The radiation belts of Jupiter at 13 and 22 cm

Transcription

1 Astron. Astrophys. 319, (1997) ASTRONOMY AND ASTROPHYSICS The radiation belts of Jupiter at 13 and 22 cm I. Observations and 3-D reconstruction Y. Leblanc 1, G.A. Dulk 1,2, R.J. Sault 3, and R.W. Hunstead 4 1 CNRS URA 264, DESPA, Observatoire de Paris, F Meudon, France 2 Department of Astrophysical, Planetary and Atmospheric Sciences, University of Colorado, Boulder, CO 80309, USA 3 Australia Telescope National Facility, CSIRO, Epping, NSW 2121, Australia 4 Departement of Astrophysics, University of Sydney, NSW 2006, Australia Received 11 April 1996 / Accepted 17 June 1996 Abstract. We present Australia Telescope observations of Jupiter in July 1995 at 13 and 22 cm with a resolution of 3 8 at 13 cm and 5 14 at 22 cm. Images averaged over 10 days of observation clearly show the two populations of energetic electrons, one concentrated at the magnetic equator, and the other reaching high latitudes. The average separation between the east and west limb peaks is 2.9 R J at both 22 and 13 cm, and the radiation extends to 4 R J with good signal to noise. A 3-D reconstruction of the belts is presented, showing vividly the warping of the magnetic equator as manifested in the radiation belts around the planet, and in the mirror regions at high latitudes. From a series of images at different longitudes, the E-W brightness distribution as a function of CML is shown in a new way, demonstrating how the brightness on the two sides of the belt changes with Jupiter rotation. The bright spot crosses the east limb when CML 120, located at System III longitude λ III 210. When it crosses the west limb, less than 180 later, the same spot is fainter. When the E-W brightness is plotted in terms of λ III, the ratio of east-to-west limb brightness takes on a simple, sinusoid-like form. The ratio is greater than unity in the λ III range 180 to 40 for these observations, made at D E = 3.3. In Paper II we relate the observations to the warping of the magnetic equator and obtain further insight into the magnetic field of Jupiter. Key words: planets and satellites: Jupiter continuum: solar system techniques: image processing 1. Introduction Jupiter s synchrotron radiation is emitted by trapped, relativistic electrons as they spiral in Jupiter s magnetic field. The emission depends on the magnetic field strength and direction, and on the Send offprint requests to: Y. Leblanc number, energy distribution and pitch angle of the relativistic electrons. There are two populations of electrons, one with pitch angles near 90 giving radiation concentrated at the magnetic equator, where the intensity is largest near R 1.5R J, and one with a wide pitch-angle distribution giving radiation at a large range of magnetic latitudes (e.g. reviews by Berge & Gulkis 1976; Carr, Desch & Alexander 1983; de Pater 1990). High resolution images show two lobes of the radiation belts, one to the left (east) and one to the right (west). One lobe of the belts is more intense than the other, and the appearance of this asymmetry changes with Jupiter s rotation. This east-west asymmetry has often been described by the ratio of the peak intensities of the east and west lobes as a function of Central Meridian Longitude (CML). The east-west asymmetry was first observed by Branson (1968) who made two dimensional maps at 21 cm for three different longitudes, each integrated over 120. As noted by Branson and by Conway & Stannard 1972, these maps revealed the existence of a hot spot or a region of peak intensity near λ III 220 (about 6 smaller if converted to System III ). Later, de Pater made high resolution images with the Westerbork telescope in 1973 (de Pater & Dames 1979) and 1977/78 (de Pater 1980), and with the Very Large Array (VLA) telescope in 1981 (de Pater & Jaffe 1984). From the analysis of the observations, de Pater (1983) reported that the hot region moved to the longitude λ III = 255. The asymmetry was very soon attributed to the non-dipolar character of the magnetic field (Conway & Stannard 1972, Gerard 1976). De Pater (1981) made an extensive, pioneering study of Jupiter s synchrotron radiation with a model that incorporated existing knowledge of the magnetic field (the P11 and O4 models), the energy and pitch angle distributions of the relativistic electrons, their diffusion inward from large distances, their losses at small distances, and the effects of satellites. She compared the results of her model calculations with the distribution in CML of the east-west ratio, i.e. the difference of intensity on the east and west side of the belts. However, she

2 Y. Leblanc et al.: The radiation belts of Jupiter at 13 and 22 cm. I 275 found that the match with the observations was not satisfactory, and she invoked an overabundance of electrons in the longitude range λ III Whereas longitude drift of electrons in the O4 and P11 leads to a 3% density increase, an increase of about 30% was found to be needed. With this and another effect (the dusk-to-dawn directed electric field in the inner magnetosphere generated by the wind system in the upper atmosphere) she succeeded in obtaining a better fit with the observations. When Comet Shoemaker Levy-9 (SL9) hit Jupiter in July 1994, changes in brightness of the belts were observed at 13 and 22 cm with the Australia Telescope (Dulk, Leblanc & Hunstead 1995, Leblanc & Dulk, 1995) and at 21 cm with the VLA (de Pater et al. 