Io s auroral limb glow: Hubble Space Telescope FUV observations

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. A8, 1333, doi: /2002ja009710, 2003 Io s auroral limb glow: Hubble Space Telescope FUV observations K. D. Retherford, 1 H. W. Moos, and D. F. Strobel Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, Maryland, USA Received 27 September 2002; revised 1 May 2003; accepted 28 May 2003; published 30 August [1] Observations of Io s FUV aurora were obtained using the Space Telescope Imaging Spectrograph (STIS) on several occasions between 1997 and The STIS first-order long-slit spectroscopy mode and 2 00 wide slit were used to produce nearly monochromatic images of Io. These images reveal several distinct auroral features, including limb brightened emissions at the poles, designated limb glow. A detailed study of the limb glow brightness in OI] 1356 Å images is reported. The limb glow on the hemisphere facing the plasma torus centrifugal equator is consistently brighter than on the other hemisphere, and is brighter by a factor of 2 when Io is farthest from the plasma torus centrifugal equator. We determine that this behavior is consistent with there being more electrons and more electron energy in the part of an intersecting plasma torus flux tube located above the brighter hemisphere. Since most of the electrons in an intersecting flux tube have time to travel along the field line and collide with Io before the flux tube moves downstream across Io s poles, more torus electron energy is transferred into the polar hemisphere with brighter aurora. We demonstrate that a Kappa distribution model of the plasma torus electron column density above each hemisphere predicts to first order the ratio of north and south limb glow brightnesses as a function of Io s distance from the plasma torus centrifugal equator. This finding illustrates the importance of field-aligned torus electron energy transport for producing Io s aurora. INDEX TERMS: 6218 Planetology: Solar System Objects: Jovian satellites; 2459 Ionosphere: Planetary ionospheres (5435, 5729, 6026, 6027, 6028); 2732 Magnetospheric Physics: Magnetosphere interactions with satellites and rings; 5780 Planetology: Fluid Planets: Tori and exospheres; KEYWORDS: Io, aurora, FUV, Jupiter, plasma torus, HST Citation: Retherford, K. D., H. W. Moos, and D. F. Strobel, Io s auroral limb glow: Hubble Space Telescope FUV observations, J. Geophys. Res., 108(A8), 1333, doi: /2002ja009710, Introduction [2] Io s auroral emissions reveal much about the interaction between its atmosphere and Jupiter s magnetospheric plasma. Volcanos are the ultimate source of material for Io s atmosphere, which is much denser near the equator [Feldman et al., 2000; Strobel and Wolven, 2001] and composed mostly of sulfur and oxygen compounds. Ions created in Io s atmosphere, corona, and extended clouds is the primary source of plasma in Jupiter s magnetosphere. This plasma from Io is confined in a torus surrounding its orbit and corotates with Jupiter s magnetic field. Since the plasma torus rotates faster than Io orbits Jupiter, plasma sweeps past Io, impacts the atmosphere, and produces aurorae. As a result, torus electron impact ionization is a much more important source of Io s ionosphere than photoionization [Saur et al., 1999]. Both the flow of plasma past Io and the transfer of electron energy along magnetic field lines determine the brightness and morphology of the aurora [Saur et al., 1999, 2000] and are fundamental to our understanding of Io s ionosphere. Several distinct auroral features are observed in FUV images obtained with the 1 Now at Southwest Research Institute, San Antonio, Texas, USA. Copyright 2003 by the American Geophysical Union /03/2002JA009710$09.00 Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS). These features include equatorial spots, limb glow, wake, and extended corona [Roesler et al., 1999; Retherford et al., 2000; Wolven et al., 2001]. Analysis of the auroral limb glow feature, defined as the polar limb brightened emissions labeled in Figure 1, is reported here. Hereafter, limb glow refers to north and south limb emissions only. [3] Roesler et al. [1999] reported STIS images which show that the limb glow is qualitatively brighter on the hemisphere facing the plasma torus centrifugal equator. This effect has also been observed in HST/WFPC2 [Trauger et al., 1997], Galileo Solid State Imager [Geissler et al., 1999, 2001a], and Cassini Imaging Science Subsystem images [Geissler et al., 2001b]. Roesler et al. [1999] reported that the greater electron column density above the hemisphere facing the centrifugal equator could be responsible for this effect since most of the electrons in a flux tube could potentially reach Io as the plasma slows down near Io. However, recent efforts to model the interaction between Io s atmosphere and Jupiter s magnetosphere, such as those of Saur et al. [1999] and Linker et al. [1998], have not included this effect. Furthermore, this idea has not been tested with the observations. [4] Measurements of the auroral limb glow feature in STIS images of Io s OI] 1356 Å emissions are reported. A SIA 7-1

