2.1. The narrow-band events
The main observations reported in our analysis are taken from the Riihimaa catalogue (1992) by selecting typical examples of narrow-band emissions (NB). In Table 1, we give the main observational characteristics for three events recorded in 1987 (22nd Oct.) and in 1988 (12th and 19th Nov.). Those events are associated to Io-B and Io-C sources which are known as regions of high occurrence probability of Jovian millisecond bursts. To take into consideration the observational conditions, we report in Table 1 the observation date, the type of source, the start and end of observation time associated to each event. The narrow-band emissions were recorded by the acousto-optic spectrograph (AOS) in two frequency bands (20-27 MHz and 20-30 MHz) with time and frequency resolutions of about 7 ms and 70 kHz, respectively. For the description of the characteristics of the NB, we should notify here that the selected bands of our study represent only a part of the emission as reported by Riihimaa in his catalogue.
The event of Nov. 12, 1988 : The evolution of the narrow-band is more clearly shown in Fig. 3 where three dynamic spectra associated to the same event were recorded in the frequency range from 20 MHz to 27 MHz. In Fig. 3a, we show a quasi-continuous NB with weak fluctuations and an instantaneous frequency band of about 200 kHz. It is important to note the absence of individual S-bursts and sometimes the presence of a small gap in the narrow-band with a time duration of about tens of milliseconds. Few minutes later and as shown in Fig. 3b, small perturbations in the narrow-band appear with some gaps three times larger than in the previous dynamic spectrum. Those perturbations are named type V because of their shape (described in more detail by Riihimaa & Carr 1981) and they present a frequency bandwidth of about 1.5 MHz, seven times larger than the instantaneous frequency band of the narrow-band. Then the perturbations of the narrow-band generate more type V structures as shown in Fig. 3c where we also note the absence of individual millisecond structures.
The event of Nov. 19, 1988 : The same source (Io-B source) is observed one week later in the region of high occurrence probability of Jovian S-bursts. In this event shown in Fig. 4, the evolution of the narrow-band is associated to the evolution of type V where one component is still connected to the NB whereas the second one disappears. The gap in the narrow-band is found to be larger than 100 milliseconds. In the last part of the event we observe individual structures totally disconnected from the narrow-band, the total frequency bandwidth extends more than 4 MHz when the instantaneous bandwidth is still of the order of 300 kHz.
The event of Oct. 22, 1987 : The dynamic spectrum shown in Fig. 5 presents several discontinuity (gap in time of about 70 ms) where the individual S-bursts are similar to those reported in the Riihimaa classification. The small arrows in Fig. 5 are associated to some typical structure as type g (first arrow), type r (second arrow) and type h (third arrow). It is important to note that the R-center parts of individual S-bursts appear at a nearly constant frequency of about 22.5 MHz with a frequency bandwidth equal to 200 kHz. The R-up and R-down of individual S-bursts present quasi-equal drift-rate of about - 25 MHz/s. The total frequency bandwidth of one individual S-burst is, on average, about 3.5 MHz.
2.2. Temporal evolution steps of the narrow-band emissions
The previous selected events displayed in Fig. 3, Fig. 4 and Fig. 5 are very representative of a general behaviour of the NB for which four steps can be detected in the temporal evolution. In the first step, the NB appears as a continuous band emission with an instantaneous frequency bandwidth of about 200 kHz, and sometimes as discontinuous emissions with a variable gap duration of about 20 to 40 ms. In the second step, perturbations appears in the NB where the type V is the fundamental structure. In the same time the duration of the gaps increases up to more than 80 ms. However the instantaneous narrow-band frequency is still similar to the previous one. In the third step, the number of type V increases, some of them remain connected to the NB while the others have only one component connected to the narrow-band. The frequency bandwidth of the type V is five times bigger than the instantaneous frequency bandwidth of the narrow-band. The gap in the narrow-band is found to be proportional to the frequency band of the type V . It is important to note the absence of millisecond radio bursts in the first previous steps. In the last step the type V is totally disconnected from the NB and finally the Riihimaa structures appear. In this case the drift rates of the structures are found to be similar and the frequency bandwidth increases up to about 3 MHz.
The temporal evolution of the narrow-band generates in the end the Riihimaa structures. The R-center part of the individual S-burst is the remainder of the narrow-band and the R-up and R-down parts are associated to the discontinuity of the NB.
© European Southern Observatory (ESO) 2000
Online publication: December 5, 2000