Ulysses Encounter Research Articles

Solar wind effects on Jupiter non-Io DAM emissions during Ulysses distant encounter (2003–2004)

Aims. We analyze solar wind data from the Ulysses spacecraft during the distant Jupiter encounter from 2003 November to 2004 March as well as Nancay decametric (DAM) radio observations non-controlled by Io. Methods. The Ulysses solar wind data are balistically propagated towards Jupiter and are correlated with Nancay non-Io DAM emissions. Results. It is found that the average solar wind dynamic pressure around the time of DAM emissions is 1.7 times higher than its average value during the Ulysses encounter. The occurrence of fast forward (FS) and reverse (RS) interplanetary shocks and heliospheric current sheet crossings (HCS) is correlated with the occurrence of non-Io DAM emissions. We note an enhanced probability of occurrence of non-Io DAM emissions after the expected arrival of FS, RS and HCS at Jupiter. However, about half emissions (54%) did not seem to be associated with these interplanetary structures. We also note that the average duration and power of non-Io DAM emissions are enhanced during periods associated with those interplanetary structures. Conclusions. From the results obtained in this work it seems that non-Io DAM emissions occur during intervals of enhanced solar wind dynamic pressure. Yet, there is no direct correlation between the non-Io DAM emissions duration or power versus the solar wind pressure values and the interplanetary shock Mach number.

An overview of Jovian electrons during the distant Ulysses Jupiter flyby

Since the 1970's interplanetary electrons in the MeV energy range, of Jovian origin, have been extensively studied from close to the Sun to beyond the Kronian orbit, near the ecliptic. The Ulysses trajectory allowed to study the propagation of these particles, in a wide range of heliographic latitudes. The location of Jupiter with respect to the structure of the heliospheric magnetic field is precisely determined and non-central. This makes Jovian electrons an ideal opportunity for studying the particle propagation parallel and perpendicular to the heliospheric magnetic field. 12 years after its first encounter in February 1992, the Ulysses mission encountered Jupiter for a second time in February 2004 at a distance of 1684 Jovian radii. The first flyby took place at a distance of closest approach of 6 Jupiter radii ( R J ) and changed the inclination of the Ulysses trajectory so that it would pass above the Sun's polar regions. During the 2004 encounter, in contrast to 1992, Ulysses did not enter the Jovian magnetosphere but remained upstream of it. In mid 2002, the MeV electron flux started increasing and displaying large short term variations. These features lasted throughout the encounter, making the electron intensities less obviously correlated with the proximity to Jupiter compared with the first Jovian encounter. In previous studies it has been shown that the diffusion coefficient κ ⊥ , ϑ perpendicular to the heliospheric magnetic field in polar direction increased in 1998 during the transition from solar minimum to solar maximum close to the ecliptic plane. Although the distant Ulysses encounter took place during the declining phase of the solar cycle the absence of an intensity variation with latitude indicate an unexpected further increase of κ ⊥ , ϑ . Thus the diffusion coefficients, and in particular perpendicular diffusion in the polar direction, are highly time-dependent. In this paper, we present the corresponding data and discuss the implication for particle propagation in the three-dimensional heliospheric magnetic field.

Encountering the Other: The Artwork and the Problem of Difference in Blanchot and Levinas (review)