2 SIA 7-2 RETHERFORD ET AL.: IO S AURORAL LIMB GLOW As illustrated in the work of Roesler et al. [1999], images of the OI] 1356 Å, 1359 Å emission doublet using the G140M mode were separated from each other well enough to isolate the auroral features. Long-slit extended source calibrations require the 2 00 wide images to be two-dimensional rectified and flux calibrated using the sensitivity at each emission line wavelength. Table 1 lists observational and Io ephemeris-based information (calculated at the midpoint times of the exposures) for each exposure in the data set. See Retherford [2002] for more details. Figure 1. STIS image of Io s OI] 1356 Å emission (o49d01a30; also shown in Figure 1 Retherford et al. [2000]) with limb glow regions labeled. These north and south limb glow regions contain the pixels used for the brightness measurements listed in Table 2. The brighter equatorial spot features, apparent in this figure, were discussed by Retherford et al. [2000]. simple plasma torus model is used to illustrate how the ratio of north and south limb glow brightness varies with Io s location relative to the plasma torus centrifugal equator. A more realistic model is then used to better fit the data. Also, we show that the average of north and south limb glow brightness decreases with Io s distance from the plasma torus centrifugal equator and therefore with the local plasma torus density, as we found for the equatorial spot emissions [Retherford et al., 2000]. These results provide evidence that the limb glow is linked to the interaction between the Jovian magnetosphere and Io s atmosphere. The behavior of the limb glow reported here provides information on the nature of this complex electrodynamic interaction. In particular, the limb glow behavior elucidates the importance of field-aligned torus electron motions for the transfer of energy from the plasma torus to Io s ionosphere. 2. Observations [5] Io s FUV oxygen emissions were imaged in 32 separate exposures during 16 HST orbits. STIS s slit was used to image Io s disk in a slitless first-order long-slit spectroscopy mode. In this mode, each emission line produced an individual monochromatic image of Io, with the separation between images depending on the dispersion of the grating selected. Grating modes G140L and G140M (with central wavelength 1371 Å) were used to image Io s OI] 1356 Å emissions with varying degrees of isolation and overlap between their multiplet components. 3. Data Analysis [6] The north and south limb glow brightnesses were measured for each OI] 1356 Å image. The extended corona emissions in the images were first excluded using the same effective-background subtraction for the equatorial spot brightness measurement described by Retherford et al. [2000]. Then, we determined the average brightness of pixels within two arcs (illustrated in Figure 1) with angular ranges of 90 centered at Io north and Io south, and with inner and outer radii of 0.75 and 1.25 R Io. These radii were chosen to include the brightest limb glow emission and exclude the equatorial spot and wake auroral features. The 90 ranges about Io s north and south poles also generally exclude the other auroral features, which do not significantly contribute to the measured limb glow brightnesses. [7] Within G140L mode images, the arc region defined for the OI] 1356 Å image also encompasses most of the OI] 1359 Å limb glow feature since it is offset by only 5 pixels. For higher dispersion G140M(1371) mode images the OI] 1359 Å limb glow feature is well separated from the OI] 1356 Å limb glow feature but is at times contaminated by part of the much brighter OI] 1356 Å equatorial spot feature. Within the G140M mode images we therefore measure only the OI] 1356 Å limb glow feature and scale it by to allow for consistent comparisons between total doublet brightnesses. The optically thin ratio of emission excitation rates for this spin forbidden doublet depends on the ratio of Einstein A coefficients, 0.298, hence the scale factor of [8] Sources of systematic measurement error include the overlap of OI] 1359 Å and SI 1389 Å images and the uncertainty of Io s location in the images. These systematic errors are significant enough to warrant their careful estimation and inclusion with our reported measurement error estimates. We estimate the systematic error from the overlapping images by using each observed OI] 1356 Å image to simulate the overlap effect. The OI] 1356 Å image is shifted, scaled, and added to itself for each of the overlapping emission lines to create a simulated image of OI] 1356 Å, OI] 1359 Å and SI 1389 Å. The arc brightness measurements are then repeated on the simulated images. The differences between the original and simulated measurements are calculated to determine an estimate of this systematic error for each image. We estimate the systematic error caused by the uncertainty of Io s location in the image by repeating the arc measurements on images that have been shifted by two pixels in each direction. The standard deviation from the mean of these measurements is calculated to determine an estimate of this systematic error for each image. Also, we estimate the sensitivity of the ratio of the

3 RETHERFORD ET AL.: IO S AURORAL LIMB GLOW SIA 7-3 Table 1. Observational Summary and Ephemeris-Based Information a Root Name Date, b UT T Start, UT T Exp., s Grating f Io, c arcsec Sub-Earth Longitude l III, d degrees y III, e degrees 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 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 o5h9a /10/99 12:07: G140L o5h9a /10/99 12:27: G140L o5h /10/99 04:50: G140L o5h /10/99 05:07: G140L o5h /10/99 11:17: G140L o5h /10/99 11:34: G140L o5h /02/00 09:51: G140L o5h /02/00 10:08: G140L o5h /02/00 11:09: G140L o5h /02/00 11:35: G140L a Calculated at exposure midpoint. b Read 26/09/97 as 26 September c Io s angular diameter in arcseconds. d Jovian System III (magnetic) longitude. e Jovian System III latitude (calculated with the 0 4 offset, tilted dipole magnetic field model). f Distance of Io from the plasma torus centrifugal equator in units of R Io (positive for Io north). z c, f R Io north to south brightnesses to the effective-background subtraction. This sensitivity is negligible relative to the propagated statistical errors. The measured arc brightnesses and their total estimated