equal 1/4 that from the entire disk.
In the above reappearance light curve Io is in eclipse so there is no emission from the overall disk, but only from discrete hot spots. Loki reappears at 200 seconds, with other weaker hot spots reappearing before it. That early emission can be modeled as multiple faint hot spots distributed across the disk, or as 2 to 3 moderate strength ones. The signal-to-noise ratio from this single observation is not sufficient to distinguish between those two cases. However with multiple occultation curves it should be possible to check for consistency in the position of the minor steps, and therefore determine their reality.