Reviewed by: Encountering the Other: The Artwork and the Problem of Difference in Blanchot and Levinas Warren F. Motte Alain P. Toumayan . Encountering the Other: The Artwork and the Problem of Difference in Blanchot and Levinas. Pittsburgh: Duquesne University Press, 2004. 231 pp. Alain Toumayan remarks in his introduction that his intent in this book is not to provide an overview of the work of Maurice Blanchot and Emmanuel Levinas, but rather to focus on a few exemplary intersective moments in their work, moments that testify to their mutual influence. What he finds most intriguing is the manner in which both thinkers address the problem of difference and the question of the other, in ways that make apparent the richness of their intellectual conversation. The key term in Toumayan's analysis is that of the "encounter," which points toward a variety of concerns here: the encounter of Blanchot and Levinas, certainly and most obviously, but also the encounter of philosophy and literature, and, more generally still, that of the subject and the other. In his first chapter, Toumayan meditates on the dialectic of identity and difference, and upon the manner in which the subject in Blanchot and Levinas is constituted differentially. Reading Blanchot and Levinas's texts as "case studies of difference" (43), and staging his discussion through a rehearsal of Saussure, Heidegger, and Derrida's thought, he argues that Levinas's construct of the il y a serves both of them as a useful tool in their reflections on the question of difference, even if the results of those reflections diverge. Levinas's ethical subjectivity insists upon identity as deriving from the responsibility to the other, while Blanchot constantly returns to the oppositional quality of the encounter, the way the other disrupts the subject in productive fashion. Turning to the problem of death in his second chapter, Toumayan suggests that for both thinkers, death is the privileged figure of alterity, an inevitable and irreducible problem in any interrogation of difference. In Levinas's Le Temps et l'autre, he sees an articulation of death as an absolute other, and a characteristic passivity of the subject when faced with death. In Blanchot's L'Instant de ma mort, he draws our attention to the way Blanchot suspends the instant of death, refuting any possibility of truth therein. In chapter three, Toumayan invokes two mythological narratives of encounter, involving Orpheus and Ulysses respectively, and thinks about the ways Blanchot and Levinas meet them. Blanchot's principal [End Page 122] concern as he read the Orpheus myth is the question of artistic inspiration. Pointing toward Orpheus's forgetfulness and his lack of care, he argues that inspiration results from failure. Like death, inspiration "is ultimately a relation with otherness," Toumayan suggests (99)—the crucial difference being that we survive the encounter. Both Blanchot and Levinas read Ulysses's encounter with the Sirens against the traditional grain, according to Toumayan, seeing failure instead of victory—but a failure that is ultimately productive of meaning. Like the artwork itself, meaning is something that continually and ineluctably escapes from the artist. The fourth chapter deals with the way that Blanchot and Levinas conceive the image. For the latter, Toumayan believes, the image's power "derives from a being's noncoincidence with itself; a simple, autonomous conception of identity is not sufficient to contain being" (117). The image is located in time, but represents time, for Levinas, as an entretemps, an intertime that oscillates precariously between time and timelessness, and causes us to question our status as temporal beings. For his part (and drawing liberally upon Levinas's analyses), Blanchot sees in the image both the (incomplete) disappearance of the world and a resemblance, or a doubling of the object. Here, Toumayan traces Blanchot's reflections with particular resourcefulness and attention to detail, demonstrating convincingly how they "characteristically evolve toward ambiguity" (139). The final chapter of this book is entitled "Encountering the Other Night," and it deals largely with the "obscurity or nocturnality" (145) of the il y a. Toumayan suggests that both Blanchot and Levinas meet and grapple with the il y a at moments of darkness, in sites that are largely apart from the...