errors are listed in Table 2 and plotted in Figure 2. [9] Figure 2a shows the individual north and south brightness measurements. Figures 2b and 2c show two trends that are derived from these individual brightnesses, (1) the averaged brightness of the north and south limb glow is correlated with Io s absolute distance from the plasma torus centrifugal equator; and (2) the north to south brightness ratio varies with Io s distance above or below the plasma torus equator. [10] The trend of average limb glow brightness with Io s absolute distance from the plasma torus centrifugal equator, z c, is shown as a solid line in Figure 2b. This linear least squares fit has a normalized slope of ± R R 1 Io, indicating a 2.5s trend. Note that more observations with Io at the torus equator would be useful since the one point we have near there is quite influential. This fit indicates a correlation between limb glow brightness and the absolute value of Io s distance from the plasma torus centrifugal equator (with correlation coefficient 0.6). This fit therefore also indicates a correlation with plasma torus density comparable to that found in the analysis of the equatorial spots by Retherford et al. [2000]. However, a similar plot of equatorial spot measurements has a somewhat steeper normalized slope of ± R R Io 1 (Figure 3.6b in the work of Retherford [2002]). The brightness measurements of extended corona emissions between 2 and 4 R Io also show this correlation [Wolven et al., 2001]; we similarly measured a normalized slope of ± R R 1 Io for oxygen extended corona (the measured slope for sulfur brightnesses is comparable). The disk-averaged [OI] 6300 Å brightness measurements reported by Oliversen et al. [2001] more conclusively show this correlation with a similarly measured normalized slope of ± R R 1 Io. [11] Random deviations from these trends could, in principle, reveal secondary sources of variability. However, the standard deviation from the average limb glow brightness fit is 18% and is similar to the deviation expected from the measurement uncertainties. This standard deviation is unlike those determined from the equatorial spot brightness trend (31%) and from the extended corona oxygen brightness trend (41%), which are both 5 times the deviation expected from the measurement uncertainties. Furthermore, the [OI] 6300 Å data set shows a 48% standard deviation from the general trend and particularly large deviations for certain dusk-side observations. The unidentified source(s)

4 SIA 7-4 RETHERFORD ET AL.: IO S AURORAL LIMB GLOW Table 2. Limb Glow Brightness (Rayleighs) Root Name North South Average Ratio North South o49d ± ± ± ± 0.18 o49d02a ± ± ± ± 0.20 o49d ± ± ± ± 0.17 o49d01a ± ± ± ± 0.21 o49d ± ± ± ± 0.27 o49d01a ± ± ± ± 0.33 o49d ± ± ± ± 0.36 o49d01a ± ± ± ± 0.29 o4xm ± ± ± ± 0.25 o4xm ± ± ± ± 0.33 o4xm ± ± ± ± 0.30 o4xm ± ± ± ± 0.33 o4xm ± ± ± ± 0.40 o4xm ± ± ± ± 0.53 o4xm ± ± ± ± 0.52 o4xm ± ± ± ± 0.57 o4xm ± ± ± ± 0.23 o4xm ± ± ± ± 0.20 o4xm ± ± ± ± 0.07 o4xm ± ± ± ± 0.07 o4xm ± ± ± ± 0.05 o4xm ± ± ± ± 0.06 o5h9a ± ± ± ± 0.28 o5h9a ± ± ± ± 0.26 o5h ± ± ± ± 0.34 o5h ± ± ± ± 0.40 o5h ± ± ± ± 0.08 o5h ± ± ± ± 0.09 o5h ± ± ± ± 0.25 o5h ± ± ± ± 0.29 o5h ± ± ± ± 0.21 o5h ± ± ± ± 0.25 of brightness variability exhibited by the equatorial spot and extended corona features is apparently less important for the limb glow feature. [12] The north to south brightness ratio varies with Io s distance to the plasma torus equator (Figure 2c). See figures in the works of Roesler et al. [1999] and Retherford et al. [2000] for images that show a variety of limb glow brightness ratios. We find that when Io is farthest from the plasma torus equator (at high positive or negative z c or magnetic latitude), the limb glow from the polar hemisphere facing the torus equator is a factor of 2 brighter than the limb glow from the other hemisphere. Unlike the average limb glow brightness, the brightness ratios show deviations larger than expected from measurement uncertainties (discussed in section 4.4.3). We next present our physical explanation for how the limb glow morphology changes and describe a simple model that roughly predicts this factor of 2 change in brightness ratio. 4. Discussion [13] The following discussion describes a method for roughly predicting limb glow brightness ratios based on Io s location in the Io plasma torus. We compare our observed limb glow brightness ratios to those predicted with this method. We also briefly compare the limb glow and equatorial spot brightnesses Physics of the Limb Glow [14] Io s auroral emissions are produced by the transfer of electron energy from the plasma torus to Io s ionosphere through two mechanisms. These mechanisms are (1) convection (bulk motion) of magnetospheric plasma and (2) field-aligned electron motions [Saur et al., 1999, 2000]. We show here that the polar variation in limb glow emissions with Io s location in the torus is the result of a preferential transfer of torus electron energy by field-aligned electron motion into the hemisphere of Io that is closer to the plasma torus centrifugal equator. An Io plasma torus flux tube is defined as a column of plasma along a jovian magnetic field line that convects with the magnetic field in accordance with the frozen-in magnetic flux theorem. The Io flux tube is defined as the total column of plasma along all of the jovian magnetic field lines that intersect Io at a particular instant. Io s conductive ionosphere alters the corotational electric field, strongly diverging the equipotential lines and plasma flow around both Io and the Io flux tube. See Figure 2 in the work of Saur et al. [2000] for an illustration of this plasma-atmosphere interaction. Some plasma torus flux tubes do intersect