The Io Plasma Torus: Motivation for Abandoning the "Active Sector" Concept in Favor of System IV Modulation

Abstract:

We use an extensive spectroscopic dataset of 1000 observations of Io in [OI] 6300A, presented by Oliversen et al. (2001), narrow-band images of the torus in [SII] 6731A, and HST/STIS EUV observations of Io and its environs to confirm that the Io [OI] flux is an excellent proxy for the electron density in the Io plasma to torus (IPT). Furthermore, we find:

(1) A careful statistical analysis of short-term variations in this dataset (20 min to 1 hour), previously suspected to be from flux tube interchange (Oliversen et al. 2001, Morgenthaler et al. 2012) are, in fact, consistent with the expected statistical variation of the parent population of observations.

(2) The semi-empirical IPT model developed by W. Smyth (Oliversen et al. 2001; Smyth, Peterson, & Marconi 2011) fits the overall trends in the data reasonably well, with notable exceptions.

(3) There may be a link between what was previously known the "active sector" at system III longitudes of 170 ~ 230 degrees and the modulation in IPT plasma density caused by the beating between system III and system IV, predicted by Hess et al. (2011).

Conclusion: a simple modification to the W. Smyth semi-empirical torus model to incorporate a system IV-controlled density enhancement, rather than a fixed "active sector" in system III, should enable the model to "lock in" more readily to our extensive set of [OI] 6300A observations recorded between 1990 and 2008. This will provide detailed information about the flow of mass and energy in the IPT during the Galileo, Ulysses, Cassini, and New Horizons missions. Additional observations are planned during NASA's upcoming Juno mission.

This work was supported by NASA Planetary Research Program RTOP 344-32-40 to GSFC and grants NAGW-3319 and NAG5-6787 to the University of Wisconsin--Madison, STIS contract NAS5-30131 to the University of Wisconsin--Madison and NASA Outer Planets Research Program grant NNX11AM43G to PSI.