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

7. Conclusion

For the first time, we provide a statistical study of more than 200 events observed from space by Wind/WAVES experiment and from ground by the Nançay Decameter Array (NDA). We have found that more than 50 of these events are simultaneously observed, particularly after midnight; the occurrence probability around 02 and 04:30 UT is about the same. Our analysis was influenced by two effects: (a) the observation conditions (interference and ionosphere cut-off) were totally different between the spacecraft (quasi-continuous observations of the jovian emission) and the ground radiotelescope (the NDA can track Jupiter during 8 hours around the meridian transit), (b) the common frequency band (10 MHz to 13.8 MHz) was mainly affected by interference. According to the previous effects, it appears that the Wind/Nancay data are complementary. First in term of time coverage: after midday priority must be given to the spacecraft observations. The man-made interference cause saturation of the instruments, except for the spectro-numeric receiver (Kleewein et al. 1997), not used as routine receiver. The detection of a jovian signal falls to 4 of occurrence in Nançay for f 15 MHz and 12 for f 15 MHz. The frequency band 10-40 MHz is also affected: the transparency of the ionosphere is getting worse at the end of the day. The equality of the detection occurrences in the early morning indicates that the terrestrial equipment plays a determining role in the astrophysical studies. In particular the NDA due to its characteristics has a sensitivity about 100 times better than Wind/WAVES experiment. The sophisticated receivers that we can connect to the NDA requires computing power and enormous amount of data storage, not comparable with the space experiment constraints. The Wind/Nancay data are also complementary with respect to the frequency. The 4 MHz common to both experiments makes it possible to study in the whole decameter range the phenomenology of the jovian events. We found 50 of common events between 0 and 6 UT. It is thus possible to recreate tens of complete arcs as did Lecacheux et al. (1998).

In the Io-phase-CML diagram, the Wind/WAVES events are associated to controlled and not controlled emissions and it is not possible to separate between them according to their occurrence area. From NDA events, we report mainly the emissions related to Io-controlled occurrence area contrary to Wind/WAVES emissions. With regard to previous studies, we have found the absence of the great arc with right-hand polarization associated to Io-C regions. One could explain this disappearance by the Jovicentric declination of the Earth effect which allows to observe more the Southern hemisphere than the Northern hemisphere of Jupiter. If this assumption is true the other emissions (Io-A, Io-B and not controlled emissions with right-hand polarizations) should also be not visible from the Earth. The studies of the arc shapes (Lecacheux et al. 1998) and the emission occurrences reported in our analysis seem to be incoherent with the hollow cone model (Dulk 1965) and the polarization associated to each hemisphere.

In the future we will extend our analysis to the observations made in 1997 and 1998 by the Nançay radiotelescope and Wind spacecraft. From 1995 to 1998, the Jovicentric declination of the Earth changes from -3.0^o to +1.5^o which should allow a better knowledge of the occurrence probability in the Io-phase CML diagram in particular in the frequency range from 1 MHz to 40 MHz. The complementarity of space and ground observations should be the answer to avoid the Earth observation conditions in particular due to man made interference and the diurnal Earth effect. The high sensitivity of the NDA partly compensate the absorption produced by the ionosphere, since the occurrence is about the same order in the middle of the night.

© European Southern Observatory (ESO) 2000

Online publication: February 25, 2000
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