Io's equatorial spots: Morphology of neutral UV emissions

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. A12, PAGES 27,157-27,165, DECEMBER 1, 2000 Io's equatorial spots: Morphology of neutral UV emissions Kurt D. Retherford, H. Warren Moos, Darrell E Strobel, and Brian C. Wolven Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, Maryland Fred L. Roesler Physics Department, University of Wisconsin, Madison Abstract. The first observations of Io with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) showed that the brightest ultraviolet emissions come from localized regions near Io's equator, designated "equatorial spots." This paper presents a detailed study of the location, shape, and brightness of the equatorial spots in near-monochromatic images obtained using STIS in the first-order long-slit spectroscopy mode. This study provides evidence that the equatorial emissions are linked to the interaction between the Jovian magnetosphere and Io's atmosphere. The morphology of the equatorial spots reported here provides additional information on the nature of this complex electrodynamic interaction. We find the following principal results: the locations of the equatorial spots are correlated with the Jovian magnetic field orientation at Io, but with a relation that is not 1:1; the equatorial spots are centered 10 ø- 30 ø longitude downstream from Io's sub-jovian longitude; the brightness of the emissions in this data set is correlated with Io's distance from the plasma toms centrifugal equator; and the anti-jovian equatorial spots are 20% brighter than the sub-jovian equatorial spots. ther the plasma torus or Jupiter's ionosphere. H - emissions 1. Introduction at the foot of Io's flux tube in Jupiter's atmosphere [Con- While aurorae on Earth and other planets are found near nerney et al., 1993] and the correlation of decametric radio their magnetic poles, Io's brightest auroral emissions are emissions with Io's location [Bigg, 1964] suggest the latter. found near its equator. Furthermore, the auroral emissions To study this interaction, observations using both the Hubare caused by collisions with particles trapped not along ble Space Telescope (HST) and the Galileo spacecraft have magnetic field lines intrinsic to Io, but along those of Jupiter. revealed, with unprecedented detail, the spatial distribution Referred to as "equatorial spots," these bright emission reof Io's auroral emissions [Roesler et al., 1999; Trauger gions are localized near Io's sub-jupiter and anti-jupiter et al., 1997; Geissler et al., 1999]. The equatorial spots points. are one of four general regions of UV emission we observe Io's auroral emissions arise from the interaction between at Io using HST's Space Telescope Imaging Spectrograph its atmosphere and Jupiter's magnetosphere. As Jupiter's (STIS); these regions are shown in Figure 1 and Figure 2. magnetic field sweeps past Io, charged particles in Io's coro- The changing location of the equatorial spots with the Jona are accelerated by a reduced corotational electric field and vian magnetic field orientation Io [Roesler et al., 1999] eventually move with the field. These trapped particles form is evidence for a strong association between the equatorial a torus of plasma that roughly surrounds Io's orbit and corospots and the field-aligned currents. Several authors [Saur tares with Jupiter's magnetosphere. As this plasma sweeps et al., 1999; Linker et al., 1998] have constructed theoretical past the satellite, it collisionally ionizes neutral gases in the models of the plasma-atmosphere interaction which account atmosphere. Hence the plasma torus resupplies itself as for this association. However, very little experimental inforthese newly formed ions are picked up. The pickup of these mation has been available for evaluating these models. The ions contributes to an electric conductivity perpendicular to information presented in this pape regarding the equatorial the magnetic field lines [Goertz, 1980], as do elastic collispot morphology will be of value in the assessment of desions in Io's ionosphere. The corotational electric field gen- tailed simulations based on these models. erates an electric current of the order of 10 million amps, which is though to be carried into Jupiter's magnetosphere 2. Observations along field-aligned Alfv6n wings [Neubauer, 1980; Saur et al., 1999]. The circuit for this current is then closed in ei- Io's UV emissions were imaged in 30 separate exposures during 15 HST orbits. HST observed Io on two occasions Copyright 2000 by the American Geophysical Union. ("visits") in 1997 when Io was near western elongation (the Paper number 2000JA Jovian duskside). HST also observed Io in three visits in /00/2000JA : two with Io on the duskside of Jupiter and one with 27,157