1995). The increase of brightness was almost entirely confined to one hemisphere, from λ III 100 to 240. The brightness increase was attributed to a new or a newly accelerated population of electrons in the region where the magnetic field is the stronger (Dulk & Leblanc 1995). Here we report on Jupiter observations at 13 and 22 cm made with the Australia Telescope in July 1995, one year after the comet crash. We find that the radiation belts have almost entirely regained their normal state; the comparison with the observations made on July 1994 is the subject of another paper. The purpose of this paper is to show the 1995 observations of Jupiter s radiation belts at 13 and 22 cm, to compare the belts at the two wavelengths and to present the results of the east-west asymmetry in a new way that makes it more easily understood. The interpretation of these results, based on beaming of the emission from the warped magnetic equatorial surface, is developed in Paper II (Dulk et al. 1996). In Section 2 we present the observations. In Section 3 we describe how both two-dimensional images and a threedimensional reconstruction of the radiation belts were computed and give the results, including images where we are able to visualize the warp of the magnetic equatorial surface. In Section 4 the east-west asymmetry will be shown for the first time in a 2D image, and how the bright spot is brighter when seen on the east limb than on the west limb. The peak intensities of the east and west sides of the belt will be described as a function of longitude λ III of Jupiter, and not solely by their ratio as a function of CML; this presentation is efficient in showing essential features. In Section 5 we give concluding remarks. 2. Observations and method of analysis The Australia Telescope Compact Array (ATCA) is located at latitude 31 south, near Narrabri, NSW. It consists of 6 antennas, each 22 m diameter, located along an east-west line. For our observations the shortest baseline was 153 m, the longest 6 km. The telescope operated simultaneously at two wavelengths, 12.6 and 21.7 cm, with bandwidths of 128 MHz broken into 32 channels. Each antenna measures two orthogonal linear polarizations at each wavelength, from which the four Stokes parameters are derived. Jupiter was observed on 10 days, and July The antenna locations were the same for the two, 5-day sessions. The five day gap between the sessions was chosen so that the UV coverage would be approximately uniform as a function of Jovian longitude (e.g. the CML s at transit would be spaced by about 40 ). The Earth s declination as seen from Jupiter was D E = 3.3 jovigraphic ( 2.9 jovicentric) and Jupiter s distance was 4.63 AU (giving R J = 21.3 at that distance). Observations continued for about 11.5 hours per day, the entire time that Jupiter was above the 12 elevation limit of the antennas. Given Jupiter s 9 h 55 m period, all longitudes were observed each day, and an identical 50 longitude range was observed near rise and set. With Jupiter at DEC 20 at that time, aperture synthesis produced a good degree of north-south resolution as well as excellent east-west resolution. The synthesized beam was about 3 8 at 13 cm and 5 14 at 22 cm. All four Stokes parameters were measured. The data were reduced in the Miriad system (Sault et al. 1995) using conventional editing and calibration (both gain and polarization), and multifrequency synthesis was used to incorporate the 16 independent channels across the 128 MHz bandwidth. Multi-frequency synthesis significantly reduces the sidelobe level, particularly at 22 cm. Despite our polarization calibration, for an array with linear feeds, a residual error in the XY phase and absolute feed ellipticity (both typically 1 for the ATCA) allows a small fraction of linear polarization to corrupt the circular polarization, and vice versa (e.g. Sault et al. 1996a). Jupiter has strong linear polarization ( 60%) and weak circular polarization ( 1%), so the effect is not important for linear polarization, but very important for circular. At present we have little confidence in our images in circular polarization and do not present them here. We remark that only the locations on Jupiter with a significant longitudinal magnetic field component along the line of sight should show circular polarization. 3. Results 3.1. Rotation-averaged images We first present longitude-averaged images at 13 and 22 cm, for which we incorporated all 10 days of observing, approximately 12 rotations of Jupiter. These images were built essentially by assuming that the radiation belts are circularly symmetric about a tilted magnetic axis. We use an axis tilted by 10 from the rotation axis towards λ III = 200. To account for the wobble introduced by the tilt, we applied a time-dependent rotation to the u v coordinates (and, for linear polarization, to Stokes Q and U visibilities by double the angle). Other than this, we used standard synthesis imaging and deconvolution techniques. It is assumed that there were no large temporal changes of the intensity of the radiation belts during the observations; excepting the impact of Comet SL-9, during many years of observation no short-term variations have been confirmed, although long-term variations have been established by Klein, Thompson & Bolton (1989) and by Bolton et al.(1989). Fig. 1 shows the resulting images. They have the magnetic axis vertical. Longitude-dependent features are smeared out.

