C. T. Russell1, J. J. Caldwell2, I. de Pater3, J. Goguen4, M. J. Klein5, B. L. Lutz6, N. M. Schneider7, W. M. Sinton4 and R. A. West6

1. IGPP, University of California, Los Angeles, CA 90024, U.S.A.
2. York University, North York, Ontario M3J IP3, Canada
3. University of California, Berkeley, CA 94720, U.S.A.
4. Institute for Astronomy, Honolulu, HI 96822, U.S.A.
5. Jet Propulsion Laboratory, Pasadena, CA 91102, U.S.A.
6. Lowell Observatory, Flagstaff, AZ 86001, U.S.A.
7. University of Arizona, Tucson, AZ 87521, U.S.A.

Originally Published in:
Adv. Space Res., Vol 10, No. 1, (1)239-(1)242, 1990.



          The International Jupiter Watch is a program for the encouragement and coordination of the study of temporal variations in the Jovian system. It consists of six discipline working groups concerned with: the Io torus under N. Schneider; the Jovian atmosphere under R. West; the magnetosphere and radio emissions under I. de Pater and M. Klein; aurora under J. Caldwell; the Galilean satellites under W. Sinton and J. Goguen; and laboratory measurement and theory under B. Lutz. To date the IJW has held two workshops and selected several Jupiter Watch periods for coordinated observations. The next Jupiter Watch workshop is tentatively scheduled for 1990 in association with the next COSPAR meeting.


          The International Jupiter Watch grew out of a series of discussions in the early 1980's among a number of planetary scientists studying the Jovian system. They expressed concern that while major spaceflight missions had been flown to Jupiter and while a new one was being prepared, there was not the monitoring of the highly variable Jovian system needed to place these measurements in their proper context, nor was there sufficient monitoring to understand the source of much of this variability. An organization was needed to coordinate and encourage studies of the variability of the Jovian system, especially because the necessary long-term observations, often receive less support than those that appear to have a more rapid payoff. Moreover, the Jovian system requires study over a wide range of wavelengths, with quite disparate techniques at widely separated sites. Coordination was in order.

          A small grant was obtained from the California Space Institute to hold a workshop which would prepare a report on the feasibility of starting a Jupiter Watch program. This was followed by an initial meeting of interested parties at the October 1985 DPS meeting in Baltimore and then by a workshop at the Pasadena Convention Center in May 1986, attended by over 60 scientists. (Plans for having the meeting at the Kennedy Space Center in conjunction with the launch of Galileo were cancelled after the loss of Challenger).

          As a result of that meeting an International Steering Committee was appointed and a report written outlining the science needs and current programs which contribute to the study of the variability of the Jovian system. The steering committee presently consists of M. Belton, I. de Pater, D. M. Hunten, T. V. Johnson, Y. Leblanc, G. Morfill, C. T. Russell, F. Taylor and R. West. To further delineate the present status of this field of study, its present achievements and remaining objectives, a second workshop was held in August 1987 in Flagstaff, Arizona. The proceedings of this workshop are presently being edited by M. J. Belton, J. Rahe and R. West for publication later this year.

          In the discussions that followed this workshop it was decided that the leaders of each of the discipline working groups of the IJW would prepare working papers on a possible program for the study of Temporal Variations in the Jovian System (TVJS). A program plan would then be assembled from these working papers. Such a program would optimize the scientific return from existing facilities and research efforts, increase the scientific understanding of the Jovian system and pave the way for understanding the observations of the Galileo spacecraft when it arrives at Jupiter in late 1995. The purpose of this paper is to summarize these working papers and outline both the rationale for such a program and how it might be undertaken.


          The Jovian system is the most complex of our planetary systems. From the intense banded circulation pattern at the deepest visible levels in the atmosphere, to the volcanic activity on Io, to the powerful radio signals generated in the magnetospheric plasma, Jupiter presents an ever changing visage to our sensors. No one instrument operating by itself can hope to unravel the complex interplay of forces that shape the Jovian environment. The study of Jupiter must be done at multiple wavelengths from x-ray to radio. It must be done with a variety of instruments from imagers to photometers, to spectrometers, to radio telescopes, It requires not only a multiplicity of instrumentation but also a multiplicity of sites, including Earth orbiting satellite data. Moreover, since the system is ever changing on time scales as long as the longest baselines for which data are available we need to monitor Jovian activity for long periods of time, not just take snapshots.

