Iizima, M.

Konno, S.

Nakajo, T.

Ono, T.

Oya, H.

Oya, M.

Elektronisches Publikationsportal der Österreichischen Akademie der Wissenschaften

DEVELOPMENT OF AN ARRAY ANTENNASYSTEM AND A MULTI–FREQUENCYINTERFEROMETER NETWORK FOR THEJOVIAN DECAMETRIC RADIATIONM. Oya∗, T. Nakajo∗, S. Konno∗, T. Ono∗, M. Iizima∗, and H. Oya†AbstractTo understand the energy source and the radiation process of Jovian DecameterRadiations (DAMs) being related to the Jovian auroras, ionosphere–magnetospherecouplings and interactions with satellite Io, it is important to obtain the informationon the source location and the polarization of DAMs. The array antenna systemconsists of 9 antennas covering a frequency range from 20 MHz to 30 MHz. Thenew long baseline interferometer system employs the multi–frequency interferometermethod by which ionosphere scintillation effect can be largely reduced.1 IntroductionJovian decametric radiations (DAMs) are understood to be generated in the Jovian polarionosphere being associated with the electromagnetic processes in the Jovian magneto-sphere. In spite of the long history of DAM studies, generation mechanisms of DAMs arestill unknown. The polarization and source location are, then, fundamental informationin the studies on the generation processes of DAM.To identify the source location of DAM directly, in early phase of DAM studies in 1960’sand 1970’s, several experiments by using the long baseline interferometer observations hadbeen carried out by Sydney group, Florida group and Colorado group [Carr et al., 1965;Dulk, 1970; Lynch et al., 1976; Phillips et al., 1987]. Carr et al. [1965] have carried out theinterferometric observation using 55.3 km baseline at Florida at the frequency of 18 MHz.Dulk [1970] also carried out the very long baseline interferometer (VLBI) observationusing baselines more than 1000 km spacing; in their works, the interferometer analyses ofDAM signals have been made by using the tape recorder data. They had obtained a highcorrelation by using the interferometer with more than 1000 km baseline and showed that∗Geophysical Institute, Tohoku University, Sendai 980–8578, Japan†Fukui University of Technology, Fukui, Japan12 M. Oya et al.(a) (b)Figure 1: The long base line interferometer system (a) and the array antenna system (b) ofTohoku University. The array antenna system is located at Iitate station, which is also one ofthe four stations of the interferometer system.source size was less than 0.1 arc sec. However, no fixed source position can be identifiedbecause of the fast fringe change due to the ionospheric scintillation effects. Lynch et al.[1976] also observed Jovian S–burst using 6980 km baseline interferometer whose antennapair was extended between the north and the south American continents, and concludedthat the source size of the S–burst is as small as 0.05 arcsec. In their analyses, no fixedposition could be identified on the Jovian disk. Phillips et al. [1987] have attemptednew observation of DAM event at 18 MHz with 46 km cross–polarized interferometerwhich is consisted of two stations: one station is arranged to observe RH (right–handed)polarization, and the other is arranged to observe LH (left–handed) polarization. Theythought that DAM waves are elliptically polarized and the RH and LH emissions havesome coherency with each other. They concluded that in the later case two or moresources were active simultaneously. In the multiple source case, they also analyzed thescintillation index of detected power for RH and LH emissions. They pointed out thattwo sources for LH and RH emissions have different source sizes, and the LH sources arelocated in both hemispheres simultaneously, while the RH emission sources are located inthe northern hemisphere.In Tohoku University, interferometer observations using 100 km baselines have been devel-oped [Tokumaru, 1985; Murao, 1995; Misawa, 1997]. In these works, they try to measurethe source positions and motions in the Jovian disk by analyzing the interferometer data.Although there were some cases where they could detect the source movement betweenthe north and the south hemisphere of Jupiter, they have also pointed out that terrestrialionospheric scintillation effects are important for the interferometer observations of DAMsignals [Tokumaru, 1985; Murao, 1995]. Therefore, to identify the precise source loca-tions, it has been required to establish a method to eliminate the terrestrial ionosphericDevelopment of an array antenna system and an interferometer for DAM 3XYHybrid CircitRHLHABC IRH  PC(A/D)A ant.LHABCIABCIFrequencyControler1st. localphasecontroleradditionaddition3sec sweep22mH22mH22mH0.1µ0.1µ0.1µ0.1µ2k2kPHASE CONTROL VOLTAGETO SUBTRACTIONPrinciple of pi/2 controller mainampbackampFigure 2: Block diagram and principle of the voltage controlled phase shifter