3 276 Y. Leblanc et al.: The radiation belts of Jupiter at 13 and 22 cm. I Fig. 1. Rotation-averaged images of Jupiter at 22 and 13 cm. At the top are images in total intensity and at the bottom in linear polarization. At 13 cm the thermal radiation of Jupiter s disk is bright in total intensity and absent in linear polarization. At 22 cm the disk is relatively faint. The resolution at 13 cm is At 22 cm the resolution in total intensity is 6 3 (super-resolved) and in linear polarization it is Fig. 3. Two-dimensional images of Jupiter at 22 and 13 cm for two longitudes. The resolution at 13 cm is and at 22 cm it is The color table was chosen to cover the full brightness range of 100 K to 1320 K at 22 cm and 50 K to 530 K at 13 cm. The circle shows the size of Jupiter s disk and the ellipses show the half power beams.

4 Y. Leblanc et al.: The radiation belts of Jupiter at 13 and 22 cm. I 277 In addition, the up-and-down rocking of the radiation belt in front of the disk adds a smeared-out component of synchrotron radiation to the brightness of the thermal emission. Images of Jupiter at 13 cm are rare, the only previous ones being those of Kenderdine (1980). Fig. 1 shows features with unprecedented clarity. At 13 cm the thermal radiation of the disk produces the bright central region, with the synchrotron radiation producing the extensions. Thermal radiation, which is unpolarized, is absent in the linearly-polarized image. At 22 cm the thermal disk appears much less bright than at 13 cm, whereas in reality its brightness temperature is slightly higher; the synchrotron radiation is much brighter (by a factor of approximately (22/13) 2 3). The images clearly show the radiation from the two populations of energetic electrons, one with large pitch angles producing radiation concentrated at the magnetic equator, and the other one with smaller pitch angles producing radiation up to high latitudes. Even though these are averaged maps, the east side of the belt is brighter than the west one. This is not unexpected; de Pater & Jaffe s (1984) observations made in 1981 also show the belt on the east to be brighter than the west for most CML. We will develop an explanation for this asymmetry in Paper II. Fig. 2 shows one-dimensional brightness scans as a function of position along the magnetic equator. The scans show that the equatorial belts extend to 4 R J at both 13 and 22 cm; at that distance the signal is still above the noise, estimated to less than T B = 10 K. We note that the slope of brightness of the belt is very regular from the peak to the noise level: neither at 2.5 R J nor at 3 R J (positions of the orbits of Amalthea and Thebe) is there a distinct change of slope, showing that they produce no large change in the synchrotron-emitting electron population. The peak to peak separation is 2.9 R J at both wavelengths. De Pater & Klein (1989) and de Pater (1991) summarized previous measurements of this separation, which is not always the same. However, the most recent measurements, those of 1989 with the VLA, were close to our value of 2.9 R J. We note that the separation we measure is the average over 12 rotations. The distance of the peaks from the center of Jupiter changes with CML, as is described below and documented in Paper II Images at different longitudes For aperture synthesis, a significant portion of the u v plane must be sampled before a Fourier transform can produce a reliable image. For an east-west array, such as the ATCA, only a very limited coverage at a particular longitude is obtained on any one day. However, on successive days different parts are covered. With our 10 days of observations, (5 days, a 5 day gap, and 5 more days), the resulting coverage for a given longitude is quite reasonable, provided that we consider bins of longitude. We have formed images at 18 nominal values of CML (0, 20,..., 340 ), 9 of which are independent, by using data within ±20 of the CML. We make images in the four Stokes parameters, CLEAN them, and construct images of the total and linearly Fig. 2. One-dimensional cuts of T B along the magnetic equator of the total intensity images at 13 and 22 cm of Fig. 1 showing the distance between the peaks, the rate of brightness decrease outward and the extent of detectable radiation to 4 R J. The vertical scale of brightness is for 13 cm emission (full line); it must be multiplied by 3.3 to give the brightness at 22 cm (dashed line). polarized intensity. Within the 40 of rotation for each image, we apply corrections (as in the previous section) to remove some of the effects of the magnetic field wobble. Fig. 3 shows maps at 13 and 22 cm for two longitudes 180 apart. At CML = 20 the east side of the belt is dimmer than the west, while at CML = 200 the east side is brighter than the west. The color table was chosen to span the full range of brightnesses, 100 K to 1320 K at 22 cm and 50 K to 530 K at 13 cm. The non-uniformity of the brightness of the