          To undertake a successful study of such a complex system requires coordination and it requires adequate resources. The realization of the community that the science return will be enhanced through coordination led to the establishment of the worldwide International Jupiter Watch effort. However, by itself the IJW can supply only enthusiasm and coordination. It has no resources with which to support or enhance existing programs for the study of these objectives. To do so requires the support of national agencies. This paper outlines some of those objectives which need such support.

          It is our belief that a budget of about $2 million per annum could enable several new research thrusts in each of the Jovian disciplines. Since Jupiter varies on all time scales, the optimum length of the program is not easy to define. It seems opportune to begin now because a dedicated and enthusiastic community exists. The program should last through the prime mission of Galileo in order properly to place the Galileo observations in context relative to the long term variability of the system. Thus we envision an 8 or 9 year program.

          In the sections below we present recommendations in each of the areas covered by the IJW discipline working groups. The recommendations were prepared by the following people: on the Jovian atmosphere, by R. A. West; on the satellites of the Jovian system, by W. M. Sinton with contributions from J. Lunine, J. Gougen and J. E. Arlot; on aurorae on Jupiter, J. Caldwell; the plasma torus, neutral clouds and the atmosphere of Io were prepared by N. Schneider with contributions from J. Morgan, L. Trafton, W. Smyth, T. Skinner, F. Bagenal, W. Sandel, D. Shemansky, P. Feldman, F. Scherb, D. Strobel and G. Ballester; on Jupiter's magnetospheric and radio emissions, by M. Klein with contributions from I. de Pater and Y. Leblanc; and on laboratory measurements and theory, by B. Lutz.


          The program for the study of the Temporal Variations of the Jovian System should be a balanced one. It should promote the acquisition of new data. It should support the reduction and analysis of both old data and new data. It should involve theory, modeling and simulation, and laboratory measurements. The program should be complete in that all elements of the variability should be studied, including atmospheric phenomena, volcanic eruptions on Io, the Io torus, neutral cloud and atmosphere, the Jovian aurorae, and the Jovian radio emissions, both decimetric and decametric. Many of these phenomena are closely coupled so that we can not satisfactorily study any one of them in isolation.


          Although some first order questions remain unanswered we now have a basic understanding of the average properties of the Jovian atmosphere such as the mean temperature profile, bulk composition and zonal flow. Substantial gains in understanding the complex and highly coupled meteorological processes will require coordinated observations and sophisticated models. A program to coordinate observations of the Jovian atmosphere at many wavelengths and over several years would be extremely valuable. The scientific objectives motivating such an effort are to improve our understanding of the meteorological parameters (e,g. temperature, relative humidity of condensable species, winds, etc.) which influence cloud albedo, color, and optical depth and to develop an understanding of why some large-scale regions vary with time.

          It will be essential to monitor Jupiter from the Earth over a period of years, using telescopes with such diverse characteristics as, but not limited to, the NASA Infrared Telescope Facility, which can record spatially resolved thermal emission and the NASA Hubble Space Telescope (scheduled for launch in mid-1989), which will provide ultraviolet spectroscopy of selectable regions of the planet and visual wavelength and near-infrared methane band images at 0.1 arc-sec resolution or better. Even radio interferometers, such as the Very Large Array, are important because they can give information on ammonia below the clouds, a region not otherwise accessible.


          The two basic purposes of these studies are to understand the causes of the variability of Io and to verify the suspected variability of Europa. In order to achieve these objectives observers are studying the composition of the surface of Io, its geomorphology and tectonics, explosive volcanism, hot spots, tidal and other internal heating mechanisms and how the volcanism of Io affects the rest of the Jovian system.

          Since many of the most interesting phenomena are rare, it is desirable to have an automatic tracking and data acquisition network which could monitor explosive volcanism of the Pele class. This tracking could be accomplished using several existing 24-inch telescopes with some refurbishment of those telescopes. Coordination of an amateur network of photometric observations is also strongly recommended. Infrared observations should continue to be made and the instruments improved to their theoretical maximum. Lastly, full advantage should be taken of the 1991 mutual eclipse opportunity. This maximum advantage can be obtained with the construction of a transportable multi-channel IR occultation photometer and data system.


          The aurorae on Jupiter are of interest because they allow us to infer the nature of the precipitating energetic particles which cause the auroral radiation. The study of aurorae allows the testing of models of the planetary magnetic field. It permits us to determine the rate of energy deposition from magnetospheric sources. By determining where the aurorae occur, one can ascertain the magnetospheric source regions of the particles and possible causative mechanisms. In particular, it is of significant interest to determine the causes for the differences between the northern and southern aurorae. Finally, particle precipitation near the magnetic poles induces unique chemistry that is important for understanding some aspects of aerosol formation.