2 . 27,158 RETHERFORD ET AL.' IO'S EQUATORIAL SPOTS Sub-Earth Longitude 271 ø j< Limb. GLOW :-'.:. : : ; :.:.:: ;... ; -.-:.-... : :: :,.... IoN reasons, images of the O I] 1356/, 1359/ emission dou- blet have primarily been used throughouthis paper, except as noted. The images were corrected for line of sight velocity shifts. Io's velocity relative to Earth was as high as,-,,40 km/s for some observations. This resulted in emission line Doppler shifts of,-,,3 pixels for observations using the G140M and G230M grating modes but of only,-,,0.3 pixels for observations using the G 140L grating mode. :? Data Analysis :-...::( :::. & ::...::...:: : -: Equatorial Spot Locations The equatorial spots are not uniformly shaped. We therefore describe their central locations relative to Io using the centroids of the image intensities, determined as follows. First, we encompass one equatorial spot by selecting a 41 x 41 pixel square subimage (41 pixels m 1 Io diameter) cen- Sub-jOVa tered on its brightest pixel. This subimage always excludes Equatorial S... EquatOrial Spot the other equatorial spot. Next, pixels with brightnesses lower than that of an effective background brightness (described below) were assigned this brightness to eliminate Extended Corona their contribution. After summing the image in the spatial direction, the following formulas were used to find the cen- Figure 1. STIS image of OI] 1356 A emission with Io troid position < x > with uncertainty a<x> along the disperat sub-earth longitude 270 ø (western elongation), October sion axis: 14, Three of four general regions of auroral emis-.41 XiIx i sion are labeled here. The fourth region (wake) is labeled in <x>= Z/4 '--- 1 ;x,' Figure 2. The images have been smoothed for display only. IoN, J, and B label the directions of Io north, Jupiter, and the nominal Jovian magnetic field, respectively. The location of the O I] 1359 A image is denoted by the gray circle. Contours IJ<x> -" 41 IJ ciifi Jr- Ix i (xi-- < x >, Zi= Ix v i= show 500 R increments in brightness. where Xi is the distance of the pixel along the dispersion axis, Ixi is the flux value of the pixel (having been summed in the spatial direction), and the summation is performed over Io on the dawnside of Jupiter. The satellite angular diame- the number of pixels along the dispersion axis. The uncerters were 1 16 and 1! 10 during the two visits in 1997 and tainty a<x> is the propagated centroid error and depends on 1! 25, 1! 25, and 1 26 during the three visits in Ta- axi, the nominal relative astrometric uncertainty (0.25 pixels, ble 1 lists observational and Io ephemeris-based information [Space Telescope Science Institute (STScI), 1998]), and (calculated at the midpoint times of the exposures) for each the propagated statistical error for flux (propagated through exposure in the data set. the calibration and the subsequent summation in y). The cen- STIS's 52" x2" slit was used to image Io's disk in the "slit- troid position < y > with uncertainty a<y> along the spatial less" first-order long-slit spectroscopy mode. In this mode axis was similarly determined. Other methods for determineach emission line produced an individual monochromatic ing the equatorial spot locations, such as using the brightest image of Io, but the separation between images depended on pixel position, gave simila results. the dispersion of the grating selected. Grating modes (with To quantify the change in equatorial spot location, we central wavelengths in parentheses) G 140L (1425), G 140M used two different approaches that have different advantages (1371), G140M (1470), and G230M (1933) were used to and disadvantages but provide