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Astron. Astrophys. 347, 1039-1045 (1999)

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4. Discussion and conclusion

We have considered observations of Jupiter's synchrotron radiation belts over a period of several years while the Earth's declination [FORMULA] changed from near its maximum southerly value of -2.9o to near 0o. We have concentrated on the component of the radiation that outlines the magnetic equator, and particularly on the locus of brightness maxima that follows the magnetic equator corresponding along a fixed value of B of approximately 1.2 G. We have demonstrated that, when [FORMULA] is only 1 or 2o away from zero, there is an east-west asymmetry: the brightness of a given location when it traverses the east limb is distinctly different from when it traverses the west limb, 180o of rotation later.

The investigation of this east-west asymmetry and its changes with [FORMULA] has led to an understanding of the form of the magnetic equator and its warp as described by the magnetic declination [FORMULA]. With the 1995 ATCA data and knowledge of [FORMULA], Dulk et al. (1997) were able to predict that the brightness distribution of the equatorial radiation would change with [FORMULA] as found here, and that the east-west asymmetry would tend to zero as [FORMULA] approaches zero and then reverse as [FORMULA] becomes positive. Similarly the longitude [FORMULA] of maximum brightness, which was different by about 30o for the two limbs at [FORMULA], was predicted to become the same at [FORMULA] and then to separate in the opposite sense as [FORMULA] becomes positive.

Comparisons with models of the magnetic field leave little doubt that the radius, latitude and brightness of the locus of maximum intensity is due to relativistic electrons with pitch angle [FORMULA], and its variation with longitude depends entirely, or almost entirely, on the magnetic field of Jupiter, and not on the energy distribution of the relativistic electrons.

At the present time the Earth is moving to the north of Jupiter's rotational equator, so [FORMULA] is becoming positive. The east-west asymmetry should reappear, but in the opposite sense, i.e. at most longitudes [FORMULA] the brightness at west limb passage will be higher than at east limb passage. For the magnetic equatorial component of synchrotron radiation, it is now understandable why, according to an oft-quoted dictum, "With Jupiter, [FORMULA] explains all".

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© European Southern Observatory (ESO) 1999

Online publication: June 6, 1999