Io and its ionosphere however. The transfer of electron energy from the torus to Io occurs as electrons traveling up and down along these flux tubes collide with the atmosphere. [15] Since the intersecting flux tubes are divided by Io, each polar ionosphere has access to different total amounts of energy contained within each part of the flux tube. These total amounts of accessible energy depend on the distribution of plasma along each half of an intersecting torus flux tube, which varies with Io s location relative to the torus centrifugal equator plane. The limb glow brightness ratio may be proportional to the ratio of accessible energy above each polar hemisphere. However, the total energy actually transferred to the ionosphere depends on a balance between the timescale for energy transfer along the length of a torus flux tube by field-aligned electron motions and the timescale for an intersecting torus flux tube to convect across Io. If most of the electrons in a torus flux tube convecting past Io have time to collide with it, then the ratio of accessible energy should approximate the limb glow brightness ratio. We will demonstrate in section 4.3 that this condition generally applies for >5 ev electrons Prediction of Brightness Ratios [16] We quantitatively investigate this possible explanation for the limb glow morphology by approximating the relative electron energy column densities above each hemisphere as a function of Io s distance to the plasma torus centrifugal equator. The electron energy column densities above each hemisphere are accordingly assumed to be directly proportional to the electron energy flux that produces each polar limb glow feature. The ratios of approximated electron energy column density above each hemisphere represent a model of the limb glow brightness ratios that we test with the data. [17] To calculate the ratio of electron energy column density (N E ) above each polar hemisphere as a function of distance to the plasma torus centrifugal equator, z, we use Z N E ¼ n e ðþ z 3 2 T eðþdz; z and a simple and convenient analytical formula for the plasma torus thermal electron number density distribution ð1þ

5 RETHERFORD ET AL.: IO S AURORAL LIMB GLOW SIA 7-5 Figure 2. OI] 1356 Å limb glow brightness variability. Values are listed in Table 2. (a) Plot of north and south limb glow brightnesses versus Io s distance to the plasma torus centrifugal equator, z c (in R Io ). Io is observed north of the torus equator for z c > 0. The combination of trends shown in Figures 2b and 2c determine these brightnesses. (b) Plot of averaged north and south limb glow brightnesses versus the absolute value of Io s distance to the torus equator, jz c j (in R Io ). The solid line represents a linear fit to the data. (c) North to south limb glow brightness ratio versus Io s distance to the torus equator. The three curves represent the predicted brightness ratio based on the ratio of electron energy column density above each polar hemisphere calculated with a simple torus model using torus scale heights of 1, 2, and 3 R J for illustration. A torus scale height of H 2R J best matches the data (solid curve). (d) Similar to Figure 2c but calculated with a Kappa distribution torus model reported by Meyer-Vernet et al. [1995]. A torus scale height of H k 1.6 R J best matches the data for k = 2.5, and corresponds to a more realistic H = 1.0 R J. provided by Hill and Michel [1976],x n e ðþ¼n z e;0 exp z 2 : ð2þ H Here n e,0 is the electron density at the torus equator and H is the torus electron density scale height (further defined in section 4.3). Typically, n e, cm 3, but this divides out of our energy column density ratios. For our simple calculation we assume that local electron energy, 3 2 T e(z), is a constant along a field line, so this also divides out of our energy column density ratios. Integrating equation (1) numerically by summation from z c to large z, we obtain the electron energy column density above the northern hemisphere, N E(North), and integrating from largely negative z to z c we obtain the energy column density above the

6 SIA 7-6 RETHERFORD ET AL.: IO S AURORAL LIMB GLOW Figure 3. Illustration of how the electron energy column densities above each polar hemisphere depend on the geometry of Io s location in the plasma torus. The analytical formula used for the plasma torus thermal electron number density distribution is provided by Hill and Michel [1976], n e (z) =n e,0 exp[ ( z H )2 ], and T e (z) is assumed constant. The torus scale height H is exaggerated. The electron energy column densities integrated above the northern hemisphere, N E(North), and southern hemisphere, N E(South), change with Io s distance from the plasma torus centrifugal equator plane, z c. The electron energy column density ratio N E(North) /N E(South) is assumed proportional to the brightness ratio of the limb glow aurora. southern hemisphere, N E(South) (z c is Io s distance from the plasma torus centrifugal equator plane). The geometry for this integration is illustrated in Figure 3. This simple model predicts a factor of ^2 difference in electron column density when Io is farthest from the torus equator and could therefore explain the observed factor of 2 difference in limb glow brightness. In section 4.4 we discuss our model fit to the data and describe a more realistic model that better predicts the limb glow brightness ratios for typical torus conditions Comparison of Torus Electron Field-Aligned Motion and Convection Timescales [18] We next show that it is reasonable to assume that most electrons in a flux tube have time to collide with Io before the flux tube convects past Io. The field-aligned electron energy transport from the torus to Io s atmosphere is related to the