Short Term Variation in Oxygen Emission from Io: A First Census of "Departure Events"

The [OI] 6300 Å flux from Io has been measured spectroscopically during 12 Jovian oppositions since 1990. During each opposition, Io has been observed on 8-45 nights, for a total of 235 nights. A total of more than 3500 spectra have been recorded. Using ~1000 of these observations, Oliversen et al. (2001) showed that the [OI] intensity is related to the instantaneous electron density at Io's position in the Io plasma torus (IPT). This is, in general, a double-peaked distribution with peaks in the vicinity of the IPT centrifugal equator plane crossings. Oliversen et al. used a semi-empirical model of the IPT, recently improved by Smyth, Peterson, & Marconi (2011), to help reach this conclusion. In addition to the smoothly varying [OI] signal, predicted by the Smyth et al. model, the [OI] data also show statistically significant positive perturbations, or "departure events." In this work we analyze the shortest timescale "departure events," which occur at the sampling period of the data (~20 minutes). These can be spotted in the data in a model-independent way using simple statistical techniques. Using the Oliversen et al. (2001) subset of ~1000 points, we find the system III distribution of the short-term "departure events" is fairly uniform, with an excess near system III = 130 ~ 160 degrees. The peak of the torus plasma density happens to occur in this system III longitude range (e.g., Brown 1995, Steffl et al. 2004ab, 2006, 2008). We will discuss various processes which might give rise to this apparent correlation.