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

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4. East-west asymmetry

For each of the 18 images obtained at [FORMULA] increments in CML, we have taken a slice through the Jovian magnetic equator. These 18 slices were then stacked together to show the east-west 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.

[FIGURE] 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 [FORMULA], [FORMULA].

The east-west asymmetry is very evident in Fig. 5. For example, the bright spot shows up on the east limb at [FORMULA] ; when it is on the west limb, less than [FORMULA] later at [FORMULA], 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.

4.1. 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 [FORMULA] range centered on [FORMULA]. The peak brightness of [FORMULA]  K on the east limb occurs at [FORMULA] ; the same peak when crossing the west limb at [FORMULA] has [FORMULA]  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.

[FIGURE] 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 [FORMULA] 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 [FORMULA].

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 [FORMULA], and those of the VLA were made when [FORMULA] to [FORMULA]. De Pater and Klein (1989) state that there is no obvious differences between the observations from - [FORMULA] to [FORMULA] ; however, in the CML range 300- [FORMULA] there is a small positive bump when [FORMULA] that is missing when [FORMULA]. Later, de Pater (1991) stated that there are differences which she attributed to the viewing geometry. Our observations, made when [FORMULA] are very similar to those of the VLA at - [FORMULA], with the same positive bump near [FORMULA]. Our peak at [FORMULA] is wider due to our lower resolution in longitude.

4.2. East-west asymmetry vs. longitude [FORMULA]

At this point we emphasize the difference between CML and [FORMULA] 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 [FORMULA] 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 [FORMULA]. To show the data vs. [FORMULA], we add [FORMULA] to the CML for east limb data, and subtract [FORMULA] for west limb data, thus converting them to their [FORMULA] longitudes. From here on we present our results in [FORMULA], which is helpful in understanding the observations.

The peak intensities of the east and west lobes vs. [FORMULA] 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 [FORMULA] 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.

[FIGURE] Fig. 7. Same as Fig. 6 except for brightness vs. [FORMULA].

The curve for east limb passage is brighter than the one for west limb passage in the range [FORMULA] to [FORMULA] to [FORMULA], while the west limb curve is brighter only in the range 40 to [FORMULA]. Hence, most of the time the east limb is brighter than the west limb, which accounts for the asymmetry in the rotation-averaged map of Fig. 1. The bright spot is observed at [FORMULA] on the east limb, while on the west limb it is observed earlier, at [FORMULA]. This is true at both 22 and 13 cm. We recall that these results concern observations made at [FORMULA].

In the bottom panel of Fig. 7 we show the east-west ratio at 22 and 13 cm as a function of [FORMULA]. 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.

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998