number of electrons in an intersecting torus flux tube that travel along the field line and collide with Io. The timescale for field-aligned electron energy transport from the torus to Io s atmosphere primarily depends on the timescale for a torus electron to travel along the field line and collide with Io, rather than the timescale for torus electron self-collisions (74 s) [Sittler and Strobel, 1987]. While a few electron self-collisions will occur along the way, the energy transferred through these electron selfcollisions is generally negligible for a quasi-collisionless plasma. Instead, if the round trip period of an electron s motion along a torus flux tube (parallel to the magnetic field line) is shorter than the time for the torus flux tube to convect across Io, the electron will collide with Io and deposit its energy into the atmosphere. These collisions ionize the neutral atmosphere, maintaining the ionosphere, and excite auroral emissions [Saur et al., 1999]. While the energy contained within a plasma torus flux tube is depleted through such collisions, it could be replenished either from neighboring flux tubes that diffuse into it or from ionospheric heating. However, both the perpendicular energy transport in the torus and the (reverse) heating from Io s ]2000 K (0.2 ev) ionosphere are negligible [Saur, 2000]. Therefore once the flux tube becomes totally depleted of energy it stops producing aurora. We therefore need to estimate this field-aligned electron energy transport timescale to understand the limb glow morphology and time variability. [19] An individual electron s motion along a field line depends in part on the polarization electric field that maintains charge neutrality in the torus plasma. Centrifugal forces dominate the ion motions, confining both ions and electrons about a centrifugal equator. The resulting plasma torus scale height for a simple Gaussian plasma distribution is sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2k Z i T e þ T ik H ¼ 3M i 2 ; ð3þ J where T ik, Z i, and M i are the ion parallel temperature, charge state, and mass measured in situ by Voyager, respectively, and J is the rotation rate of Jupiter [Bagenal and Sullivan, 1981; Bagenal, 1985]. The plasma torus electron density distribution along magnetic field lines is actually a superposition of ion distributions with different scale heights [Bagenal, 1994], different velocity filtration effects [Meyer-Vernet et al., 1995], and different anisotropies (1 < T i? /T ik <5[Moncuquet et al., 2002]) for each ion species in diffusive equilibrium. p [20] A torus scale height H = ffiffiffi 2 RJ, reported by Bagenal [1985] (to correct the H 1R J reported by Bagenal and Sullivan, [1981]), can be used to roughly estimate the round trip period for an electron s motion along a field line. The average electron temperature at Io is 5 ev [Sittler and Strobel, 1987], which corresponds to an average velocity of 1300 km/s. For electrons with low pitch angles (a = tan 1 (v? /v k )) this is also approximately the parallel velocity. The distance that an electron travels in a round trip is 1 2 torus scale heights times four (10 R J ). The round trip travel time is therefore T trip 550 s. Note that the round trip period for electrons p with pitch angles a ] 45 is only modestly longer at ] ffiffi 2 Ttrip. [21] The >9.1 ev electrons required to excite the OI] 1356 Å emissions travel faster than the 5 ev electrons we consider for our estimation. The energetic electrons which produce secondary electrons through collisional ionizations once in Io s SO 2 atmosphere (> ev) are also faster. The estimated T trip is therefore quite conservative for the sake of this discussion. Note that electrons with energies higher than several hundred ev mostly penetrate Io s polar atmosphere without transferring their energy through ionospheric collisions since Io s polar SO 2 column

7 RETHERFORD ET AL.: IO S AURORAL LIMB GLOW SIA 7-7 density is ]10 15 cm 2 [Strobel and Wolven, 2001] and the SO 2 electron impact ionization cross section decreases below cm 2 at energies above 500 ev [Lindsay et al., 1996]. [22] A plasma torus flux tube will convect across Io s diameter (f Io = 3640 km) in time t ¼ f Io =v plasma : As mentioned above, the convective plasma flow velocity near Io is greatly altered by the plasma-atmosphere interaction relative to corotation (v corotation = 57 km/s). Within the most conductive part of the ionosphere the plasma is slowed to less than corotation and just outside this region it is decelerated on the upstream side, accelerated to faster than corotation on the flanks (the equatorial spots), and decelerated again on the downstream side (see Figure 7.26 in the work of Saur [2000] or Plate 4d in the work of Linker et al. [1998]). Roesler et al. [1999] reported that if the plasma slows to 1 2 km/s, as observed with the Galileo PLS instrument in Io s wake [Frank et al., 1996], then almost all of the electrons in a torus flux tube would reach Io s atmosphere while the flux tube intersects the satellite. Frank and Paterson [2002] recently reported Galileo PLS measurements of the plasma flow over Io s northern polar region with velocities of <2 km/s in the downstream direction, and average velocities of only several km/s. These measurements are roughly consistent with the plasma velocities of 7 km/s over Io s poles predicted by Saur et al. [2002] (who reproduce plasma velocities at Io s flanks that are consistent with Galileo PLS velocity measurements of 74 km/s [Frank and Paterson, 2000] at 1500 km altitude). Also, a suitable path for a torus flux tube s convection across a pole over our defined limb glow region (shown in Figure 1) starts and ends at 45 latitude along the upstreamp ffiffiffi and downstream