consistent results. In the first isolate selected atomic oxygen and sulfur emissions with approach, we calculated the angle 0 between a line connectvarying degrees of overlap between their multiplet com- ing the two equatorial spot centroids (as seen in the twoponents. These grating modes have dispersions of dimensional image) and Io's equatorial plane. This avoids,3dpixel, 0.053,3dpixel, 0.053/!Jpixel, and 0.087,3dpixel, re- referencing the individual equatorial spot locations to the spectively. As illustrated by Roesler et al. [ 1999], images of center of Io in the images, our determination of which could the O I] 1356/, 1359/ emission doublet using the G140L be off by 1-2 pixels. However, at certain viewing geomemode were reasonably well separated from other emission tries, one of the equatorial spots was obstructed. Therefore line images. When using the G140M mode, these O I dou- this approach could be used only for observations in which blet emission line images were sufficiently separated from Io's sub-earth longitude was within +30 ø of western eloneach other to isolate the equatorial spot features. For these gation. (Seen from Earth, Io's geocentric orbitalongitude

3 RETHERFORD ET AL.: IO'S EQUATORIAL SPOTS 27,159 A o4xm02010 ' B o4xm01080 Sub-Earth Longitude 28 ø IoN Sub-Earth Longitude 327 ø T IoN ß ::!-': v v:-..-cu " : :. -, :!':"::: ;:":' :":;:i:: :: 'i!':i:'""%i!,:: ::' :.,/':'r'-"-'> ;.'?..i:..:::., ::.:: :C::%.. X:... "-:,:'i?,':---' ::;:i?'.:?::-" :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::: ":'. : '. -.-.: : :... ::: :/:':-:.':-..'.-'-- :... [.:i: ; ;:": i::: i;..'... :-::::,:>:'i" ::: :.'... :: -":,. :,,: '"', ::::::::::: ;.," '" ::i?::::.-.. :;:.../ :...'- "--" - :: ::: ::...-.:i::,: i ::--... ' i : :a b- n> :::: :l... ::::... r'":-'"' "::::.i:::::. Figure 2. Dawnside image and duskside image of Io's OI] 13563, emission obtained in 1998 with Io at sub-earth longitudes,-,30 ø and,- 330 ø, respectively (using the G140L grating mode). The sub-jovian longitude is indicated by a dashed meridian, while the downstream and upstream longitudes (90 ø and 270 ø, respectively) are indicated with dotted meridians. The sub-jovian equatorial spot appears to be shifted downstream from the sub-jovian longitude. is approximately equal to Io's sub-earth longitude because latitude at which the nominal Jovian magnetic field is tanof its synchronous rotation, so Io has a sub-earth longitude of,- 270 ø when at western elongation.) We used the six orbits of data from 1997 used by Roesler et al. [1999] but gent to Io along the sub-jovian and anti-jovian meridians (calculated with the 04 offset, tilted dipole model [Acuna et al. 1983, p. 7]). Linear fits to the sub-jovian and antimeasured each exposure per orbit separately (12 exposures) Jovian measurements are shown as dotted and dashed lines and added five exposures from Figure 3a shows 0 plotted versus the angle between the local Jovian magnetic field lines at Io and Jovian south in the image; the error bars shown are the propagated statistical uncertainties for 0. A with slopes of and , respectively, while a 1:1 relation is shown as a solid line for comparison. The latitudes of the sub-jovian equatorial spots are on average equivalent to the latitudes of the magnetic field line tangent linear fit to the measurements is shown as a dotted line with points, but the latitudes of the anti-jovian equatorial spots slope , while a 1:1 relation between the orientation of the equatorial spot locations and the orientation of the nominal Jovian magnetic field is shown as a solid line. This indicates that while a correlation exists, the true relationship is not adequately described by a simple 1:1 relationship. In the second approach we calculated the Iocentric latitude of each equatorial spot centroid location. This calcuare on average closer to the equator than the magnetic field line tangent points, though only with one sigma certainty. Taken together, however, a linear fit to all of the measurements has a slope of , which is consistent with a slope of 1.0 for a 1:1 relation. Note thathis slope agrees with that found using the first approach shown in Figure 3a. However, the statistical uncertainties this second approach lation