          To achieve these goals, the data base from the International Ultraviolet Explorer satellite should be extended as long as feasible, and the HST should be brought into this effort as soon as it is launched. Ultraviolet spectroscopy with high spectral and spatial resolution will be increasingly important and should be encouraged. Existing facilities should be coordinated to achieve optimal efficiency, and continued observations over long temporal baselines must be undertaken.


          Researchers in this area are attempting to understand the plasma torus and neutral clouds through a study of the variations in their morphology, density and energy distribution. They are attempting to explain the state, variability and stability of the system and to identify the primary source, sink and transport processes for mass and energy. Some of the specific problems that need to be addressed are as follows: to determine why there is a higher density plasma near System III magnetic longitude 200° ; to determine whether two magnetic longitude systems are required to explain periodic variations; and to examine the long-term variability of the Io torus. It is also necessary to monitor the neutral sodium cloud for long-term changes; to determine the size of east/west asymmetries in the sodium cloud; and to understand the source of the newly discovered sodium jets. In order to understand the overall behavior of the system, one must investigate: its stability, including feedback mechanism; the interaction between Io and the plasma; the relationship of the torus to the other components, in particular, Io's volcanic eruptions.

          To achieve these objectives we must improve the temporal coverage of both the plasma torus and neutral clouds. We must increase simultaneous wavelength coverage and stimulate greater interdisciplinary cooperation. High resolution spectroscopy and narrow band imaging should be encouraged. Existing equipment and facilities which are in many cases idle should be utilized and more data acquired and analyzed. Observations from Earth-orbit including the current International Ultraviolet Explorer, the imminent Hubble Space Telescope and future missions such as Lyman will greatly benefit these studies.


          The primary focus of the magnetospheric and radio emission researchers is the two non-thermal components which are observed over broad intervals of the decimetric and decametric wavelength bands. Decimetric observations are being used to determine the source and mode of transport of the energetic particles in Jupiter's magnetic field, and to study the variability of Jupiter's magnetic field. Monitoring programs such as the weekly 13-cm total flux density observations (Jupiter Patrol) with the DSN and the annual high resolution mapping with the VIA need to be continued. Microwave observations need to be expanded to measure the synchrotron spectrum, the polarization parameters at several wavelengths and to examine short-term (days to weeks) variability. Correlative studies of the intrinsic decimetric variability with parameters external to the Jovian system should be continued. Improved modeling needs be undertaken.

          At decametric frequencies, the problems to be solved include determining the processes that produce the emissions, the locations and apparent sizes of the sources and the geometry of the source beaming patterns. To do this we must expand the ground-based decametric network to enable 24-hour coverage at several frequencies. This will test the long-term stability of the inner magnetosphere, its magnetic field and rotation period. Further, these data will allow the radiation characteristics with respect to the Jovicentric declination of the Earth to be measured, any solar cycle effects to be deduced and the emission beam models to be improved. Lastly, a VLBI network should be established to observe Jupiter in the 18-26 MHz frequency range. This network could resolve source locations to a few hundred km at Jupiter.


          Laboratory measurements are essential to any study of the Jovian environment in order to provide adequate knowledge of the fundamental properties of atoms and molecules and the physical and chemical processes that affect them in their extraterrestrial environments. These measurements thus allow proper interpretation of remote sensing (and eventually) in-situ data. Similarly, theoretical analyses, computer simulations and models are essential if we are to extend the empirical knowledge obtained through observations to a thorough understanding of the physical and chemical mechanisms underlying the phenomena.

          A few examples of needed laboratory measurements include: near IR spectra of CH4 and NH3 at temperatures relevant to the Jovian atmosphere: ultraviolet spectra Of C2H2, H2S and S02; the radio spectrum of NH3 and NH4SH; optical properties of ices and crystals; rate coefficients for chemical reactions; sputtering yields of candidate Jovian satellite surface materials. The transition probabilities and collision strengths for many atomic transitions of interest in the Io torus (especially for S+) should be determined both experimentally and theoretically.

          Extensive simulation, modeling and theory need to be undertaken as well. Computer models of the Jovian circulation, and the interaction of the Io torus with Io are needed. Analytical studies which help us understand the mechanisms behind the observed phenomena such as the tidal heating of Io, the interaction of eddies and shear layers and the radial diffusion of energetic charged particles in the Jovian magnetosphere are all needed.


          The International Jupiter Watch program consists totally of researchers interested in the Jovian system who have volunteered their time to help coordinate a program to study the temporal variability of the Jovian system. It is open to all such volunteers. Any persons interested in participating in this effort should contact the discipline working group leader whose interest most closely matches their own.

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