meridians. This path has length f Io / 2. Conservatively adopting a velocity of 7 km/s and a path length of 2570 km, we find an average polar convection time t polar = 370 s. [23] Most electrons make less than half a trip before colliding with Io. Since t polar >T trip /2, most of the 5 ev electrons in a plasma torus flux tube will collide with Io. The greater column density of electrons above the hemisphere facing the centrifugal equator should result in brighter limb glow in that hemisphere Comparison of Model to Observations Simple Gaussian Distribution Torus Model [24] We compare our measured north to south limb glow brightness ratios in Figure 2c with our calculated model N E(North) /N E(South) ratios for Io at corresponding z c with a least squares c 2 minimization fit and find H = 2.2 R J. Since the 30% standard deviation of the data from this model curve fit is 3 times the deviation expected from measurement uncertainties, we simply report that a torus scale height H 2R J best describes the time averaged behavior of the data. [25] This inferred torus electron density scale height is larger than currently accepted average values of H R J. Our knowledge of H at Io s orbit is limited and currently depends on torus models constrained by ion measurements and charge neutrality. Bagenal and Sullivan [1981] reported a scale height of H 1 R J based on ð4þ Voyager measurements, pffiffiffi but Bagenal [1985] identified a missing factor p of 2 in this work, changing their result to the H ffiffiffi 2 RJ referenced in section 4.3. A refined theoretical analysis reported by Bagenal [1994] results in H 1R J (see their Figure 9). Using the formula for H in section 4.3 with typical thermal and suprathermal ion energies of 60 ev to 100 ev respectively, a range of M i from 16 m to 25 m (from the variable O/S/H ratio), Z i = 1.5, and T e = 5 ev, we find a range for H of R J. The observed S ++ torus ribbon scale height of 1.2 R J [Kuppers and Jockers, 1997] is within this range. However, note that the ribbon is typically cooler than the warm torus and therefore has a smaller scale height and that torus ribbon S + scale heights have been observed to vary from R J [Schneider and Trauger, 1995]. [26] The difference between the average model H 2R J and a more realistic H R J is likely caused by secondary physical mechanisms not included in our simple model. For example, the secondary physical mechanisms responsible for the observed variability of measured ratios generally between the H = 1 R J and H = 3 R J model curves in Figure 2c (see section 4.4.3) may also cause this difference. Therefore the Gaussian model fit, while inaccurate, suggests that the variation in limb glow brightness ratio is caused by the torus electron energy transport process rather than local plasma convection processes, which likely can not produce limb glow ratios of 2 and the observed trend with z c. We demonstrate next that this difference may also result from our use of a plasma torus model that is too simple, with a Gaussian (Maxwellian) distribution of electron density and energy along a field line instead of, for example, the Kappa (non-maxwellian) distribution reported by Meyer-Vernet et al. [1995] Kappa Distribution Torus Model [27] We find that a more realistic plasma torus model based on a Kappa distribution of electron energy and density fits the data with a more realistic average scale height. Meyer-Vernet et al. [1995] applied the theory of velocity filtration for non-maxwellian plasmas reported by Scudder [1992] to a centrifugal force potential and found that a Kappa-like distribution of electrons along a field line best explains Ulysses spacecraft observations of torus electron density and temperature over 3 torus scale heights at 7 9 R J. [28] Assuming, like in section 4.2, that the electron energy column density ratio is proportional to the brightness ratio of the limb glow aurora, we integrate the electron energy column density (equation (1)) using from Meyer- Vernet et al. [1995] the relation 1 1 T e ðþ/n z e ðþ z 2 k ; ð5þ instead of the constant T e assumed in section 4.2, and where n e ðþ¼n z e ð0þ 1 þ " z 2 #1 2 k H 2 k ¼ k k 3 2 k 3 2 H 2 k H 2 ; ð6þ ð7þ

8 SIA 7-8 RETHERFORD ET AL.: IO S AURORAL LIMB GLOW and 1.5 < k <6[Moncuquet et al., 2002]. Predicted limb glow brightness ratio profiles for this model with k = 2.5 are plotted in Figure 2d. [29] Using values of k = 2.0, 2.5, and 3.5, the best fit scale heights are found to be H k = 1.1, 1.6, and 1.8 R J, respectively. These best fit H k s in terms of H are 0.5, 1.0, and 1.4 R J, respectively. Models with k = have scale height fits which agree with torus observations and are therefore physically realistic. Meyer-Vernet et al. [1995] found that k 2.4 best describes Ulysses data and Moncuquet et al. [2002] similarly found that k 2.0 best describes Voyager data (using a bi-kappa distribution). However, there are currently only in situ observations of T e near the torus equator at Io s orbit (5.9 R J ), which cannot yield the Kappa distribution nor the best value of k for torus electrons within the Io flux tube. We have therefore demonstrated that the variation in limb glow brightness ratio to first order likely results from the changing rate of torus electron energy transfer to each hemisphere of Io s ionosphere as proposed, but note that other physical mechanisms are likely important at second order (e.g., ion temperature anisotropy and those discussed next) Additional Sources of Brightness Variability [30] Deviations of the measured north to south brightness ratios from the time averaged trend for H = 2 R J shown in Figure 2c (or for H k = 1.5 R J shown in Figure 2d) further indicate that real variability is likely caused by additional physical