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

<body> <H2 ALIGN=CENTER>Simultaneous HST/STIS and Groundbased Observations of Sulfur in the Io Plasma Torus </H2> <CENTER> <A HREF="/rcwj/" target="_top">R. Carey Woodward, Jr.</A>,<SUP>1,5</SUP> Fred L. Roesler,<SUP>1,5</SUP> Ronald J. Oliversen,<SUP>2,5</SUP> William H. Smyth,<SUP>3</SUP> H. Warren Moos,<SUP>4</SUP> and Frank Scherb<SUP>1,5</SUP> </CENTER> <p> <I> This work was originally presented as a "poster paper" at the <A HREF="http://www.agu.org/meetings/sm00top.html" target="_top">Spring 2000 meeting</A> of the <A HREF="http://www.agu.org/" target="_top">American Geophysical Union</A>. Here you may view the hypertext version of the paper (though it looks better in a frames-capable browser), or download a <a href="/rcwj/reprints/AGUS2K.ps.gz">compressed PostScript</a> version or an <a href="/rcwj/reprints/AGUS2K.pdf">uncompressed PDF</a> version (you may have to shift-click those links). In each figure, you can view a larger version of a graphic by clicking it (or download the larger version by shift-clicking it if your browser works that way). Address queries on this work to <a href="mailto:woodward@physics.wisc.edu"> woodward@physics.wisc.edu</a>. <a href="#copy">Copyright statement</a> follows. </I> <a name="abs"> <h3>Abstract</h3> 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. <P> 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. <a name="ref"> <h3>References</h3> <DL> <DT>[Roesler et al. 1999] <a name="R9"> <DD> Roesler, F. L., H. W. Moos, R. J. Oliversen, R. C. Woodward, Jr., K. D. Retherford <EM>et al.</EM>. "<A HREF="/rcwj/IoSTIS.html" target="_top">Far-Ultrviolet Imaging Spectroscopy of Io's Atmosphere with HST/STIS</A>." <A HREF="http://science-mag.aaas.org/" target="_top"><EM>Science</EM></A>, <STRONG>283</STRONG>, 353, 1999. <DT>[Scherb et al. 1998] <a name="S8"> <DD>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.", <a href="http://www.agu.org/pubs/eos.html"><em>Eos</em></a>, <STRONG>79</STRONG>(17), S201. <DT>[Woodward et al. 1999] <a name="W9"> <DD> Woodward, Jr., R. C., R. J. Oliversen, F. Scherb, A. Steffl, G. M. Hilton, and N. E. Doane. "<A HREF="/rcwj/AAS99abs.html" target="_top">Synoptic Broadband Imaging of the Io Plasma Torus in SII: First Results</A>." <EM><A HREF="http://www.aas.org/publications/baas/baas.html" target="_top">Bull. American Astron. Soc.</A></EM>, <STRONG>31</STRONG>(3), 849, 1999. <DT>[Woodward and Smyth 1994] <a name="W4"> <DD> Woodward, Jr., R. C. and W. H. Smyth. "Modelling the Io Plasma Torus: Effect of Global Parameters." <CITE><A HREF="http://www.aas.org/publications/baas/baas.html" target="_top">Bull. Amer. Astron. Soc.</A></CITE>, <STRONG>26</STRONG>(3), 1139, 1994. </DL> <h3>Motivation</h3> 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 <a href="splash.html#R9" target="figure">Roesler et al. 1999</a>, 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 <a href="fig1.html" target="figure">(fig. 1)</a>, since we simultaneously acquired [O I] 6300Å spectra of Io <a href="splash.html#S8" target="figure">Scherb et al. 1998</a> and [S II] 6731Å images of the plasma torus <a href="splash.html#W9" target="figure">Woodward et al. 1999</a> <a href="fig2.html" target="figure">(fig. 2)</a> 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. <a name="mod"> <h3>Modelling</h3> We have available a semiempirical model of the Io plasma torus <a href="splash.html#W4" target="figure">Woodward and Smyth 1994</a>, 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 <img src="epsilon.xbm" alt="<strong>e</strong>">, the ratio of convectional to corotational electric fields. We modeled the [S II] 6731Å data for several possible values of <img src="epsilon.xbm" alt="<strong>e</strong>"> and correlated them with the "ribbon" feature in the data; the best fit, shown in <a href="fig3.html" target="figure">fig. 3A</a>, is <img src="epsilon.xbm" alt="<strong>e</strong>"> = 0.040 ± 0.005. This is an unusually (but not unprecedentedly) large value, but the generally accepted average value of <img src="epsilon.xbm" alt="<strong>e</strong>"> = 0.025 is clearly not right <a href="fig3.html" target="figure">(fig. 3B)</a>. 