1. Modulation features on the dynamic spectra of the Jovian sporadic DAM emission.
Litvinenko, G. V. (1); Rucker, H.O. (2); Lecacheux, A. (3); Konovalenko, A. A. (1); Vinogradov V.V. (1); Shaposhnikov V.E. (4); Taubenschuss, U. (2)
Institute of Radio Astronomy, Kharkov, Ukraine; (2) Space Research Institute, Graz, Austria;
(3) CNRS & Observatoire de Paris, France; (4) Institute of Applied Physics, Nizhny Novgorod, Russia.
ABSTRACT: Analysis of the large number of the Jovian sporadic decameter emission dynamic spectra allows confidently concluding that practically all observed radiation is in some way modulated. In each specific case the nature of modulation can be different. A mechanism that generates S-bursts can itself create modulation processes as well as the initial emission characteristics can be changed along the propagation path from the source region to the observer (Jupiter magnetosphere, interplanetary medium, Earth ionosphere). Depending on the time and frequency resolution achieved in the experiment and also on the visualization time scale, different features of the radiation spectra appear on the Jovian dynamic spectra. In spite of an extensive investigation of DAM radiation, detailed description and analysis of the modulation events with the different frequency-time scales are not fully available.
In connection to this the following questions are interesting: 1) the statistical set of the modulation events: their dependence or independence on Jupiter – Io – observer position, season time, day-night time, the Solar activity; 2) the strong definition of the observed parameters: sign and value of the frequency drift, lane’s curvature, modulation depth, distances between the nearest lanes and their variety, scale of the modulation; 3) the careful study of the reliability of already proposed mechanisms, i.e. diffraction model of Riihimaa and Imai, Cotton-Muton effect, Faraday rotation of the polarization ellipse, scintillation on the Earth ionosphere disturbance (for instance, on the structure of the Traveling Ionospheric Disturbances) and so on. The major attention in the present work is centered on the different unusual features of the modulation effects observing in the dynamic spectra of the Jovian decameter emission.
INTRODUCTION
Different modulations effects observed on the Jovian S-emission dynamic spectra were discovered and studied by several authors [Riihimaa, 1979; Genova et al., 1981, Imai et al., 1997]. For the first time Riihimaa described the so called “modulation lanes”, groups of lanes drifting in the time-frequency plane. He founded that on the spectra these lanes can have positive or negative sign of drift, and, in some cases, their superposition. Firstly Riihimaa and then Imai proposed an “interference screen” model for the explanation of modulation lanes origin. Genova et al. depicted a new class of modulations, “high frequency lanes”. Moreover, these authors, based on the analysis of big amount of the experimental data, suggested the selection criteria for the modulation events produced either by the Earth’s ionosphere or by the interplanetary medium oscillations. In the work of Ryabov [2001], the evolution of S-bursts fine structure was studied against a background of the multi-scale spectra of Io-depending radio-burst emission. Ryabov [2001] marked a repetition of the self-similar burst images with the different scale on the frequency-time plane. Carr and Reyes [1999], Litvinenko et al., [2004] using the different original procedures of time resolution improvement (till 1 μs) firstly showed that the simple Jovian S-bursts have an internal microstructure, which also presents some kind of DAM emission modulation.
Progress in electronics, computer and information technologies allows to create the effective registration systems with high frequency and temporal resolutions (for example, Digital Signal Processors (DSP), the Waveform Receiver (WFR) and so on). Using these back-ends in connection with large radio telescope the results can be unique. Some improvements can be also reached by applying special mathematical methods. These new technologies give the possibility to found and investigate the specific features of the Jovian emission on the more high quality and quantity levels.
In the present work special attention is paid on the modulation events which can be presented at the Jovian S-emission dynamic spectra in dependence on the time resolution achieved in the experiment and also on the visualization time scale. It is obvious that the dynamic spectra image is changing with the visualization time scale.
The wide-band data taken for the present analysis have been obtained with the high frequency and time resolution equipment of a Digital Spectropolarimeter [Kleewein et al., 1997] and a Waveform Receiver [Leitner and Rucker, 2001] installed into the world’s largest decameter band radio telescope UTR-2 [Konovalenko et al., 2001]. The DSP provides the ultimate spectral analysis capability performing real time digital signal processing. The WFR reaches a maximum theoretically possible time-frequency resolution. The main concept of the waveform receiver is to store the signal’s original waveform. The frequency resolution of DSP is 12.5 kHz, the bandwidth is 12.5 MHz and dynamic range is 70 dB. During the experiments a time resolution was varied from 1ms to 100 ms.