processes. These additional processes could include variability in the plasma torus scale height, inhomogeneities in Io flux tube content, or variability in the density of the neutral polar atmospheres (perhaps from ephemeral volcanic activity) at the time of observation. Also, changes in the limb glow brightness ratio could result from a decrease in the strength of the plasma-atmosphere interaction below a point where the plasma velocity is no longer slowed to perhaps ]10 km/s at Io s poles. The torus electron energy transfer may not always be completely efficient like we assume. We do not have sufficient temporal coverage and measurement accuracy to determine if this potential variability is correlated with longitudinal (azimuthal) asymmetries in the plasma torus or radial variations in scale height like the ribbon feature. No correlation was found between the north to south brightness ratio and either torus inclination angle (roughly the angle of the nominal jovian magnetic field projected into the line of sight at elongation) or sub-earth longitude, ruling out variability from viewing geometry effects Complete Model of Limb Glow Brightness [31] The auroral brightness depends on the atmospheric neutral density, the ionospheric electron density and an ionospheric electron energy dependent emission rate coefficient. The ionospheric electron densities and energies depend on the convection of torus flux tubes and fieldaligned electron transport into Io, since the delivered energy both creates and energizes the ionospheric electrons. The average ionospheric electron energy is proportional to the total torus electron energy transported to Io. As a result, the average ionospheric electron temperature of ^1 ev is far above neutral temperatures (]0.2 ev), but below the local torus electron temperature (5 ev). The model for average limb glow brightness (Figure 2b) combined with the model for limb glow brightness ratio (Figure 2d) completes a first order model of Io s north and south limb glow brightnesses (the data shown in Figure 2a) Comparison of Limb Glow and Equatorial Spot Brightnesses [32] The relative brightness between the limb glow and equatorial spots requires explanation. Strobel and Wolven [2001] reported that while Io s equatorial SO 2 atmosphere has column density N SO2 = cm 2, its polar SO 2 atmosphere is at least an order of magnitude less dense with N SO2 ] cm 2. Were the oxygen mixing ratio constant across Io and the relationship between density and auroral brightness linear, this factor of ^10 difference in column density between the polar and equatorial SO 2 atmosphere would be inconsistent with polar limb glow brightnesses that are only 3 5 times less bright than equatorial spot brightnesses. Also, a sharp decrease in density from ±(30 45 ) latitude is required to account for the morphology of HST/STIS Lyman-a images [Feldman et al., 2000]. However, a sharp decrease in auroral brightness poleward of these latitudes is not seen. [33] The auroral brightness dependence on neutral density is complicated and nonlinear and may explain this discrepancy. Since 5 ev electrons have a penetration depth through SO 2 gas of cm 2, the aurora at the equator is limited to high altitudes while the aurora at the poles is not. In other words, the field-aligned torus electron energy transport is limited to its maximum at the equator, regardless of the extra gas there. Most plasma torus flux tubes are diverted around Io and only penetrate its atmosphere near the equator at high altitudes where the density is lower. This self shielding effect causes the equatorial spots to be located 100 km above the surface [Retherford et al., 2000; Saur et al., 2000] and has a nonlinear dependence on neutral density. Flux tubes that pass through Io and over the polar regions have access to the full columns of gas there, including the densest regions near the surface. Initial models of Io s aurora reported by Saur et al. [2000] used a spherically symmetric atmosphere and did not predict the relative brightness of 3 5 between the limb glow and equatorial spots [see Saur et al., 2000, Figure 1]. However, recent models that include a dense equatorial atmosphere and a thin polar atmosphere better simulate the limb glow (J. Saur, personal communication, 2002). This distribution of Io s atmosphere apparently allows more efficient fieldaligned electron energy transport at the poles, which increases the relative brightness of the limb glow. [34] Alternatively, this discrepancy may be explained by a more global distribution of atomic oxygen and sulfur, which are likely the primary sources of the atomic aurorae. The expected distribution of the atomic gases is consistent with the morphology of the aurorae. Higher mixing ratios for these constituents in the polar regions could account for the relative brightness between the limb glow and equatorial spots. 5. Summary [35] We report measurements of the auroral OI] 1356 Å limb glow brightness in HST/STIS images. We find that the polar hemisphere closest to the plasma torus centrifugal equator is a factor of 2 brighter than the other polar