applied in theinterferometer.effects.To identify the source locations and generation processes of DAM, we have developed anew array antenna system for a sensitive polarimeter and a new long baseline interferom-eter system designed by using multi–frequency interferometer method by which we canlargely reduce the effect of fluctuations in the terrestrial ionosphere.2 Array antenna systemThe new array antenna system is installed at Iitate observation station of Tohoku Univer-sity in Fukusima prefecture, Japan. The array antenna consists of 9 log–periodic antennaswith spacing from 15 m to 150 m (Figure 1(b)). Observation frequency range is set from20 to 40 MHz for both RH and LH polarizations. The front–end of the observation systemconsists of orthogonal cross log–periodic antenna, a hybrid circuit for dividing receivedDAMs into RH and LH polarization signals, 20 MHz–40 MHz BPF (Band Pass Filter),and low noise wide–band preamplifiers with NF (Noise Figure) of 1 dB. After separationof the polarization (RH and LH), the signals are fed to 18 three–stage super heterodyne4 M. Oya et al.single log-periodic antenna          (E component)Array Antenna(Beam width 6deg)0-3-10dB180O90O0O0-3-10dB180O90O0OFigure 3: Beam Patterns of a single log–periodic antenna and the array antenna.receivers. Figure 2 shows the block diagram of one element of this array antenna system.These receivers act as spectral analyzers by sweeping the 1st local signals with a frequencystep of 100 kHz and a sweep time of 3 s. All of the local signals used in 18 receivers aregenerated by a signal oscillator, which is divided for all receivers. In this array antennasystem, the phase control of the received signals is carried out at each frequency of theobservations by calculating the phase difference of the observed signals from the sourceat the moment. The phase control of the receivers has been made in the intermediatefrequency stage (455 kHz) by phase control bridge circuit. The phase control bridge cir-cuit includes voltage controlled capacitances the control voltage for which is fed from themicrocomputer (see Figure 2 for principle) [Oya et al., 1999]. As shown in Figure 3, thearray antenna has the main beam with a size of 6 degrees, which is one tenth of the size ofthe main beam of a single log–periodic antenna. In Figure 4 we show one example of thefirst observations of DAM emissions. As shown in this figure, the array antenna system issensitive and can observe detailed features of the dynamic spectra of DAM. However, thefirst observation shown in Figure 4 was carried out by using only 6 antennas, therefore,the corresponding gain of this observation is about 8 dB, compared with the observationwith a single antenna.3 Multi–frequency interferometer networkThe new long base–line interferometer system consists of four stations which are locatedin Miyagi and Fukusima prefecture (see Figure 1(a)) where we have 6 combinations ofbase lines ranging from 45 km to 115 km. At each observation station, an log–periodicantenna for 20–30 MHz range and Cs (cesium) vapor frequency standard are installed.Development of an array antenna system and an interferometer for DAM 5First Observation (Io-B)10:30 11:00 11:30 12:00 12:304035302540353025MHzMHz(a)Array Antenna(6 antennas)(b)Single Antenna (RH)(RH)0246dB2001/1/9(UT)Figure 4: Comparison between observations of the array antenna (a) and the single antenna (b).This observation of the array antenna was carried out by using only 6 antennas.The signals from four stations are transmitted directly to the central station at Sendaithrough the telemetry channels.For a long base–line interferometer, the effect of fluctuations in the terrestrial ionospherebecomes important for the decameter wave length range. Observed phase difference be-tween two stations (see Figure 5(a)) are given by∆φ =2pifcL cos θ −Kf∆NTEC (1)K = const.,where ∆φ and L represent observed phase data difference at frequency f and a base linedistance, respectively. ∆NTEC is a total electron content along the ray path of the DAMsignals. The first term and the second term of Eq. (1) represent the direction of the sourcelocation and the effect of fluctuations in the terrestrial ionosphere, respectively. To reduce6 M. Oya et al.LθA BAB1    2    3                           1    2    3                           Bandwidth 10 kHzEEEEEEEEEEX fA ( )X fB( )FrequencyFrequency5HzInterference(a) (b)Figure 5: Methods of the new long–base line interferometer system.Pre.Amp.CrossLog Periodic AntennaAAPre.Amp.DDS Local 1'DDS Local 2DDS Local 3DDS Local 1Pre.Amp.10.7MHzBW=50kHz455kHzBW=8.4kHz10kHzBW=10kHz A/D50kHz samplingA/D50kHz samplingFrequency Srandard(10MHz)465KHz10.245MHzLocal 1: 30.7-40.7MHzF1F2Figure 6: Block Diagram of the new long–base line interferometer system.Development of an array antenna system and an