disk, especially notable at 13 cm, is the result of synchrotron radiation of the equatorial belt in front of it. In the images at CML = 20 where the magnetic dipole points away from the observer, the belt is north of disk center, and conversely for the images at CML = Reconstruction of the belts in 3 dimensions Studying a series of two-dimensional projections as above is not the optimal way of understanding three-dimensional objects, so we also have produced cubes of Jupiter with three spatial axes. Only a brief description of the technique used to produce these will be given here full details can be found in Sault et al. (1996b). Consider imaging a rotating object where the source is optically thin, and where the emission is radiated isotropically. In this case, an interferometer measures an instantaneous sample in a three-dimensional Fourier domain. With many such measurements combined, the use of a three-dimensional Fourier transform will produce a cube with three spatial axes. This cube can then be deconvolved using a three-dimensional algorithm. We have implemented such an imaging and deconvolution scheme, incorporating a MX -like CLEAN algorithm. In this form of CLEAN, source components are subtracted from the

5 278 Y. Leblanc et al.: The radiation belts of Jupiter at 13 and 22 cm. I Fig. 4. Three-dimensional images of Jupiter s radiation belts at 22 cm. At the top are images in total intensity and on the bottom in linear polarization. The values of CML for the left and middle images are 90 and 270. The right-hand image is at CML = 90 and viewing angle declination 55. The warping of the magnetic surface is most evident in the images at CML = 90. Fig. 5. Brightness on each side of the equatorial belt as a function of CML at 13 cm. On the left is the total intensity and on the right the linearly polarized intensity. This new presentation shows how the brightness evolves with Jupiter s rotation. The position of peak intensity moves slightly inward and outward with CML. The bright spot on the east limb is at CML 120,(λ III = 210 ).

6 Y. Leblanc et al.: The radiation belts of Jupiter at 13 and 22 cm. I 279 visibility data (rather than the images) during so-called major cycles. Using this characteristic, we are able to relax the optically thin assumption by determining when a particular component is shadowed behind the disk of Jupiter, and not subtracting it if it is. As we are not interested in imaging the disk, we subtract a constant temperature elliptical disk model from the data before imaging the total intensity data (we have used temperatures of 280 and 350 K at 13 and 22 cm respectively; see de Pater et al. 1995). The assumption that is not readily relaxed is that the emission is radiated isotropically. For the synchrotron radiation, where beaming is involved, this is not satisfied. With the isotropic assumption, the resultant cubes represent the average emission at each 3-D position. Asymmetries in the emission of a given longitude between east and west limb passage are averaged out. However, as will be seen later these asymmetries are of order 20 to 30% and do not obscure certain important features of the radiation belts Total intensity Fig. 4 (top row) shows sample views of the cube from three different angles in total intensity. The thick disk coincides with Jupiter s magnetic equator and is a result of emission from the electron population with large pitch angles. The inner extensions to higher latitudes are due to the population with smaller pitch angles. In this presentation the disk of Jupiter has been removed. A very important feature seen in Fig. 4 is that the equatorial belt is not cylindrically symmetric or planar, but is warped, with some portions being tilted relative to the average magnetic equator by 10 or more. In addition, some portions are at larger radii than others. Further discussion of these features is given in Paper II Linear polarization The same procedure was used to build 3-D images of the linearly polarized emission (Fig. 4 bottom). The most striking feature is that the low pitch angle emission is seen at high latitudes, north and south, like two little rings, but is weaker (absent in the visualization) at medium latitudes. The linearly polarized emission at high latitudes corresponds to mirror regions where the electrons are reflected when the magnetic field strength reaches the critical value; it is there that the electrons spend most time, that the emission is the most intense. We see also the warped, asymmetric shape of the two little rings, as in the main belt. De Pater & Jaffe (1984) first noted maxima at high latitudes in VLA images in total intensity. As seen in Fig. 1, they exist in our 2-D images in total intensity with low contrast, and in linearly polarized radiation with somewhat higher contrast simply because the total dynamic range is smaller. At 13 cm we find that the degree of linear polarization is constant at 0.6 ± 0.1 over most of the image and the brightness temperature is 15 20% higher at the high latitude peaks than at intermediate latitudes, both in total intensity and in linear polarization. The high contrast in the 3-D reconstructions of linear polarization in Fig. 4 arises because effects of different regions along the line of sight are largely eliminated. 