Complex modulation structure of the signals makes the natural difficulties for the data analysis and interpretation. In Fig.1 the consecutive zooms of one dynamic spectrum are presented. An initial spectrum was obtained with UTR-2 and DSP. The spectrum image changes with varying the time interval. It needs to be stressed that a variety of evident different scale modulation effects can be marked even for such simple example.
Fig. 1
SOME OBSERVATIONAL RESULTS
In the Fig.2 and Fig.3 some kind of extraordinary modulation events were presented. In the work of Ryabov, 2001 was mentioned that the long series of S-bursts are concentrated inside spectral bands with width from 1.5 to 5 MHz and a distance between central frequencies of bands in order of 3.5 MHz. Bands can have a positive and also a negative drift. During the experiments with UTR-2 and DSP the similar band structures were obtained. The high sensitive and frequency-time resolution of receiving facilities in our investigation allow to get the more specific information and, in some cases, more complex content. As an example, in Fig.2 the dynamic spectrum with clear expressed two spectral bands, containing the long series of S-bursts, is illustrated. Distance between central frequencies of bands is approx. 6 MHz, and there is not frequency drift of bands. In the spectrum a non-regular frequency modulation in each band is clearly seen. In the same time the strong correlation of large scale events and absolute absence of correlation between small scales effects in the bands is visible. The spectrum presented in the Fig.3 is also interesting due to the uncommonness: the S-burst radiation is modulated in the form of cross-lane structure consisting of spots of ~ 0.4 ms duration. The crossed-lanes demonstrate a positive and negative frequency drift simultaneously, with the faster negative drift reaching 40 MHz/sec and slow positive drift ~ 1.2 MHz/s. In Fig.4 internal structure of simple S-burst obtained by the changing visualization scale is shown. It is clear seen that burst consists of series of millisecond pulses which appear due to the complex modulation processes as in frequency as well as in time. In the Fig. 5 one more specific spectrum which consists frequency drifting consecutive quasi-harmonic structures which exist against a background of classical S-burst emission is shown. Of course, more detail investigation needs to be carried out for demonstration if this phenomenon belongs to the Jovian origin, in particular the experiments in ON-OFF regime or with distant antennas can clarify this question.
Fig.3
Fig.2
Fig.5
Fig.4
CONCLUSION
In the present work the wide-band dynamic spectra of the Jovian decameter emission which have been obtained during last years with the high-frequency and high-time-resolution equipment on the UTR-2 radio telescope have been analyzed. Main attention was paid to the detection and consideration of the different kinds of modulation effects, as known before and also firstly discovered. Several new intriguing kinds of spectrum modulation have been found Without any doubt the theoretical and experimental investigations of these phenomena are necessary to be continued with the largest existing instruments and with the future LOFAR and LWA.
REFERENCES
Riihimaa, J.J., Modulation Lanes in the Dynamic Spectra of Jupiter‘s Decametric Radio Bursts, A&A, 78, L21-L23, 1979.
Genova F., M.G. Aubier, and A. Lecacheux, Modulations in Jovian Decameter Spectra: Propagation Effects in Terrestrial Ionosphere and Jovian Environment, A&A, 104, , 1981.
Imai, K., L. Wang, T.D. Carr, Modeling Jupiter‘s decametric modulation lanes, JGR, 102, , 1997.
Ryabov, B.P., Jovian Decametric Emission. Multiscale Dynamic Spectra, Radiophysics and Radioastronomy, 6, , 2001.
Carr, T.D., and F. Reyes, J.Geophys.Res.-Space, 104, 127, 1999.
Litvinenko, G.V., H.O. Rucker, V.V. Vinogradov, M. Leitner, and V.E. Shaposhnikov, Astron. Astrophys., 426, 343, 2004.
Kleewein, P., C. Rosolen, and A. Lecacheux, New digital spectrometers for ground-based decameter radio astronomy, in: Planetary Radio Emission IV, Eds. H.O. Rucker, S.J. Bauer, and A. Lecacheux, Austrian Academy of Sciences Press, Vienna, 51, 1997.
Leitner, M., and H.O. Rucker, in: Planetary Radio Emissions V, Eds. H. O. Rucker, M. L. Kaiser, Y. Leblanc, Austrian Academy of Sciences Press, Vienna, 91-96, 2001.
Konovalenko, A.A., A. Lecacheux, C. Rosolen, and H.O. Rucker, ibid., 63-76, 2001.