interferometer for DAM 7the effect of fluctuations in the terrestrial ionosphere, the double frequency interferometrywith multi sub–band method is introduced to the new long base line interferometer system.3.1 Double frequency interferometryTo reduce the effect of fluctuations in the terrestrial ionosphere, we apply double fre-quency interferometry which is used in VLBI to our long base line interferometer system.Observed phase differences between two stations by using double frequency interferometryare given by∆φ1 =2pif1cL cos θ − Kf1∆NTEC∆φ2 =2pif2cL cos θ − Kf2∆NTEC(2)K = const.,where ∆φ1 and ∆φ2 are observed phase data difference at frequencies f1 and f2, respec-tively. We assume that the difference of these two ray paths at f1 and f2 can be neglected.Then, we can determine the effect of ∆NTEC and θ in Equation (2).In this method, however, it is required that the DAM events are observed with largesignal–to–noise (S/N) ratio, because the difference between two observation frequenciesshould small for the assumption of the same ray path in the ionosphere. Therefore, toimprove the S/N ratio, we further employ the multi sub–band method.3.2 Multi sub–band methodTo achieve enough of S/N ratios for observed DAM signals, the bandwidth of the observa-tion system needs to be wide. However, when the bandwidth become wide, interferometrybecomes difficult. To solve this problem, we use multi sub–band method, shown in Fig-ure 5(b), where the observed data with a wide bandwidth is divided into multi sub band,and the observed interferometer data is analyzed in each sub band. This interferometryprocessing is given by the following equationC =∫XA(f)X∗B(f) exp[−i2pifcL cos θJC ]df, (3)where XA(f) and XB(f) are observed data at station A and B, respectively (see Fig-ure 5(b)), and θJC is the direction of Jovian center. The subtraction of fringe phase ofJovian center (−i 2pifcL cos θJC) enables the interferometry of wide bandwidth. The realdata processing is carried out each sub band of 5 Hz (see Figure 5(b)).Figure 6 shows the block diagram of the new long–base line interferometer system em-ploying the double frequency interferometer method and the multi sub–band method.8 M. Oya et al.4 ConclusionWe have developed two new observation systems for DAM, i.e., the array antenna systemand the long base line interferometer, to understand the energy source and the radiationprocess of DAMs. The new array antenna system consists of 9 log–periodic antennaswith spacing from 15 m to 150 m, which can observe RH and LH polarizations with highsensitivity. The corresponding gain of the array antenna is designed to be about 10 dBwhen we use the full set of 9 antennas. The new long base line interferometer systemconsists of 4 stations with spacing from 44 km to 116 km by using double frequencyinterferometry and multi sub–band method by which we can largely reduce the effectof fluctuations in the terrestrial ionosphere. We have estimated that it is possible todetermine the source position (North or South) for the condition when the fluctuationof ∆NTEC is less than 5.0 × 1015 [el/m2] by using a simulation study considering theray paths of DAMs. In addition, we are now developing a new monitoring method of∆NTEC for the cases of larger amplitude disturbances by using TEC data from the GPSobservation network in Japan.ReferencesCarr, T. D., S. Gulkis, A. G. Smith, H. Bollhagen, N. F. Six, Jr., and N. E. Chatterton,Recent decameter–wavelength observations of Jupiter, Radio, Sci., 69, 1530–1536,1965.Dulk, G. A., Characteristics of Jupiter’s decametric radio source measured with arcsecondresolution, Astrophys. J., 150, 117, 1970.Lynch, M. A., T. D. Carr, and J. May, VLBI measurements of Jovian S–bursts, Astrophys.J., 207, 325, 1976.Misawa, H., Studies on the origin of Io–related Jovian decametric radiations, Ph. D.thesis Tohoku University, 1997.Murao, H., Identification of the Jovian decameter wave radiation sources using two–frequency interferometer network system, Ph. D. thesis Tohoku University, 1995.Oya, H., and M. Iizima, Cluster of super massive black holes in the central region of ourgalaxy observed by decameter radio wave pulses: - Discovery of 24 super massiveblack holes and their motions, Sci. Rep. Tohoku Univ., Ser. 5, 32, 1–78, 1999.Phillips, J. A., T. D. Carr, W. B. Greenman, and J. Levy, Cross–polarized interferometryof a Jovian decametric radio storm, J. Geophys. Res., 92, 9971–9977, 1987.Tokumaru, T., Studies on the source location of Jovian decametric radiation using 1 kmand 75 km interferometer, Ph. D. thesis Tohoku University, 1985.

Development of an Array Antenna System and a Mulit-frequency Interferometer Network for the Jovian Decametric Radiation

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Elektronisches Publikationsportal der Österreichischen Akademie der Wissenschaften