4. East-west asymmetry For each of the 18 images obtained at 20 increments in CML, we have taken a slice through the Jovian magnetic equator. These 18 slices were then stacked together to show the eastwest brightness distribution as a function of CML. The result is shown in Fig. 5 for 13 cm, in total and polarized intensity. This new presentation shows the brightness on each side of the belt and how it evolves with Jupiter rotation. The images at 22 cm (not shown) are similar to these except in total intensity where the disk is much less bright than the radiation belts. The east-west asymmetry is very evident in Fig. 5. For example, the bright spot shows up on the east limb at CML 120 ; when it is on the west limb, less than 180 later at CML 270, the same bright spot is distinctly less bright. The figure also shows the E-W extension of the belt and how the radius of peak intensity moves slightly inward and outward as a function of CML Brightness maxima vs. CML We now concentrate on the maximum brightness temperatures in the east and west belts and how they change with CML. Fig. 6 shows the results. The top panel, for 22 cm, shows that the curves for the east and west limbs are quite different, and that the east limb brightness is higher than the west limb brightness except for an 80 range centered on CML 30. The peak brightness of T B 1300 K on the east limb occurs at CML 120 ; the same peak when crossing the west limb at CML 270 has T B 1200 K, about 10% lower. The middle panel, for 13 cm, has all of the same features as at 22 cm. The brightness temperatures are about half as large, as expected. The third panel of Fig. 6 shows the ratio of brightness of the east limb to that of the west limb for each CML. These curves can be compared with the ones reported by de Pater (1983) from Westerbork 1977 observations, and by de Pater & Klein (1989) and de Pater (1991) from VLA observations. The Westerbork observations were made when the Earth s declination as seen from Jupiter was D E =+2.2, and those of the VLA were made when D E = 2.3 to De Pater and Klein (1989) state that there is no obvious differences between the observations from 2.3 to +2.2 ; however, in the CML range there is a small positive bump when D E = 2 that is missing when D E = +2. Later, de Pater (1991) stated that there are differences which she attributed to the viewing geometry. Our observations, made when D E = 3.3 are very similar to those of the VLA at 2.3, with the same positive bump near 320. Our peak at CML 200 is wider due to our lower resolution in longitude.

7 280 Y. Leblanc et al.: The radiation belts of Jupiter at 13 and 22 cm. I Fig. 6. Variation of brightness with CML of regions on the east and west limbs at 22 cm (top panel) and 13 cm (middle panel). The symbols showing the data are larger than the ±10 residuals in the maps; repeatibility using different clean parameters is larger, about 50 K. The bottom panel shows the ratio of brightness, east limb to west limb. The observations were made at D E = East-west asymmetry vs. longitude λ III At this point we emphasize the difference between CML and λ III since the two have not been clearly distinguished in some published papers. CML concerns the longitude of the central meridian with respect to the observer, while λ III is independent of the observer. The consequence is that when an observer at a given CML compares the intensity of the east and west limbs of Jupiter, the comparison is of regions of different λ III. To show the data vs. λ III, we add 90 to the CML for east limb data, and subtract 90 for west limb data, thus converting them to their λ III longitudes. From here on we present our results in λ III, which is helpful in understanding the observations. The peak intensities of the east and west lobes vs. λ III are shown in the top panel of Fig. 7 for 22 cm and in the middle panel for 13 cm. At the two wavelengths the curves are nearly identical. The curves for east limb passage have peaks and valleys near the same λ III as those for west limb passage, but the detailed shapes and brightnesses differ. This presentation shows clearly that each Jovian longitude has a somewhat similar brightness when seen on the west limb as on the east limb, and it makes clear the differences. The curve for east limb passage is brighter than the one for west limb passage in the range λ III = 180 to 360 to 40, while the west limb curve is brighter only in the range 40 to 180. Hence, most of the time the east limb is brighter than the west limb, which accounts for the asymmetry in the rotation-averaged Fig. 7. Same as Fig. 6 except for brightness vs. λ III. map of Fig. 1. The bright spot is observed at λ III 210 on the east limb, while on the west limb it is observed earlier, at λ III 180. This is true at both 22 and 13 cm. We recall that these results concern observations made at D E = 3.3. In the bottom panel of Fig. 7 we show the east-west ratio at 22 and 13 cm as a function of λ III. The two curves are very similar and have a simple, almost sinusoidal form. As will be developed in Paper II, this is mainly the result of the warping of the equatorial magnetic field surface. 