Residual and Emergent Cultures in Joyce Studies

Residual and Emergent Cultures in Joyce Studies Sheldon Brivic (bio) Derek Attridge and Marjorie Howes, eds. Semicolonial Joyce. Cambridge University Press, 2000. 269 pp. Kimberly J. Devlin and Marilyn Reizbaum, eds. Ulysses: En-Gendered Perspectives: Eighteen New Essays on the Episodes. University of South Carolina Press, 1999. 345 pp. Peter Francis Mackey. Chaos Theory and James Joyce’s Everyman. University Press of Florida, 1999. 234 pp. $49.95. John S. Rickard. Joyce’s Book of Memory: The Mnemotechnic of Ulysses. Duke University Press, 1999. 240 pp. $17.95 paper. R.J. Schork. Joyce and Hagiography: Saints Above! University Press of Florida, 2000. 239 pp. $49.95. Raymond Williams observes that in addition to the dominant culture, modern societies generally contain residual and emerging cultures. Residual cultures, such as aristocracy and religion, consist of residues of previous social formations. In emergent cultures, such as feminism and psychoanalysis, new meanings are continually created. Either residual or emergent cultures may be either alternative—capable of co-existing with the dominant one—or oppositional—in conflict with it. The oppositional culture of Modernism combined the residual in its mythological aspect with the emergent in its avant-garde features. 1 It has long been customary for Joyceans to oppose the traditional foundations on which Joyce built his works to his revolutionary skepticism, and almost all works of Joyce criticism involve some alignment of the two sides. Because the attitude of opposing the dominant order is shared by residual and emergent cultures, they often overlap, as in the apocalyptic aspect of the revolutionary. It behooves the Joycean to be aware of how the two sides dialogue with each other in critical works on Joyce and in passages by him. Even a straightforward presentation of traditional material, such as R.J. Schork’s Joyce and Hagiography, a valuable compendium of information on Joyce’s references to Christian saints, ends up having an [End Page 575] oppositional side because Schork maintains an awareness of the often scathing irony with which Joyce handles religious elements. This is Schork’s third scholarly study of Joyce’s archaic backgrounds. He is saving the Virgin Mary for his fourth, which will be on scripture and Catholic ritual. Peter Francis Mackey’s Chaos Theory approaches residual culture through emerging ideas. Mackey attempts to use the recent mathematical concepts known as chaos theory to describe Ulysses, and, especially, the role of Leopold Bloom. This theory of complexity recognizes that the events of the world are subject to so many variable forces that they cannot be reduced to a formula of linear logic with determinable cause and effect. Mackey’s account of Bloom’s quest to understand and act on the contingencies of his world is often revealing, but it may impose a frame of its own. The science of complexity, which is often used to describe elaborate physical processes like the weather, develops equations that allow for unpredicatable changes caused by the interaction of many different factors. Chaos theory tends toward the view that if patterns of life are incomprehensible, it is not because we need to know more about them, but because the processes of nature, even if totally known, are inherently incomprehensible. On this basis, Mackey draws parallels between his ideas and poststructuralism (which he calls “postmodernism”) in its emphasis on the indeterminacy of truth. Seeing parallels between the obscurity of Bloom’s life experience and the obscurity that readers of Ulysses encounter, Mackey argues that Bloom follows a method of Aristotelian inquiry into physical reality that Stephen Dedalus develops in the “Proteus” episode. This overlooks Stephen’s rather hostile attitude toward the physical, but it reveals many striking examples of Bloom’s experimental procedures for confronting the complexity of physical reality and explaining it—a new twist on the experimental novel. Mackey emphasizes the idea that tiny changes can result in huge results through random connections, making it clear that this is an important idea in Ulysses. He sees Bloom as recognizing the potential of the improbable and acting on this sense of contingency. In fact, he sees Bloom as using free will to make choices that may cause liberating changes: Mackey emphasizes the possibility of hope for Bloom at the...

O‐Mode Emission at the Io Torus: A Real or Virtual Source?

We examine the nature of an ordinary (o‐mode) emission observed near the Io plasma torus during the 1992 Ulysses encounter with Jupiter. The emission appears to intensify by factors of 10–100 in local torus density depletions. We examine whether these intensifications are due to local generation within the density depletion or reflection of a remotely generated emission. Based on the observed emission spectra, we find that a virtual source is most consistent with the character of the local enhancements. This virtual source observation near the dense torus may be relevant to the understanding of a newly discovered torus‐related hectometric emission by the Galileo orbiter.

An explanation for the absence of sounder‐stimulated gyroharmonic resonances in the Io plasma torus by the Ulysses relaxation sounder

Plasma resonances were excited by the relaxation‐sounder component of the unified radio and plasma wave instrument (URAP) during the Ulysses encounter with Jupiter's Io plasma torus. The observed resonant spectra were quite different than expected, based on earlier published observations from topside sounders in the terrestrial ionosphere and relaxation sounders in the terrestrial magnetosphere, in that the URAP sounder spectra did not contain resonances at the harmonics of the electron gyrofrequency (which are commonly excited in the ionosphere and magnetosphere). Arguments are presented here to support the earlier suggestion that the absence of sounder‐excited gyroharmonic resonances in the Io plasma torus was due to the relatively long receiver dead time and to the inability to satisfy the required echo rendezvous conditions.

Octupole model of Jupiter's magnetic field from Ulysses observations

A spherical harmonic model of the planetary magnetic field of Jupiter is obtained from the Ulysses encounter observations. The magnetic field in the Jovian magnetosphere is represented using a degree and order 3 spherical harmonic for the planetary (“internal”) field, and an explicit model of the magnetodisc for the field (“external”) due to distributed currents. The model is obtained by partial solution of the underdetermined inverse problem using generalized inverse techniques. Dipole, quadrupole, and a subset of the octupole spherical harmonic coefficients are determined and compared with those obtained using Voyager and Pioneer observations. The model requires a less intense magnetodisc current, in early 1992, compared with that observed during the Voyager 1 encounter in 1979 and the Pioneer encounters of 1973 and 1974.

