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Astron. Astrophys. 319, 274-281 (1997)

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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 number, energy distribution and pitch angle of the relativistic electrons. There are two populations of electrons, one with pitch angles near [FORMULA] giving radiation concentrated at the magnetic equator, where the intensity is largest near [FORMULA], 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 [FORMULA]. 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 [FORMULA] (about [FORMULA] smaller if converted to System III 1965.0). 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 [FORMULA].

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 found that the match with the observations was not satisfactory, and she invoked an overabundance of electrons in the longitude range [FORMULA]. 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 [FORMULA] to [FORMULA]. 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 three-dimensional 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 [FORMULA] 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.

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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