relies on our determination of the center of Io in the limit further insight into the relation between the equatorial images. We thereforestimate the error for the measured spot locations and the orientation of the local Jovian maglatitudes as 4-5 ø on the basis of the uncertainty in our determination. The advantage of this approach is that obsernetic field at Io. Images obtained at certain viewingeometries also allow vations in which one of the equatorial spots was obstructed determination of the Iocentric longitude of the sub-jovian are not excluded. All of the OI] 1356 ] and SI] 1900 ] images were used, resulting in 43 measurements of equatorial spot latitude (26 sub-jovian, 17 anti-jovian) and providing equatorial spot centroid location. From the images in Figure 2 we conclude that these equatorial spots are centered about 10 ø- 30 ø longitude downstream from Io's sub-jovian more than twice as much information as the first approach. meridian. The apparent offset from the sub-jovian longitude Figure 3b shows these measured latitudes plotted versus the in the duskside image (with sub-earth longitude of,- 330 ø,

4 27,160 RETHERFORD ET AL.: IO'S EQUATORIAL SPOTS Table 1. Observational Summary and Io Ephemeris-Based Information a Root Name Date, UT Sub-Earth b c d e rstart, UT TExp., s Grating Longitude 'III VIII gc o49d /09/97 10:29: G230M o49d02a10 26/09/97 10:47: G230M o49d /09/97 11:51: G230M o49d02a20 26/09/97 12:18: G230M o49d /09/97 13:34: G140M o49d02a30 26/09/97 13:56: G140M o49d /10/97 02:44: G140L o49d01a10 14/10/97 03:03: G140L o49d /10/97 04:06: G140L o49d01a20 14/10/97 04:33: G140L o49d /10/97 05:43: G140L o49d01a30 14/10/97 06:10: G140L o4xm /08/98 18:53: G140M o4xm /08/98 19:12: G140M o4xm /08/98 20:25: G140M o4xm /08/98 20:52: G140M o4xm /08/98 22:07: G140M o4xm /08/98 22:30: G140M o4xm /08/98 23:42: G140M o4xm /08/98 00:05: G140M o4xm /08/98 17:35: G140L o4xm /08/98 18:00: G140L o4xm /08/98 19:15: G140L o4xm /08/98 19:38: G140L o4xm /08/98 20:51: G140L o4xm /08/98 21:14: G140L o4xm /08/98 17:15: G140L o4xm /08/98 18:20: G140L o4xm /08/98 18:39: G140L o4xm /08/98 19:58: G140L acalculated at exposure midpoint. bread 26/09/97 as September 26, CJovian System III (magnetic) longitude. d Jovian System III latitude (calculated with the 04 offset, tilted dipole magnetic field model). edistance of Io from the plasma toms centrifugal equator in units of Rio (positive for Io north). right side of Figure 2) could potentially result from the twodimensional projection of the brightest part of the equatorial spot. However, this would require a height of 500 km above Io's surface, which is much more than heights of 100 km that we measured in images at elongation; large ranging from 1.5 to 3 Io diameters away from the center of Io along the length of the slit nearest Io's upstream hemisphere; this avoids any downstream wake-related emission. In the O I] 13563, images, this effective background brightness ranged from 20 to 56 rayleighs (R), which is of the uncertainties prevent a more quantitative height measure- order of the faintest emission seen on Io's disk. In the ment though. Furthermore, the similar offset in the dawnside image (with sub-earth longitude of 30 ø, left side of S I] 19003, images this background brightness ranged from 2320 to 3100 R, which is a measure of the much higher de- Figure 2) surely could not result from the projection of the tector dark rate associated with the use of STIS's NUV deequatorial spot's brightest part, as it is seen almost straight tector (with the G230M grating mode [STScl, 1998]). on Equatorial Spot Brightness To determine the brightness of each equatorial spot, we averaged the brightness of 9 pixel wide square boxes (81 pixels) centered on the centroid locations. This reduces Prior to determining the brightness of an equatorial spot the effects of statistical noise on the measurement. Anfrom the image data, an effective background brightness other advantage of this method is that the 9 pixel width was subtracted in order to exclude contributions from extended cloud emission from Io, instrumental scattered light, and detector background. This effective background was of the box reduces the effects of multiplet smearing in the G140L images (the O I] 1356/k image is separated from the OI] 1359 /k image by 5 pixels). In the G140M imdetermined by averaging the brightness of an image region ages the OI] 1356/k, 1359/k brightnesses were measured