Solar Wind Control of Jupiter's H+3Auroras

A program of observations of Jupiter's H+3auroras was conducted during the first 4 months of 1992, a period that spanned the Ulysses encounter. Daily variations of auroral intensity are small in magnitude (∼20%) and well correlated with variations in the solar wind ram pressure arriving at Jupiter. A much larger (factor of 3) time variation is observed to occur on a time scale of approximately 2 months. During the first 2 months of observation, the inferred total intrinsic intensity of the northern aurora exceeded that of the southern aurora by a factor of 2. Throughout the latter 2 months, characterized by less intense auroral activity, the intensity of the northern aurora was comparable to that of the southern aurora.

Numerical simulations of non-adiabatic particle motions in the Jovian magnetosphere; comparisons with Ulysses observations

The Ulysses encounter with Jupiter gave a unique opportunity for the study of ions in the Jovian day side in the range 0.3 < ( E nuc ) < 6 MeV . The EPAC experiment has already provided a wealth of interesting results concerning the composition and the anisotropies in the magnetosphere (Krupp et al., Planet. Space Sci. 41, 953, 1993), however a more complete understanding of the observations is still lacking from the scientific literature. This paper contributes to the field with the analysis of numerical simulations of non-adiabatic trajectories in Jupiter's current sheet and the calculation of the appropriate anisotropies. The results presented here differ from the existing numerical results in that they seem to be in good agreement with the observations of both the Ulysses and Voyager encounters.

Further studies of waves accompanying the solar wind pick-up of interstellar hydrogen

Data collected by the magnetometer onboard the Ulysses spacecraft are surveyed for the occurrence of waves generated during the pick-up of interstellar hydrogen. Thirty one wave events were found during a 640 day study period, between March 21, 1992 and December 20, 1993 (after the Ulysses encounter with Jupiter). It is found that observation of the waves does not depend on the magnitude of the background magnetic field, but is a strong function of the angle between the magnetic field and the solar wind flow direction, with small angles being favored.

Direction finding study of Jovian hectometric and broadband kilometric radio emissions: evidence for their auroral origin

Taking advantage of the direction finding capabilities of the Ulysses unified radio and plasma wave (URAP) experiment we derive the source locations and emission characteristics of the Jovian hectometric (HOM) and broadband kilometric (bKOM) emissions. Unlike previous studies we additionally determine the systematic error of the source direction due to the uncertainty of the antenna/ receiver calibration parameters. To obtain maximum accuracy when doing direction finding, we use HOM and bKOM events observed close to the Ulysses encounter with Jupiter. It is found that both emissions have their sources at the auroral zones of Jupiter, at dipole L shells between 7 and 11 for HOM in the northern hemisphere, and between 9 and 15 for the bKOM radiation observed at high southern latitudes after the Ulysses Jupiter encounter. For the analyzed HOM events the source location is usually between 40° and 130° central meridian longitude (CML) and 2200 and 0600 local time (LT), but probably there exist emissions from other longitudes and local times too. In contrast, the bKOM is emitted from a wide range of longitudes and local times as well. There is some evidence that both the kilometric and hectometric emissions are composed of a dominant component in the fast extraordinary (R-X) mode and a weaker ordinary (L-O) mode component. Due to the uncertainty in source direction determination the emission's cone half-angle (i.e. half-angle of the cone in which the emission is beamed) cannot be accurately determined. It is some 30°–90° for the HOM and about 40°–80° for the bKOM. The source location of bKOM is likely to be associated with open magnetospheric field lines whereas the HOM is located at field lines that connect the HOM radio sources with the inner Jovian plasma sheet and/or outer plasma torus. There is some evidence that the bKOM radio emissions are correlated with ultraviolet auroral activity.

