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Astron. Astrophys. 363, 316-322 (2000)

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3. Discussion

The study of the temporal evolution of Jovian narrow-band events allows us to explain how the Riihimaa structures are related to the narrow-band. The analysis of temporal variations of the narrow-band has shown that the S-bursts seem to be sub-structures generated inside a more global structure, i.e. the narrow-band. This new approach is different from the previous ones which consider that the narrow-band looks like trains of S-bursts (Carr et al. 1983) which is not usually the case as we show from the selected events of our analysis. In general, the narrow-band and the individual structure (also so-called sub-structure or fine structure) are considered to be two different types of emissions. In the previous investigations of Jovian millisecond radio bursts there is no model which reported temporal variation of the narrow-band involving the generation of inherent sub-structures. However, few models attempt to describe how the narrow-band or the sub-structures are generated. In the framework of our study we discuss in the following the classic model (Ellis 1965) and two other models which seem to account for a greater number of observational characteristics of the narrow-band as reported in our analysis. The feedback model (Calvert 1982) and the filamentary source model (Louarn 1997) attempted to explain the discrete spectrum of auroral kilometric radiation (AKR) observed by ISEE-1 (Gurnett et al. 1979) and to clarify the small scale structure of the AKR source regions detected by Viking (Bahnsen et al. 1985), respectively. It is important to note that both models could not explain how the sub-structures are generated in the narrow-band. However it is interesting to discuss the similarity, when it exists, between the physical parameters derived from each model and the main features of our analysis, i.e. the narrow-band emission, the individual S-bursts and their corresponding drift-rates.

3.1. Ellis model

Since the discovery of the Jovian millisecond radio bursts several studies have been devoted to the analysis and the interpretation of the negative drift rates of the S-bursts (Boudjada et al. 1996 and references therein). The classic model (Ellis 1965) considers that the observed negative drift rates are due to the adiabatic motion of electrons along magnetic field lines in the Jovian magnetosphere and radiating when they stream outwards from their mirror point. Contrary to previous studies, the recent investigations (Boudjada et al. 1997; Galopeau et al. 1999) show that the instantaneous drift rate of one individual S-burst cannot be fitted by fixing the initial pitch angle and the electron speed as in the case of the Ellis model. According to our results, the S-bursts drift rates seem to be related to a mechanism inherent and totally depending on the temporal variation of the narrow-band.

3.2. Feedback model

For the feedback model (Calvert 1982) the sub-structures are due to self-excited wave oscillations generating inside the source spontaneous emissions without any external excitation; such a model is comparable to an optical laser oscillator. The discrete components (so-called in the feedback model) could be compared to the individual S-bursts reported in the Jovian dynamic spectra. The frequency bandwidth of the discrete components (associated to AKR emissions) is about 20 kHz, at least 10 times smaller than the same one in the case of Jupiter. On the other hand, the time scale of the discrete components and the generation of such structures inside the source make this model interesting to explain the Jovian millisecond radio bursts. The drift rate is associated to the changing product of the source width and refractive index. However, this model does not take into consideration the presence of the fundamental narrow-band as observed in the studied events.

3.3. Filamentary model

In the case of the filamentary model (Louarn 1997), the author considers that spectral structures, e.g. narrow-band structures and spectral fine structures, are related to the spatio-temporal organization of the emissions source with regard to the observer. It is the case of the Jovian narrow-band emission which is observed for specific geometric configurations between the observer (i.e. the Earth), the position of Io and the Jovian magnetic field. When the magnetic footprint of Io is observed on the edge of the planetary disk, i.e. [FORMULA]= 90o and [FORMULA]= 250o, the occurrence probability of the narrow-band and also the millisecond radio bursts is important. However it seems from our analysis that the narrow-band features because of the short time scale, in particular the gap (about few tens of milliseconds) and the individual S-burst duration (about few hundred of milliseconds), are intrinsic to the source contrary to the filamentary model predictions. Nevertheless the occurrence probability is totally modulated by the geometric conditions.

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© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000