which describes a variation of wave frequency ω(t) in a geometric-optic approximation [4]. Here n(ω) is the refractive index of the medium, is the vector of the group velocity of the wave. In Figure 5 the result of laboratory experiment modeling the wave propagation through a nonstationary medium is shown. In this experiment the initially quasi-monochromatic wave propagated in a magnetized plasma with strong frequency dispersion and periodic disturbance of the magnetic field. It is seen in Figure 5 that the initially continuous quasi-monochromatic signal was modulated both in frequency (Figures 5d-f) and in amplitude (Figures 5a-c,f). The magnitude of the frequency deviation increases as the frequency dispersion increases. Fig. 3. Train of pulses with two different periods [3]. Observations. The fine structure of the dynamic spectra containing narrowband decametric radio emission is very diversified. This emission can appear as S-bursts which have usually the negative frequency drift and a small frequency interval f occupied by the burst,  f/f ~10 −2 − 10 −1. The pulses of such S emission often form quasiperiodic trains with one or several repetition frequencies in a range of about 2 to 400 kHz in the dynamic spectrum. An example of the dynamic spectrum with a train of S-bursts observed in July 6, 1999 (Kharkov, Ukraine) with using radio telescope UTR-2 (Ukrainian T-shape Radio telescope) is shown in Figure 1 [1]. Riihimaa [2] discovered one more type of decametric The time frequency structure of Jovian narrowband decametric radio emission as a probe of the ionosphere of Jupiter V. Shaposhnikov 1, S. Korobkov 1, M. Guschin 1, A. Kostrov 1, H. Rucker 2, and G. Litvinenko 3 (1) Institute of Applied Physics RAS, Nizhny Novgorod, Russia. (2) Space Research Institute AAS, Graz, Austria, (3) Institute of Radio Astronomy, Kharkov, Ukraine 1.Litvinenko G. et al. Astron. Astrophys. 426, 343, Riihimaa J.J. Wide-range high resolution S-burts spectra. Univ. of Oulu. Finland, Krausche D. et al. Icarus, 29, 463, Stepanov N.S. Radiophys. Quantum Electr., 12, 227, Zaitsev et al. Astron. Astrophys. 169, 345, Hinson et al. Geophys. Res. Lett. 24, 2107, EPSC Fig. 1. Dynamic spectrum with a train of S bursts [1]. Aim of report. In this report we would like to point out that the narrow band emission can a source of information about the conditions in the Jovian ionosphere. Using of this radiation for the above task is interesting because the generation mechanism of the NB radiation significantly different from generation mechanism for ionospheric IR and UV radiation, and this fact can be used to independently estimate the Jovian ionosphere conditions. References radiation, which he named narrow-band L-emission. Similar type of the emission was observed by Flagg et al. [3]. Authors named such type of emission as a narrow-band (NB) emission. The mention above types of emissions are quasi- continuous and have narrow frequency spectra. A characteristic feature of the these narrow-band emissions is time variation of dynamic spectra. Fluctuations of the frequency of the emission band appear with the time and sometimes the narrow-band emission transforms into the train of S-bursts, and vice versa, as the noise storm develops (Figure 2). Fig. 2. Dynamic spectra of the NB emission recorded in November 19, 1988 [2]. It is important to mention that the transformation of the continuous emission into a train of S- bursts can be accompanied by a significant increasing of the frequency range occupied by the emission (up to about 3 MHz), and the appearance of a frequency drift characteristic of S-bursts. More complex dynamic spectra of the narrowband emission ware described in [3]. The dynamic spectra appear as quasi periodic train of pulses with two different periods. An example of the spectra is shown in Figure 3. An interesting example of the dynamic spectrum of narrow band emission was presented in [1]. It is shown in Figure 4. In this spectrum, the NB emission shows a quasi-harmonic oscillation of the frequency band with a characteristic frequency. Besides, one more characteristic time scale of repetition is seen. The oscillation gradually drifts towards the lower frequencies. Fig. 4. The narrow band emission with an oscillating frequency spectrum [1]. Theoretical grounds. We believe that the narrow band emission is generated in the Jovian ionosphere or/and in the low magnetosphere and the formation of S-bursts and other fine structures of narrow band emission dynamic spectra is result, in particular, the propagation of the radiation in the ionosphere and low magnetosphere of the planet. The point is that the propagation of waves in a medium with nonstationary variation of parameters (planetary magnetic field disturbance, for example) can be accompanied by the signal amplitude-frequency modulation and the broadening of its frequency spectrum. It is easy see from the equation Fig. 5. Dynamic spectra of the initially continuous quasi-monochromatic wave after the passage across the region with periodic disturbance of the magnetic field. Numerical experiment modeling narrowband radiation propagation in the Jovian ionosphere. Following [5], we assumed that the source of the narrowband emission is located in the Jovian ionosphere, near the maximum of the ionospheric plasma. From equation (1) it is seen that strong frequency dispersion is needed to get a significant variation in the dynamic spectrum. Strong frequency dispersion can be expected only in the ionosphere of the planet. Outside the ionosphere, the refractive index of the extraordinary electromagnetic wave closes to unity and only weakly depends on frequency. The dynamic spectra of the radiation were calculated at the point corresponding to the point of the radiation on the boundary of the Jovian ionosphere. For calculation we assume that the ionospheric peak electron density is N 0 =2x10 4 cm -3, and ionospheric thickness is H=1000 km [6]. Nonstationarity of the ionosphere is modeled by a variation of planetary magnetic field. Calculated dynamic spectra. Below we present examples of calculated dynamic spectra of the initial continuous quasi-monochromatic extraordinary electromagnetic wave at output of the Jovian ionosphere, obtained for the different conditions of generation and propagation. is the cutoff frequency of the extraordinary wave is the emission frequency, L i is the emission path in the ionosphere Conclusion. From the presented examples we see that initial continuous quasi-monochromatic extraordinary electromagnetic wave generated in the Jovian ionosphere at frequencies close to the cutoff frequency demonstrates at place of observation different forms of fine structure of dynamic spectra. These forms depend on generation and propagation conditions in the Jovian ionosphere. Therefore we have opportunity to get an information on conditions in the ionosphere. For example, information about low frequency waves in the ionosphere of Jupiter.