A survey of low frequency waves at Jupiter: The Ulysses encounter

We report the results of a survey of low‐frequency (LF) plasma waves detected during the Ulysses Jupiter flyby. In the Jovian foreshock, two predominant wave periods are detected: 102‐s and 5‐s, as measured in the spacecraft frame. The 102‐s waves are highly nonlinear propagate at large angles to (typically 50°), are steepened, and sometimes have attached whistler packets. For the interval analyzed the 102‐s waves had mixed right‐and left‐hand polarizations. We argue that these are all consistent with being right‐hand magnetosonic waves in the solar wind frame. The 102‐s waves with attached whistlers are similar to cometary waves. The trailing portions are linearly polarized and the whistler portions circularly polarized with amplitudes decreasing linearly with time. The emissions are generated by ∼2‐keV protons flowing from the Jovian bow shock/magnetosheath into the upstream region. The instability is the ion beam instability. Higher Z ions were considered as a source of the waves but have been ruled out because of the low sunward velocities needed for their resonance. The 5‐s waves have , are compressive and are left‐hand polarized in the spacecraft frame. Local generation by three different resonant interactions were considered and have been ruled out. One possibility is that these waves are whistler mode by‐products of the steepened lower‐frequency magnetosonic waves. Mirror mode structures were detected throughout the outbound magnetosheath passes. For these structures, the θkB values were consistently in the range of 80° to 90°, exceptionally high values. Assuming 1‐keV protons, the spacing between the magnetic decreases is ∼10‐20 rp. Close to the magnetopause, small amplitudes , transverse waves were detected. Within the wave packet, right, left‐ and linear polarizations were found. These may be similar to waves detected in the Earth's plasma depletion layer. Within the Jovian low‐latitude boundary layer, enhanced transverse spectral power was detected at frequencies just below the proton cyclotron frequency. A total magnetic power of 10−1 nT2 was determined. Cross‐field diffusion of ∼1‐keV protons yields a calculated boundary layer thickness of 0.15 RJ. This is in excellent agreement with previous measurements. Transverse left‐hand (spacecraft frame) waves were detected within the Jovian magnetosphere. These waves have periods of 5‐8 min, are elliptically to circularly polarized and propagate in a range of 10° to 43° relative to . It is most likely that these waves are generated by an ion beam instability. Assuming S+, we get a resonant parallel energy of 60 keV.

Introduction to the Ulysses Encounter With Jupiter

The Ulysses spacecraft encountered Jupiter in February 1992, passing within 6.31 radii of the planet. For approximately 8 days it was inside the Jovian magnetosphere, and for several days before and after that, Ulysses was in the interaction regions formed by the solar wind (the magnetosheath and boundary layer). The inbound trajectory was at a ≈ 1000 LT and the outbound trajectory was at 1800 LT, that is, dusk, a unique feature of the flight path. Three regions interior to the magnetosphere were identified as on previous missions both inbound and outbound. In addition, the spacecraft twice penetrated a cusplike region at high latitude in the inner magnetosphere. Following closest approach, Ulysses traversed the Io plasma torus in basically a north‐south direction. Although Ulysses is a heliospheric mission, the experiments were well suited to an investigation of Jupiter's magnetosphere and have returned much new information. TMs introduction to the accompanying articles by the Ulysses investigators provides basic information on the experiments, the spacecraft, and the trajectory. In addition, the scientific context of the encounter is reviewed on the basis of the preliminary analyses of the Ulysses observations and a rudimentary comparison with the earlier Pioneer and Voyager results. Some important scientific questions raised by the encounter, along with some tentative answers, are presented.

Low-resolution spectra of the Io plasma torus 2 days after the Ulysses encounter

Low-resolution spectra of the Io plasma torus have been obtained on 10 and 11 February 1992 (2 days after the Ulysses encounter) using the 2 m telescope of the Bulgarian National Observatory. The spectra show the forbidden line emissions of S + (λλ 6716, 6731 Å) and S 2+ (λ 6312 Å). Measured intensities are compared with a Voyager-type model. The intensity distribution of [SII] is found to deviate from the model predictions which indicates a change in the torus at the Ulysses encounter when compared with the Voyager epoch. A corotating structure was observed, both in [SII] and [SIII], at λ III = 170°, showing that the torus was not azimuthally symmetric. The λ 6716/λ 6731 and λ 6731/ λ 6312 line ratios indicate a higher electron density at the time of the Ulysses observations. Additionally, the shift of the torus caused by the dawn-dusk electric field could be observed. Peak intensities in [SII] were found at 5.66 ± 0.02 R J on the West ansa and 5.91 ± 0.04 R J on the East.

The Ulysses encounter with Jupiter: an introduction

The Ulysses encounter with Jupiter: an introduction

Ulysses observations of the planetary depletion layer at Jupiter

The repeated samplings of the Jovian magnetosheath during the Ulysses encounter with Jupiter provided an opportunity to probe the planetary depletion layer. Of the 10 complete crossings of the Jovian magnetopause, only three contained clear signatures of an overlying depletion layer. All of these occurred on the flanks of the magnetosphere near the dusk terminator; crossings on the dayside were ambiguous or clearly lacked a depletion layer signature. In this paper we present a detailed analysis of the observations by the Ulysses solar wind plasma and magnetometer experiments and discuss conditions favorable and unfavorable for depletion layer observation.