9 RETHERFORD ET AL.: IO S AURORAL LIMB GLOW SIA 7-9 hemisphere when Io is farthest from the centrifugal equator. This brightness ratio is consistent with the field-aligned transport of twice as much thermal torus electron energy into the brighter polar hemisphere than the other. A Kappa distribution model of plasma torus electron density with k = 2.5 and scale height H k =1.6 R J (H = 1 R J ), dependent on the reasonable assumption that most of the electrons in a torus flux tube intersecting Io are able to collide with Io s atmosphere, can be used to predict to first order the varying ratio of limb glow brightness as a function of Io s distance from the torus equator. This finding emphasizes the importance of field-aligned torus electron energy transport for producing Io s aurora. [36] Acknowledgments. This work was supported by NASA Guaranteed Time Observer funding to the STIS Science Team under NASA contract NAS and is based upon observations obtained with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS DFS was partially supported by NASA grants NAG-4168, and NAG The authors thank Joachim Saur, Brian Wolven, Paul Feldman, Melissa McGrath, Ronald Oliversen, William Smyth, Fred Roesler, and Fran Bagenal for useful discussions. [37] Arthur Richmond thanks Fran Bagenal and Gilda E. Ballester for their assistance in evaluating this paper. References Bagenal, F., Plasma conditions inside Io s orbit Voyager measurements, J. Geophys. Res., 90, , Bagenal, F., Empirical model of the Io plasma torus: Voyager measurements, J. Geophys. Res., 99, 11,043 11,062, Bagenal, F., and J. D. Sullivan, Direct plasma measurements in the Io torus and inner magnetosphere of Jupiter, J. Geophys. Res., 86, , Feldman, P. D., D. F. Strobel, H. W. Moos, K. D. Retherford, B. C. Wolven, M. A. McGrath, F. L. Roesler, R. C. Woodward, R. J. Oliversen, and G. E. Ballester, Lyman-a Imaging of the SO 2 Distribution on Io, Geophys. Res. Lett., 27, , Frank, L. A., and W. R. Paterson, Return to Io by the Galileo spacecraft: Plasma observations, J. Geophys. Res., 105, 25,363 25,378, Frank, L. A., and W. R. Paterson, Plasmas observed with the Galileo spacecraft during its flyby over Io s northern polar region, J. Geophys. Res., 107(A8), 1220, doi: /2002ja009240, Frank, L. A., W. R. Paterson, K. L. Ackerson, V. M. Vasyliunas, F. V. Coroniti, and S. J. Bolton, Plasma observations at Io with the Galileo spacecraft, Science, 274, , Geissler, P. E., A. S. McEwen, W. Ip, M. J. S. Belton, T. V. Johnson, W. Smyth, and A. Ingersoll, Galileo imaging of atmospheric emissions from Io, Science, 285, , Geissler, P. E., W. H. Smyth, A. S. McEwen, W. Ip, M. J. S. Belton, T. V. Johnson, A. P. Ingersoll, K. Rages, W. Hubbard, and A. J. Dessler, Morphology and time variability of Io s visible aurora, J. Geophys. Res., 106, 26,137 26,146, 2001a. Geissler, P., A. McEwen, and C. Porco, Cassini imaging of auroral emissions on the Galilean satellites, Eos Trans. AGU, 82, Spring Meet. Suppl., 51A03, 2001b. Hill, T. W., and F. C. Michel, Heavy ions from the Galilean satellites and the centrifugal distortion of the jovian magnetosphere, J. Geophys. Res., 81, , Kuppers, M., and K. Jockers, A multi-emission imaging study of the Io plasma torus, Icarus, 129, 48 71, Lindsay, B. G., H. C. Straub, K. A. Smith, and R. F. Stebbings, Absolute partial cross sections for electron impact ionization of SO 2 from threshold to 1000 ev, J. Geophys. Res., 101, 21,151 21,156, Linker, J. A., K. K. Khurana, M. G. Kivelson, and R. J. Walker, MHD simulations of Io s interaction with the plasma torus, J. Geophys. Res., 103, 19,867 19,877, Meyer-Vernet, N., M. Moncuquet, and S. Hoang, Temperature inversion in the Io plasma torus, Icarus, 116, , Moncuquet, M., F. Bagenal, and N. Meyer-Vernet, Latitudinal structure of outer Io plasma torus, J. Geophys. Res., 107(A9), 1260, doi: / 2001JA900124, Oliversen, R. J., F. Scherb, W. H. Smyth, M. E. Freed, R. C. Woodward, M. L. Marconi, K. D. Retherford, O. L. Lupie, and J. P. Morgenthaler, Sunlit Io atmospheric [OI] 6300 Å emission and the plasma torus, J. Geophys. Res., 106, 26,183 26,194, Retherford, K. D., Io s UV Aurora: HST/STIS Observations, Ph.D. thesis, Johns Hopkins Univ., Baltimore, Md., Retherford, K. D., H. W. Moos, D. F. Strobel, B. C. Wolven, and F. L. Roesler, Io s equatorial spots: Morphology of neutral UV emissions, J. Geophys. Res., 105, 27,157 27,165, Roesler, F. L., H. W. Moos, R. J. Oliversen, R. C. Woodward Jr., K. D. Retherford, F. Scherb, M. A. McGrath, W. H. Smyth, P. D. Feldman, and D. F. Strobel, Far-ultraviolet imaging spectroscopy of Io s atmosphere with HST/STIS, Science, 283, , Saur, J., Plasma interaction of Io and Europa with the jovian magnetosphere, Ph.D. thesis, Inst. für Geophys. und Meteorol. der Univ. zu Köln, Saur, J., F. M. Neubauer, D. F. Strobel, and M. E. Summers, Threedimensional plasma simulation of Io s interaction with the Io plasma torus: Asymmetric plasma flow, J. Geophys. Res., 104, 25,105 25,126, Saur, J., F. M. Neubauer, D. F. Strobel, and M. E. Summers, Io s ultraviolet aurora: Remote sensing of Io s interaction, J. Geophys. Res., 27, , Saur, J., F. M. Neubauer, D. F. Strobel, and M. E. Summers, Interpretation of Galileo s Io plasma and field observations: I0, I24, and I27 flybys and close polar passes, J. Geophys. Res., 107(A12), 1422, doi: / 2001JA005067, Schneider, N. M., and J. T. Trauger, The structure of the Io torus, Astrophys. J., 450, 450, Scudder, J. D., On the causes of temperature change in inhomogeneous low-density astrophysical plasmas, Astrophys. J., 389, , Sittler, E. C., and D. F. Strobel, Io plasma torus electrons Voyager 1, J. Geophys. Res., 92, , Strobel, D. F., and B. C. Wolven, The atmosphere of Io: Abundances and sources of sulfur dioxide and atomic hydrogen, Astrophys. Space Sci., 277, , Trauger, J. T., et al., HST observations of [OI] emissions from Io in eclipse, Bull. Am. Astron. Soc., 29, 1002, Wolven, B. C., H. W. Moos, K. D. Retherford, P. D. Feldman, D. F. Strobel, W. H. Smyth, and F. L. Roesler, Emission profiles of neutral oxygen and sulfur in Io s exospheric corona, J. Geophys. Res., 106, 26,155 26,182, H. W. Moos and D. F. Strobel, Department of Physics and Astronomy, The Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21218, USA. (hwm@jhu.edu; strobel@jhu.edu) K. D. Retherford, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78228, USA. (KRetherford@swri.edu)