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

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1. Introduction

The radio synchrotron radiation from Jupiter, produced by relativistic electrons trapped in Jupiter's magnetic field, provides otherwise unobtainable information on the magnetic field and the energy and pitch angle distributions of the electrons. The radiation mechanism is well understood, but the magnetic field at small radii and low latitudes is not well determined, nor are the detailed properties of the electrons.

In Paper I (Dulk et al. 1999) we used a 3-D reconstruction of the radiation belts to derive some properties of the magnetic field. Certain aspects of these derived properties are not in accord with present models of the field, which is not surprising because past observations have provided few constraints at small radii and low latitudes. Future models of the field may benefit from the constraints provided by the synchrotron radiation.

In addition to tracing out Jupiter's magnetic field, our observations permit us to understand how changes in [FORMULA], the declination of the Earth as seen from Jupiter, affects the longitude profile of the magnetic equatorial radiation. This is the subject of this Paper II.

Our observations span the changing [FORMULA] from approximately its maximum southerly value of -2.9o in July 1995 to essentially 0o in May and November 1997. As shown by Leblanc et al. (1997) and Dulk et al. (1997), changes in [FORMULA] strongly affect Jupiter's appearance in a way that depends almost entirely on [FORMULA] and on the warp of the magnetic equator. This is because the synchrotron radiation is beamed in the direction of the relativistic electron motion, and that the major population of electrons with pitch angles [FORMULA] emit most intensely perpendicular to the field, i.e. tangent to the local equatorial surface. Thus an observer as little as two or three degrees off this surface receives a lowered intensity. When [FORMULA] the warp of the magnetic equatorial field produces a pronounced east-west asymmetry, where the brightness of a given location when it traverses the east limb differs from the brightness of the same location when it traverses the west limb, 180o of rotation later. The variation with longitude of this asymmetry depends only on the magnetic field and not on the energy or pitch angle distributions of the relativistic electrons. Hence the asymmetry provides a good diagnostic of the magnetic equatorial field at the small radii and low latitudes where the field strength is about 1.2 G.

In Sect. 2 we describe the observations, where we concentrate on the east-west asymmetry of the magnetic equatorial radiation, and its changes with [FORMULA]. We do not study the radiation from high latitudes on the planet which is due to electrons with smaller pitch angles than are of concern here. In Sec. 3 we compare our observations with predictions of different magnetic field models and show that, while they account qualitatively for the observations, there are discrepancies that shed light on the inadequacies of the field models. Then in Sec. 4 we summarize and conclude.

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© European Southern Observatory (ESO) 1999

Online publication: June 6, 1999