Measurements of hot plasmas in the magnetosphere of Jupiter

This paper presents an overview of a number of the principal findings regarding the hot plasmas ( E ≳ 50 keV) in Jupiter's magnetosphere by the HISCALE instrument during the encounter of the Ulysses spacecraft with the planet in February 1992. The hot plasma ion fluxes measured by HI-SCALE in the dayside magnetosphere are similar to those measured in the same energy range in this region by the Voyager spacecraft in 1979. Within the dayside plasma sheet, the hot-ion energy densities are comparable with, or larger than, the magnetic field energy densities; these hot ions are found to corotate at about one-half the planetary corotational speed. For ions of energies ≳ 500 keV/nucleon, the protons contributed from 50–60% to as much as 80% of the energy content of these plasmas. Strong, magnetic-field-aligned streaming was found for both the ions and electrons in the high-latitude duskside magnetosphere. The ion and electron pitch-angle distributions could be characterized by cos 25 α throughout many of the high anisotropy intervals of the outbound pass. There is some evidence in the ion pitch-angle distributions for a corotational component in the hot plasmas at high Jovian latitudes. While there are limitations owing to the finite geometries of the detector telescope systems on the determination of the angular spreads of the ion and electron beams, the measurements show that there are intervals when the particle distributions are not bidirectional. At such times, locally the hot plasmas could be carrying currents of ∼ 10 −4 μAm −2. The temporal variations in the streaming electron fluxes are substantially larger than the variations measured for the fluxes that are more locally mirroring. The temporal variations contain periodicities that may correspond to hydromagnetic wave frequencies in the magnetosphere as well as to larger scale motions of magnetospheric plasmas. On nearly half of the days for about a 130 day interval around the time of the Ulysses encounter with the planet, particles of Jovian origin were measured in the interplanetary medium. An event discussed herein shows evidence of an energy dependence of the particle release process from the planetary magnetosphere into the interplanetary medium.

The low-frequency interplanetary radiation

Observations of the low-frequency interplanetary radiation have continued, although no major activity has begun since mid-1991. Theoretical work has been completed which suggests that the long-speculated source at the termination shock is feasible and could at least qualitatively account for several aspects of the observations, although some difficulties exist. The Ulysses encounter with Jupiter occurred in February 1992 and has renewed discussion of a Jovian source for the emissions. We reconsider the arguments used previously to rule out Jupiter as a source and conclude that the existence of a relatively high-density boundary to the heliosphere renders most of these arguments ineffective in eliminating Jupiter as a source. We conclude, therefore, that Jupiter must be considered as likely a candidate source as the termination shock for the low-frequency emissions and, given that Jupiter is a known source of emissions in the few-kHz range, perhaps Jupiter is the more likely candidate. Even with a Jovian source, however, the low-frequency emissions continue to be useful sounders of the heliosphere which can provide information on the size of the heliospheric cavity. No conclusive evidence is available which identifies either Jupiter or the termination shock as the source.

Imaging Observations of Jupiter's Sodium Magneto-Nebula During the Ulysses Encounter

Jupiter's great sodium nebula represents the largest visible structure traversed by the Ulysses spacecraft during its encounter with the planet in February 1992. Ground-based imaging conducted on Mount Haleakala, Hawaii, revealed a nebula that extended to at least +/-300 Jovian radii (spanning approximately 50 million kilometers); it was somewhat smaller in scale and less bright than previously observed. Analysis of observations and results of modeling studies suggest reduced volcanic activity on the moon lo, higher ion temperatures in the plasma torus, lower total plasma content in the torus, and fast neutral atomic clouds along the Ulysses inbound trajectory through the magnetosphere. Far fewer neutrals were encountered by the spacecraft along its postencounter, out-of-ecliptic trajectory.

Volcanic Activity on Io at the Time of the Ulysses Encounter

The population of heavy ions in lo's torus is ultimately derived from lo volcanism. Groundbased infrared observations of lo between October 1991 and March 1992, contemporaneous with the 8 February 1992 Ulysses observations of the lo torus, show that volcanic thermal emission was at the low end of the normal range at all lo longitudes during this period. In particular, the dominant hot spot Loki was quiescent. Resolved images show that there were at least four hot spots on lo's Jupiter-facing hemisphere, including Loki and a long-lived spot on the leading hemisphere (Kanehekili), of comparable 3.5-micrometer brightness but higher temperature.