5 RETHERFORD ET AL.: IO'S EQUATORIAL SPOTS 27,161 separately and then added together. Since the image plate scales vary between the grating modes used, the 9 pixel wide square boxes correspond to solid angles of (arcsec) 2 for the G 140L mode, (arcsec) 2 for the G 140M mode, and (arcsec) 2 for the G230M mode. Combined with the effect of different angular sizes due to differences in the Earth-Io separation, this resulted in our sampling slightly different portions of the equatorial spots in each image. We estimate that this systematic error is of the order of the propagated statistical error and therefore acceptable. The measured brightness values and statistical errors are listed in Table 2 and plotted in Figure 4 versus Io's magnetic longitude, Io's distance from the plasma torus centrifugal equator, and Io's sub-earth longitude. The brightness of the equatorial spot emissions is correlated with Io's magnetic longitude (Figure 4a) and therefore also with Io's distance from the plasma torus centrifugal equator (Figure 4b). Plasma torus modeling by Bagenal [1994] predicts (using their Figure 9) a factor of 2 decrease in torus electron density when Io is at its farthest distance from the torus equator (,- 30 Rio). This is consistent with the observed decrease in equatorial spot brightness. The ratio of average equatorial spot brightness for Io less than -t-20 Rio from the torus equator divided by that for Io more than -t-20 Rio from the torus equator is 1.77-t Note that only seven of the 30 data points in Figure 4b were obtained when Io was less than -t-20 Rio from the torus equator; more data in this range would be desirable, especially with Io near magnetic longitudes of,- 290 ø (Figure 4a). The trend of increasing brightness with decreasing distance to the plasma torus equator is surely not the only trend, as the mean brightnesses with Io less than and more than -t-20 Rio from the torus equator have large standard deviations (1440-t-570 and 750-t-200, respectively). We have not identified the source of this additional variability. Figure 4c shows little correlation between brightness and sub-earth longitude; potential viewing geometry effects do not account for the additional variability. The brightness of the anti-jovian equatorial spot is generally greater than that of the sub-jovian equatorial spot. Using images with neither equatorial spot obstructed (-t-20 ø from elongation, see Table 2), we calculate that the anti-jovian equatorial spot is on average % brighter than the sub- Jovian equatorial spot. a b 20 Ol]1356 ]19oo.. '"" t o o Projected angle of magnetic field Latitude of magnetic field tangent point Figure 3. Relation between the location of the equatorial spots and the orientation of the local Jovian magnetic field: two approaches. (a) Orientation versus magnetic field orientation. The orientation of a line connecting the centroids of the two equatorial spots is compared with the orientation of the nominal Jovian magnetic field in the images. A positive angle 0 corresponds to the sub-jovian equatorial spot being located south of Io's equator, and the magnetic field orientation at Io is measured counterclockwise from Jovian south in the images. The dotted line represents a linear least squares fit to the data. (b) Latitude versus magnetic field tangent latitude. The Iocentric latitude of a sub-jovian equatorial spot centroid is compared with the latitude of the magnetic field line tangent point along Io's sub-jovian meridian (squares). Likewise, the Iocentric latitude of an anti-jovian equatorial spot centroid is compared with the latitude of the magnetic field line tangent point along Io's anti-jovian meridian (crosses). Positive (negative) latitudes correspond to Iocentric north (south). A 1:1 relation is shown with a solid line, the sub-jovian linear fit is shown with a dotted line, and the anti-jovian linear fit is shown with a dashed line. Though not indicated with symbols as in Figure 3a, eight points were derived from S I] 1900 A emission images.