5. Discussion and conclusions We have presented Australia Telescope observations of Jupiter at 13 and 22 cm with a resolution of 3 8 at 13 cm and 5 14 at 22 cm. Images averaged over 10 days of observation in July 1995 reveal clearly the two populations of energetic electrons, as first revealed by the VLA (de Pater & Jaffe 1984). The average distance of the peak intensity from the center of Jupiter on the magnetic equator is 1.45 R J at both 22 and 13 cm. The distance to the center measured over 20 years with different radio telescopes has varied from 1.6 R J in 1966 to 1.3 R J in 1974, with the latest measurement at in 1989 being 1.45 R J. Here we show that the position of peak brightness moves slightly inward and outward with the rotation of the planet (Fig. 5). This finding will be developed further in Paper II. In our 10-day averaged images the radiation extends to 4 R J with good signal to noise. In addition, the monotonic decrease of brightness outward from the peak shows that neither Amalthea at 2.5 R J nor Thebe at 3.0 R J produces a significant change in the synchrotron emitting electron population.

8 Y. Leblanc et al.: The radiation belts of Jupiter at 13 and 22 cm. I 281 The warping of the magnetic equator as manifested in the radiation belts around the planet is shown for the first time by a 3-D reconstruction of the belts. The same procedure, when used for the linearly polarized emission, produces 3-D images in which the emission from low pitch angle electrons is most evident at high latitudes, north and south, where the electrons are reflected at their mirror points. We use this information in Paper II to measure the latitude of that region and to estimate the pitch angles. The E-W brightness distribution as a function of CML agrees very well with previous observations that were also made at negative D E. The bright spot shows up on the east limb at CML 120. The same bright spot is on the west limb 150 later, not 180 as we might expect, and is 10% fainter. In terms of λ III it is located at about 210 when on the east limb. This value agrees with the longitude of anomalies reported by Roberts & Komesaroff (1965), Whiteoak, Gardner & Morris (1969), Branson (1968), and Conway & Stannard (1972), indicating that the anomaly is located at the same longitude irrespective of the wavelength or the epoch. From the beginning the anomaly has been attributed to non-dipole terms in the Jovian magnetic field. The observations at 13 and 22 cm are very similar in almost all respects. The ratio of east-to-west limb brightness has a slightly higher amplitude at 13 than at 22 cm, which may be due to the better angular resolution at 13 cm. In linear polarization it has an even higher amplitude. The longitudes where the east limb is brighter than the west limb are independent of wavelength and polarization. De Pater (1981) has also shown that the east-west asymmetry is more pronounced in amplitude in linear polarization, and somewhat stronger at 6 cm than at 20 cm. When the E-W brightness plotted in terms of λ III instead of CML, the curves for east and west limb passage have similar features but are not identical, and the curve of the ratio of eastto-west limb brightness takes on a simple, sinusoid-like form. In conclusion, these high resolution observations of Jupiter at 13 and 22 cm have updated and clarified the character of the radiation belts. In addition to forming images at individual longitudes, we have used novel imaging techniques to generate 10-day average images and cubes with three true spatial axes. A detailed analysis is given in Paper II, explaining how the warping of the magnetic equator has a strong influence on the radio emission and giving further insight into the structure of the magnetic field and the nature of Jupiter s synchrotron radiation. Acknowledgements. The Australia Telescope Compact Array is part of the Australia Telescope National Facility, CSIRO. We thank T. Oosterloo and R.E. 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Hayes eds., Astronomical Data Analysis Software and Systems IV, ASP Conf. Ser., 77, 433 Sault, R.J., Hamaker, J.P., Bregman, J., 1996a, A&A Supl., in press Sault, R.J., Oosterloo T., Dulk, G.A., Leblanc, Y., 1996b, submitted to A&A Whiteoak, J.B., Gardner, F.F., Morris, D., 1969, Ap. Lett., 3, 81 This article was processed by the author using Springer-Verlag TEX A&A macro package version 4.