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

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5. Discussion and conclusions

We have presented Australia Telescope observations of Jupiter at 13 and 22 cm with a resolution of [FORMULA] at 13 cm and [FORMULA] 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  [FORMULA] at both 22 and 13 cm. The distance to the center measured over 20 years with different radio telescopes has varied from 1.6  [FORMULA] in 1966 to 1.3  [FORMULA] in 1974, with the latest measurement at in 1989 being 1.45  [FORMULA]. 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  [FORMULA] with good signal to noise. In addition, the monotonic decrease of brightness outward from the peak shows that neither Amalthea at 2.5  [FORMULA] nor Thebe at 3.0  [FORMULA] produces a significant change in the synchrotron emitting electron population.

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 [FORMULA]. The bright spot shows up on the east limb at [FORMULA]. The same bright spot is on the west limb [FORMULA] later, not [FORMULA] as we might expect, and is 10% fainter. In terms of [FORMULA] it is located at about [FORMULA] 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 [FORMULA] 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 east-to-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.

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998