6 27,162 RETHERFORD ET AL.: IO'S EQUATORIAL SPOTS Table 2. Equatorial Spot Brightness Root Name Sub-Jovian o49d02010 a o49d02a o49d02020 a o49d02a20 b o49d02030 b o49d02a o49d o49d01a o49d o49d01a o49d o49d01a30 b o4xm03010 b o4xm03020 b o4xm03030 b o4xm o4xm o4xm o4xm o4xm o4xm o4xm o4xm o4xm o4xm o4xm c Error Anti-Jovian Error Ratio( Anti d S-- - J Error a SI] 1900 images rather than OI] 1356 doublet images. Thes exclude S I] 1915 A images whic have low signal to noise due to a large detector background. b The O I] 1356/k and O I] 1359 image brightnesses are measured separately and then added. c The anti-jovian equatorial spot was obscured for those observations not within +30 ø of western elongation and so was not measured for several images. d Observations not within +20 ø of western elongation were further excluded in determining the average brightness ratio. 4. Discussion We now summarize the explanation given by Saur et al. [2000] for why Io's UV aurora is brightest near the sub- Jupiter and anti-jupiter points. They find that the emis- sion locations depend in part on the energy balance and the plasma flow of the electrons. The convection of torus flux tubes into Io's atmosphere transfers electron energy from the torus to the ionosphere, preferentially into the flanks and the upstream side. The electrons within these flux tubes collide 25OO 2000 a b c , ,,,,r,.,,...,.... J J o c ; x x + * ß 1000' ; [ Magnetic Longitude ( ) Distance of Io from Torus equator (Io radii) Sub-Earth Longitude (") Figure 4. Plot of O I] 1356 brightness (R) in a 9 pixel wide box centered on each equatorial spot versus (a) Io's magnetic longitude, (b) Io's distance to the plasma torus equator (in Rio), and (c) Io's sub-earth longitude. The brightnesses plotted here are averages of the sub-jovian and anti-jovian equatorial spot brightnesses, for only those observations within 30 ø of western elongation; otherwise, the sub-jovian equatorial spot brightness is plotted (as listed in Table 2).

7 with the neutral atmosphere, with the maximum number of collisions occurring at the part of the flux tube closesto Io where the atmospheric density is greatest; for a flux tube tangent to a spherically symmetric atmosphere, this corresponds to the magnetic field line tangent point. As the flux tube travels downstream, these collisions continue to create atomic emissions until the flux tube energy has been depleted. Heat conduction along the flux tubes partially replenishes their energy content, allowing the collisional ion- izations and excitations to continue farther downstream. In addition, Io's conductive ionosphere alters the corotational electric field, strongly diverging the equipotential lines on Io's upstream side. Since the electron flow streamlines are the equipotentialines, and the electrons move with an E x B drift velocity, the upstream side divergence slows the electrons. On the flanks of the plasma flow (near the sub-jovian and anti-jovian sides of Io, but aligned perpendicular to both the magnetic field and the nominal plasma torus velocity vector) the equipotential lines are parallel to the nominal corotational equipotential lines. While the divergent streamlines on the upstream side spread out the electron energy, allowing it to be lost quickly through inelastic collisions with the neutral atmosphere, nondivergent plasma flow on the flanks keeps the electron energy in a much narrower area (with faster plasma flow), allowing the atomic excitations to continue farther downstream. These effects combine to produce more emission along a line of sight through the flanks (as viewed by HST), producing the equatorial spots. Furthermore, the Hall effect rotates the electric field toward Jupiter, altering the equipotentialines. As a result, the electrons on the sub-jovian side must traverse the upstream atmosphere before reaching the sub-jovian flank, while the electrons on the anti-jovian side deposit their energy along a longer RETHERFORD ET AL.: IO'S EQUATORIAL SPOTS 27,163 longitudes, owing to the magnetohydrodynamic flow of plasma past Io, as the images in Figure 2 show. Perhaps more enlightening, simulated images presented by Saur et al. [2000] show the anti-jovian equatorial spot brightness to be roughly 40% brighter than that of the sub-jovian equatorial spot, owing to the Hall effect. This is qualitatively consistent with a -- 20% difference found in the images. Both the Saur et al. [1999] and Linker et al. [1998] models confer a correlation between the