Simultaneous HST/STIS and Groundbased Observations of Sulfur in the Io Plasma Torus

Motivation

Observations of Io with the Space Telescope Imaging Spectrograph (STIS) over the last 2.5 years have yielded several interesting results in the study of Io's neutral sulfur and oxygen emission features Roesler et al. 1999, but the ion lines in these same data have been much less studied thus far, in part because of the difficulty of distinguishing between emission from ions local to Io and emission from the plasma torus in which Io is always, to some degree, immersed. We have been particularly interested in the six spectrally resolved images acquired during October 1997 (fig. 1), since we simultaneously acquired [O I] 6300Å spectra of Io Scherb et al. 1998 and [S II] 6731Å images of the plasma torus Woodward et al. 1999 (fig. 2) from the ground. As reported earlier, we found a simultaneous brightening of the neutral lines in the optical and UV data; thus, once it became clear that all or nearly all of the emission near 1250Å was S II 1256Å, rather than S I 1251Å, we wished to know if this ion line underwent the same brightening. To do this quantitatively, however, we had to remove the contribution of the underlying torus.

Modelling

We have available a semiempirical model of the Io plasma torus Woodward and Smyth 1994, but the torus is highly variable and the model accordingly has many free parameters. To reduce this ambiguity, we fit the model to the groundbased images of the torus. These images do not overlap all STIS images in time, and the seeing was so poor when the last two images were acquired that they are almost useless. Moreover, they include only the east (approaching) ansa of the torus, and Io was on the west (receding) side; the STIS data, of course, are from the west side. (We have since learned how to get useful data under some circumstances even when Io or other moons are in the field of view.) Nonetheless, these images provide significant constraints on the model. In this work, we examine only the effect of adjusting e

, the ratio of convectional to corotational electric fields. We modeled the [S II] 6731Å data for several possible values of e

and correlated them with the "ribbon" feature in the data; the best fit, shown in fig. 3A, is e

= 0.040 ± 0.005. This is an unusually (but not unprecedentedly) large value, but the generally accepted average value of e

= 0.025 is clearly not right (fig. 3B). We then modeled the S II 1256Å emission from the torus expected in the STIS slit. We expected it to be sensitive to the value of e

, because changes in e

result in changes in the location of the "ribbon," a prominent feature near Io with a sharp radial gradient, but found that, because of interactions of jovian magnetic longitude, Io orbital phase, and viewing geometry, e

had little effect on the model in the slit (fig. 4).

1256Å profile:

Examining the STIS data, we averaged the 1256Å slit image along the horizontal (dispersion) dimension of each STIS image, yielding one-dimensional spatial profiles along the length of the slit (fig. 5). Even after allowing for the background torus emission, there is significant excess emission to at least several R_Io--much more than is present in the neutral emissions in the same STIS images. Fig. 6 shows the 1256Å profile from the fourth STIS image, which is simultaneous with the fifth groundbased image (fig. 3), after subtraction of the modeled torus background contribution. There is a clear asymmetry (also unlike the neutral emissions): an excess on the positive side of Io, which is the direction that is nearer to Jupiter and to the torus centrifugal equator. Presumably, this is a consequence of the greater torus electron density in that direction; further model calculations are planned to test this hypothesis.

Brightening:

As reported earlier Roesler et al. 1999, the UV neutral emission lines in the STIS data brightened by a factor of <~ 2 (fig. 7A) over the night, while [O I] 6300Å observed simultaneously from the ground brightened fourfold (fig. 7B). Examining the S II 1256Å data (fig. 7C) after removal of the background torus shows a threefold brightening, or, in the case of emission at 4-10 R_Io, possibly sixfold. Even ignoring the earliest extended point in fig. 7C, it is clear that the increase in S II 1256Å intensity is significantly greater than that of the UV netural lines, and approaches that of [O I] 6300Å.

Summary:

Using the [S II] 6731Å synoptic torus imager data, we determined = 0.040 ± 0.005 during the October 1997 STIS observations of Io.
The spatially extended S II 1256Å emission near Io is asymmetric and significantly enhanced compared to the neutral UV emission lines.
The Io S II 1256Å emission brightens up by a factor of >~ 3, more closely resembling [O I] 6300Å than the neutral UV lines. This brightening is not simply an artifact of viewing geometry.