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 <img src="epsilon.xbm" alt="<strong>e</strong>">, because changes in <img src="epsilon.xbm" alt="<strong>e</strong>"> 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, <img src="epsilon.xbm" alt="<strong>e</strong>"> had little effect on the model in the slit <a href="fig4.html" target="figure">(fig. 4)</a>. <a name="pro"> <h3>1256Å profile:</h3> 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 <a href="fig5.html" target="figure">(fig. 5)</a>. Even after allowing for the background torus emission, there is significant excess emission to at least several R<sub><small>Io</small></sub>--much more than is present in the neutral emissions in the same STIS images. <a href="fig6.html" target="figure">Fig. 6</a> shows the 1256Å profile from the fourth STIS image, which is simultaneous with the fifth groundbased image <a href="fig3.html" target="figure">(fig. 3)</a>, 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. <a name="bri"> <h3>Brightening:</h3> As reported earlier <a href="splash.html#R9" target="figure">Roesler et al. 1999</a>, the UV neutral emission lines in the STIS data brightened by a factor of <~ 2 <a href="fig7.html" target="figure">(fig. 7A)</a> over the night, while [O I] 6300Å observed simultaneously from the ground brightened fourfold <a href="fig7.html" target="figure">(fig. 7B)</a>. Examining the S II 1256Å data <a href="fig7.html" target="figure">(fig. 7C)</a> after removal of the background torus shows a threefold brightening, or, in the case of emission at 4-10 R<sub><small>Io</small></sub>, 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. <a name="sum"> <h3>Summary:</h3> <ul> <li> Using the [S II] 6731Å synoptic torus imager data, we determined <img src="epsilon.xbm" alt="<strong>e</strong>"> = 0.040 ± 0.005 during the October 1997 STIS observations of Io. <li> The spatially extended S II 1256Å emission near Io is asymmetric and significantly enhanced compared to the neutral UV emission lines. <li> 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. </ul> <hr> <SMALL> <SUP>1</SUP>University of WisconsinMadison (Dept. of Physics)<BR> <SUP>2</SUP>NASA Goddard Space Flight Center<BR> <SUP>3</SUP>Atmospheric & Environmental Research, Inc.<BR> <SUP>4</SUP>Johns Hopkins University (Dept. of Physics & Astronomy)<BR> <SUP>5</SUP>Visiting 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 <p><a name="copy"> Copyright © 2000 <A HREF="/rcwj/" target="_top">R. Carey Woodward Jr.</a> and/or the Board of Regents of the <a href="http://www.wisc.edu/" target="_top">University of Wisconsin</a>, 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 <a href="#abs">abstract</a> published in <a href="http://www.agu.org/pubs/eos.html"><em>Eos</em></a> <strong>81</strong>(19), S290, 2000. During conversion from poster to hypertext format, an error in the data shown in <a href="fig7.html" target="figure">fig. 7C</a> was discovered; that figure, the section entitled <a href="body.html#bri" target="text">Brightening</a>, and the final summary item have been changed accordingly from the originally presented version. Other minor syntactic and stylistic changes have also been made. </small> </body>