equatorial spot location and the magnetic field orientation. However, changes in plasma flow could result from perturbations of the local magnetic field due to electromagneticurrents created by the plasma-atmosphere interaction, from an intrinsic magnetic dipole field produced by dynamo action in Io's iron core, or from an induced magnetic field in Io's outer layers of conductive hot silicates [Neubauer, 1998]. These perturbation effects have not been rigorously investigated with the Saur et al. [1999] and Linker et al. [1998] models. While the Saur et al. [1999] and Linker et al. [1998] models assumed symmetric atmospheric distributions, now there is significant evidence for an SO2 atmosphere confined to Io's equatorialatitudes and largely dependent on volcanic plume sources [Feldman et al., 2000; Strobel and Wolven, 2000]. Asymmetries in the distribution of neutral gas near the satellite's anti-jovian point could potentially affect the location of the equatorial spot. However, the distribution is complicated by a self-shielding process described by Saur et al. [2000], in which the higher local conductivities of the denser regions prevent the plasma from flowing deeper into the atmosphere. Further simulations of the equatorial spot emissions which draw on these models, but are expanded to include an equatorial SO2 atmosphere and a changing magnetic field orientation, may be required to better understand stretch of the anti-jovian flank (see Saur et al. [1999, Figure 8]); this produces the observed brightness asymmetry. Since both the locations along the flux tubes where most collisions occur (near the magnetic field line tangent points) and the electron flow streamlines (the equipotential lines) rock with Jupiter's magnetic field at Io, so do the equatorial spots. the changes in the sub-jovian and anti-jovian equatorial spot locations. Galileo Solid State Imager images of Io in eclipse have shown visible emissions from volcanic plumes [Geissler et al., 1999]. Violet filtered images presumably show emissions from SO2 gas, while red and green filtered images pre- Simulated images produced using the models described sumably show emissions from atomic oxygen and sodium. by both Linker et al. [1998] and Saur et al. [1999] (presented by Linker and McGrath [ 1998] and Saur et al. [2000], respectively) display certain observed characteristics of Io's In addition to aurorae at the volcanic plume locations, at times the visible equatorial emission regions changed latitude with the magnetic field line tangent points like the equatorial spots. For example, both show equatorial spots UV equatorial spots do. We searched for a correlation beelongated in the direction parallel to the magnetic field (ro- tween atomic emission locations and volcano locations in ughly north-south) more than in the direction perpendicular each series of consecutive O I] 1356 A and S I] 1900 A imto the magnetic field (roughly sub-jupiter and anti-jupiter). ages by overplotting the active volcanic plume and hot spot This is qualitatively consistent with average equatorial spot locations reported by McEwen et al. [1998]. We find no dimensions of and km found in these consistent correlation between the atomic emission locations respective directions in the images (determined by doubling and active volcanic plume and hot spot locations. HST imthe measured full width at half maximum of Gaussian fits ages of visible [O I] 6300 A and NaI 5890 A emissions also to 5 pixel wide slices in these directions through images do not show a correlation with volcanic plumes [Trauger with sub-earth longitudes within 4-30 ø of western elongation). However, the standard deviations for these dimensions indicate substantial variability in the shapes and sizes of the equatorial spots. Also, simulated images presented by et al., 1997; Retherford et al., 1999]. Note that UV volcanic plume emissions may be present at a brightness low enough to prevent our distinguishing them from brighter diffuse atmospheric emissions. Indeed, it is conceivable that Linker and McGrath [1998] show equatorial spots centered equatorial volcanic plumemissions cause somewhat downstream of the sub-jovian and anti-jovian deviation of the equatorial spot locations from the magnetic the equatorward

