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

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1. Introduction

1.1. Jovian millisecond radio bursts

Jupiter's dynamic spectra exhibit two main classes of structures: the L(long) and S(short) bursts. The most commonly observed are the L-bursts whereas the S-bursts account for a relatively small fraction (10[FORMULA]). S-bursts are characterized by considerably shorter duration between 1 and 50 millisecond and often more intense than the L-bursts. Both bursts (L- and S-bursts) can occur in the same storm where a transition from L- to S-bursts is observed followed by a return to L-bursts (Leblanc et al. 1980; Riihimaa & Carr 1981; Boudjada et al. 1995a). Generally, the occurrence probability of Jovian decametric emissions is depending on two parameters: the central meridian longitude (CML) and the Io-phase ([FORMULA]); one associated to the magnetic field of Jupiter and the other to the relative position of the satellite Io with regard to the observer, respectively. The representation of the emission versus CML and [FORMULA], i.e. the so-called CML-phase diagram, allows to distinguish two high occurrence probability regions of Jovian millisecond radio bursts: Io-B and Io-C sources. In both cases the Io-satellite positions are on the edge of the planet with regard to the observer at 90o for Io-B and 250o for Io-C.

Very few works have been devoted to the analysis of the narrow-band emissions because they are relatively infrequent phenomena. The first description of such emissions was reported by Riihimaa (1968) who found that the narrow-band often exhibits a slow and stable drift in frequency. Later on, receivers with higher resolutions (few tens of milliseconds) allow to find that such emission look like trains of S-bursts with a relatively small total bandwidth (Krausche et al. 1976; Flagg et al. 1976; Riihimaa 1977; Riihimaa 1985). On the other hand, Leblanc & Rubio (1982) reported another type of narrow-band, the so-called "splitting" which appears on wideband dynamic spectra as bands of emission at the higher frequency limit of the emission and associated more particularly to the Io-B region. Boudjada et al. (1995b) described similar "splitting" but in the Io-C region at frequencies lower than 22 MHz. It is important to note that the "splitting" in Io-B and Io-C regions are observed with right-hand and left-hand circular polarizations, respectively.

1.2. The Riihimaa catalogue

Since the discovery of the Jovian millisecond radio bursts, three main catalogues (Ellis 1979; Flagg et al. 1991; Riihimaa 1992) were published. Riihimaa (1991) was the first to report a very interesting classification of S-bursts based mainly on the observations described in his catalogue. The observations were made at Oulu in Finland at a latitude of about 65oN. Using two log-periodic antenna connected to a different types of equipment (a swept-frequency receiver, a multichannel receiver and an acousto-optic spectrograph), Riihimaa reported in his catalogue a summary of the main dynamic spectra based on real-time high-resolution observations of Jovian millisecond radio bursts. The main characteristics of this catalogue are: (a) observations from midnight to the early morning hours local time when the ionospheric electron density was very low, (b) a receiver band frequency from a few megahertz to 12 MHz, (c) and sometimes measurements of the sense of polarization.

1.3. The Riihimaa classification

The Riihimaa classification shows sketches of Jovian millisecond radio bursts such as they appear on dynamic spectra. As displayed in Fig. 1, Riihimaa has associated an alphabetic number to each type of structure to distinguish one S-burst from another. The classification begins with the simple structure type a which appears as a straight line in the dynamic spectra but also contains more complex structures presenting a reversal of the sign of their slopes (with positive and negative drift-rates) like in the case of type f . The analysis of the Riihimaa structure occurrences in S-patterns (Boudjada et al. 1995a) showed that more than 70[FORMULA] of the bursts belong to type a, b, e/f and g/h . The negative drift-rate is found nearly similar of about -31 MHz/s and a constant frequency duration (CFD), on average, equal to 15 ms.

[FIGURE] Fig. 1. Riihimaa classification of individual S-bursts as they appear in the Jovian decametric dynamic spectra. Those structures are got from a catalogue of observations made in Oulu (Finland) from 1974 until 1989. The drift rate is about -25 MHz/s with a frequency bandwidth of about 3 MHz.

A More precise analysis of the Riihimaa structures allows us to consider each individual S-burst to be the composition of three parts which are sometimes all observed (e.g. type q or type n ) and sometimes partially recorded (e.g. type f or type a ).

In Fig. 2 we show two examples of S-burst structures (type q and type l ) which could be considered as complete S-burst structures where the three components are observed. The first part is the upper part of both structures (hereafter R-up) which occurs usually at low frequency. The R-up appears as a straight segment in Fig. 2 for the type q and the type n . The second component is the central part of the structure (hereafter called R-center) for which we note an infinite value of the drift rate (DR) when it changes sign. The R-center appears with different forms with sub-structures for the type q or incomplete for type n . The third part is the down part of the structure (hereafter R-down) which is observed usually at frequency higher than the R-center. For the type q the R-down appears as a straight segment when for the type n it occurs as a curvature with decreasing slope.

[FIGURE] Fig. 2. Example of decomposition of type q and type n into R-up, R-center and R-down parts.

In the following we attempt to describe the relationship between the narrow-band emissions and the new vision of the Riihimaa classification. In the next section, we report on typical examples of narrow-band events selected from the Riihimaa catalogue. The characteristics of the narrow-band and their relationships with the Riihimaa structures are analysed. The confrontation of our investigation with three models proposing an interpretation (Ellis 1965; Calvert 1982; Louarn 1997) allows us to discuss and to conclude in the last section about the origin of the Jovian millisecond radio bursts.

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© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000