On 14 October 1997 Io and the Io plasma torus were simultaneously observed by
three different instruments: the HST Space Telescope Imaging Spectrograph
(STIS) obtained six spectrally resolved images in the 12001700Å
range of a 2" × 25"
rectangle encompassing Io [Roesler et
al. 1999]; a nondispersing imager at the McMath-Pierce west auxiliary
telescope [Woodward et al. 1999] obtained nine images of the east ansa of the
plasma torus in [S II] 6731Å; and the echelle spectrograph on the
McMath-Pierce main telescope obtained eighteen spectra of [O I]
6300Å from a 5" × 5"
aperture centered on Io [Scherb et
al. 1998]. Io's neutral emissions in these datasets have been discussed
elsewhere. Here we will examine the emissions of S II, both near Io
and in the torus, primarily in the STIS dataset.
Because Io was near western (receding) elongation when these data were
acquired, the torus emission observed by STIS was strongly influenced by
line-of-sight geometry, and therefore by the precise location of the
torus. The torus radial position and scale height are known to vary with
magnetic longitude, local time, and other factors in ways that are not well
understood; therefore, the groundbased torus images, in addition their
intrinsic interest, are crucial in interpreting the spatially limited STIS
data: with the aid of a semiempirical model of the plasma torus [Woodward and
Smyth 1994], these images allow us to model the underlying torus emission in
the STIS data, and thus to separate it from emission local to Io, and
distinguish true temporal variations from the effects of viewing geometry. In
particular, we will examine the behavior of the torus and near-Io ion
emissions during a brightening of neutral iogenic emissions at 06:00 UT.
- [Roesler et al. 1999]
- Roesler, F. L., H. W. Moos, R. J. Oliversen, R. C. Woodward, Jr., K. D.
Retherford et al..
Imaging Spectroscopy of Io's Atmosphere with HST/STIS."
283, 353, 1999.
- [Scherb et al. 1998]
- Scherb, F., R. J. Oliversen, F. L. Roesler,
R. C. Woodward, Jr., J. Corliss, M. Fred, and
W. H. Smyth. "Recent Ground-Based Observations of O I
6300Å Emission from Io.",
- [Woodward et al. 1999]
- Woodward, Jr., R. C., R. J. Oliversen, F. Scherb, A. Steffl,
G. M. Hilton, and N. E. Doane.
Broadband Imaging of the
Io Plasma Torus in SII: First Results."
Bull. American Astron. Soc.,
- [Woodward and Smyth 1994]
- Woodward, Jr., R. C. and W. H. Smyth.
"Modelling the Io Plasma Torus: Effect of Global Parameters."
Bull. Amer. Astron. Soc.,
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
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 , the ratio of convectional to corotational electric
fields. We modeled the [S II] 6731Å data for several possible
values of and correlated them
with the "ribbon" feature in the data; the best fit, shown in fig. 3A, is = 0.040 ± 0.005. This is an unusually (but
not unprecedentedly) large value, but the generally accepted average value of
= 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 , because changes in 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, had little
effect on the model in the slit (fig. 4).
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
RIo--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
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 RIo, 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.
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.
1University of WisconsinMadison (Dept. of Physics)
2NASA Goddard Space Flight Center
3Atmospheric & Environmental Research, Inc.
4Johns Hopkins University (Dept. of Physics & Astronomy)
5Visiting Astronomer, National Solar Observatory, National
Optical Astronomy Observatories, which is operated by the Association of
Universities for Research in Astronomy, Inc., under contract with the
National Science Foundation
Copyright © 2000
R. Carey Woodward Jr. and/or
the Board of Regents of the
University of Wisconsin,
as their interests lie. You may view and
download this work for your personal use, but may not redistribute it or make commercial use of it without permission. The
abstract published in
81(19), S290, 2000.
During conversion from poster to
hypertext format, an error in the data shown in fig. 7C was
discovered; that figure, the section entitled Brightening, and the final
summary item have been changed accordingly from the originally presented
version. Other minor syntactic and stylistic changes have also been made.