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Astron. Astrophys. 354, 1101-1109 (2000)

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6. Discussion

We analyse the jovian decameter emissions observed from ground and space by the Nancay Decameter Array (NDA) and the Wind/WAVES experiment. For the considered period, March 21 to July 08, 1995 and April 24 to August 12, 1996, the maximum values of the occurrence probability from ground and space are comparable but the NDA sensitivity is found to be affected by the observing conditions in the lower frequency ranges. It is known that the observation conditions, on ground are modulated by the interference and ionosphere conditions in particular late in the afternoon and in the evening (17 UT to 24 UT). Such observational conditions were discussed by several authors (see Genova et al. 1987 and references therein) who showed the statistical occurrence bias due to the Earth's diurnal or annual rotation and by beats between these periods and the emission periodicities. From our analysis the occurrence probability at the NDA presents same features as reported by Genova et al. (1987) in their Fig. 77 with a maximum around 04 UT. For Wind/WAVES and NDA, the maximum occurrence of detection is found to be about the same order (38[FORMULA]) in the morning between 3 and 5 UT. In the afternoon the efficiencies of the two receivers are very different. It reaches 30[FORMULA] for Wind/WAVES but it is only a few percents of the NDA (less than 4[FORMULA] at frequencies lower than 15 MHz, and less than 12[FORMULA] for higher frequencies).

It is known that the occurrence area of sources in the Io-phase CML diagram are depending on two parameters: the Jovicentric declination of the Earth and the frequency of observation. Several authors had reported the Jovicentric declination of the Earth effect deduced from a long term observations of one jovian revolution (Boudjada & Leblanc 1992 and references therein). It appears that the change in occurrence probability could be explained as an effect of beaming of the escaping radiation varying with the Jovicentric declination of the Earth between -3.4o to +3.3o over Jupiter's 11.9 year solar revolution period. The weak occurrence probability of Io-A and Io-B sources as we find in our study is mainly due to the small values of the Jovicentric declination of the Earth which changed from -3.0o to -1.8o for the periods considered in our analysis. In previous studies the occurrence probability in the Io-phase CML diagram for Io-A and Io-B sources are found to be more higher; e.g. when the Jovicentric declination of the Earth was about +1o to 0o as reported in Figs. (4), (5) and (6) of Leblanc et al. (1981).

Otherwise we have found two maxima of occurrence probability at Io-phase equal to 95o and 250o. The same maxima were reported by Kaiser & Garcia (1997) using the Wind/WAVES data selecting the most intense events and they associated them to Io-D and Io-C controlled sources. They also showed that such sources are emitted from the Southern hemisphere which is more visible because of the Jovicentric declination of the Earth. From our analysis these maxima are due to the superposition of controlled and not controlled emissions in particular at Io-phase equal 90o and 250o. For the first maximum ([FORMULA] = 90o) we distinguish three parts: first one when the CML is in the range 80o to 180o, the second between 170o and 360o and the last one for CML from 340o to 180o. Such distribution in CML is reported by Alexander et al. (1981) and Boischot et al. (1981) based on Voyager observations. The first and the third parts are associated to controlled emissions, Io-B and Io-D, respectively, when the second one is related to not controlled emissions, (non-Io-A and non-Io-C). Using the polarization measurements of Nançay data, it clearly appears that the Io-B source with right-hand polarization is shifted towards bigger values of Io-phase and covers part of Io-D source which has left-hand polarizations. In the same time, the second part, non-Io-A and non-Io-C are both observed with right-hand polarizations. For the second maximum ([FORMULA] = 250o) nearly the same distribution in CML is found but with different type of emissions for each part. Thus the second one (170o to 360o) is related to Io-A (with weak occurrence) and Io-C and the third part (340o to 180o) is associated to Io-C region which is followed by non-Io-B area. The first part (80o to 180o) includes the non-Io controlled emission (non-Io-B). On the other hand, from the NDA observations, the same distribution in polarization is kept as for the first maximum. Such distribution in CML with the corresponding polarization shows that in spite of the decrease of the occurrence probability due to the Jovicentric declination of the Earth effect, we find again the same distribution as reported from Voyager data.

For the period considered in our study two Io controlled sources, Io-C and Io-D, are recorded with left-hand polarizations by the Nançay spectropolarimeter. Considering the hollow cone beam model (Dulk 1965) one finds that both sources are localized in the Southern hemisphere as derived also from Wind/WAVES data (Kaiser & Garcia 1997). According to more recent investigations based on composite Nançay-Wind dynamic spectra, Lecacheux et al. (1998) found significant discrepancies between the arc shapes associated to Io controlled emissions (Io-B, Io-C and Io-D) and the opening angle derived from the hollow cone model. The authors reported that such difficulty is observed at high and low frequencies which could be not associated to an inadequate magnetic field model. On the other hand the left-hand polarization emissions appear at Io-phase 90o and 270o when the right-hand ones are limited to CML range 180o to 300o. Our result is similar to those reported from ground observations (see Fig. (2) of Boudjada & Genova 1991). The main difference we have noted is the quasi-absence of the great arc of Io-C source which has right-hand polarization. Boudjada et al. (1995) have shown that the great arcs are related to other arcs with the same shapes (VLA) but opposite polarization. Such association was interpreted as an effect of source located in the same hemisphere but with opposite polarizations which is not in agreement with the hollow cone model. According to this model, the Io-controlled (e.g. Io-A and Io-C sources) and non-Io-controlled emissions are related to the Northern hemisphere because of their right-hand polarizations. However for the period considered in our analysis only Io-A source and non-Io-controlled emissions are observed but not Io-C source with right-hand polarizations. This result could be interpreted as a beam effect where the Northern hemisphere is less visible from the Earth because of the low Jovicentric declination of the Earth. If this assumption is correct, the non-Io-controlled emissions should also not be visible from the Earth because of the very high frequency of Io-C source which is usually higher than the frequency associated to non-Io-controlled emissions (see Fig. (3a) and (3b) of Genova & Aubier 1987). There is still only one explanation when considering that the Io-C emissions with right-hand polarizations come from the Southern hemisphere, from regions where the jovian latitude is smaller (than those of Io-D and Io-C with left-hand polarizations). In this case the hollow cone associated to Io-C with RH polarization should be outside from the ecliptic plane which makes it not visible from the Earth.

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Online publication: February 25, 2000