8 27,164 RETHERFORD ET AL.: IO'S EQUATORIAL SPOTS field tangent points that we report (Figure 3). More work is ness for evaluating models of Io's plasma-atmosphere inneeded to resolve the discrepancy between the Galileo and teraction. Notably, the locations of the equatorial spots are HST results. correlated with the Jovian magnetic field orientation at Io, It is difficult to specify where the wake and limb glow re- but with a relation that is not 1:1. The equatorial spot logions overlap with the equatorial spots in these two-dimen- cations are centered 10 ø- 30 ø longitude downstream from sional images. (For example, see Figure 1 and Figure 2). Io's sub-jovian longitude, as reproduced in simulated im- However, since these other regions are not as bright as the ages [Linker and McGrath, 1998]. The variation in equatoequatorial spots (brightnesses of, R compared to rial spot brightness with Io's distance from the plasma torus, R), we assume that this overlap only minimally af- centrifugal equator is explained by the denser plasma at the fects our characterization of the equatorial spot morphology. torus equator. The anti-jovian equatorial spots are on av- After several years of searching for a direct correlation erage,- 20% brighter than the sub-jovian equatorial spots, between Io's atmospheric emissions and its location in the supporting the theory of Saur et al. [1999] that the Hall efplasma torus, several data sets, including this one, have now fect causes an asymmetric flow of electrons and ions around done so. The limb glow and extended cloud emission re- Io. Only,- 2x 108 W of FUV radiation is emitted from the gions in these observations also show this correlation (B.C. equatorial spots, a small fraction of the total,- 1012W in the Wolven et al., Emission profiles of neutral oxygen and sul- electrodynamicircuit between Jupiter and Io. The equatofur in Io's exosphericorona, manuscript submitted to JGR, rial spots are centered,- 100 km above Io's surface and have 2000) (there are too few data to comment on emission from dimensions of km projected along the magnetic Io's wake). It is possible that the limited scope of this field and km projected perpendicular to the magdata set prevented us from observing the few anticorrelations netic field; this is consistent with simulated images produced Ballester et al. [1997] reported after analysis of HST God- using both the Saur et al. [ 1999] and the Linker et al. [ 1998] dard High Resolution Spectrograph spectra. Scherb et al. models. While one might expect a correlation between the [1999] reported a strong correlation between Io's location locations of UV emissions and volcanos, none is apparent in in the torus and the brightness of [O I] 6300/ emissions the UV images. in numerous ground-based observations obtained through- outhe 1990s. The brightness of H - emissions at the foot Acknowledgments. This work was supported by NASA Guarof the Io Flux Tube in Jupiter's aurora also systematically anteed Time Observer funding to the STIS Science Team under NASA contracts NAS and NAS and is based upon vary with Io's magnetic longitude; however, it is currently observations obtained with the NASA/ESA Hubble Space Teleunclear whether this variation is due to Io's proximity to scope, which is operated by the Association of Universities for the plasma torus equator or due to other factors, such as the Research in Astronomy, Inc., under NASA contract NAS changing magnetic field strength at Jupiter' surface [Con- The authors thank Paul Feldman, Melissa McGrath, Ronald Olivnerney et al., 1999]. Comparison of the brightness varia- ersen, Frank Scherb, and William Smyth for useful discussions. Janet G. Luhmann thanks Michael E. Brown and another referee tions at the foot of the Io Flux Tube in Jupiter's aurora with for their assistance in evaluating this paper. the brightness variations of Io's equatorial spots could help clarify this relationship. The changes in locations of these equatorial spots, along with their changes in brightness with location in the torus, are significant, indicating that the plasma flow system must control these atomic oxygen and sulfur emissions. The References Acuna, M. H., K. W. Behannon, and J. E. P. Connerney, Jupiter's magnetic field and magnetosphere, in Physics of the Jovian Magnetosphere, edited by A. J. Dessler, pp. 1-50, Cambridge Univ. Press, New York, power available for the interaction is likewise correlated with Bagenal, F., Empirical model of the Io plasma torus: Voyager meathe local plasma torus density at Io. FUV imaging of H and surements, J. Geophys. Res., 99, 11,043-11,062, H2 emission from the foot of the Io Flux Tube in Jupiter's Ballester, G. E., J. 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