Study of source distribution of Jovian decameter radiation based on the long baseline interferometer observation

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We have developed a VLBI (Very Long Baseline Interferometer) network for determining the location or structure of Jovian decameter radiation (JDR) sources. This study has been carried out in the collaboration between Fukui University of Technology and Tohoku University. The developed VLBI system consists of Awara station in Fukui, Iitate station in Fukushima, Zao and Yoneyama stations in Miyagi. The Maximum baseline length is 540km (Awara-Yoneyama baseline) whose spatial resolution is about 5 arcsec at observation frequency of 20 MHz.It has been considered that the VLBI observation in decameter range is difficult because of the enormous influence of terrestrial ionosphere. To overcome this problem, the dual frequency technique has been tested by the group of Tohoku University from 1994 The one of the aims of this research is to evaluate the validity of the dual frequency technique based on 400km class VLBI observations.The observation system installed in each observation station consis … More ts of five parts, i. e. front-end, main receiver data recording system ant some support equipments. The front-end consists of 9 elements log-periodic antenna whose detectable frequency range is from 20 MHz to 40 MHz and pre-amplifier. The pre-amplifier consists of a bandpass filter whose passband is ranging from 20 MHz to 40 MHz and the low noise wideband amplifier The received signals by the front-end are fed into the wideband main receiver which consists of two receiving system so that the dual frequency observation is made. The frequency system of this main receiver is controlled by the reference clock of cesium or rubidium beam oscillator which ensures the phase stability of the observation system. The output of the main receiver is directly digitized and stored in each observation station. The time synchronization between the observation stations is ensured by using 1PPS signals of GPS system.The observation has been started in 2006, however we suffered from some troubles in the observation system In addition, the activity of JDR had been very low from 2006 to 2007. Therefore, the first light had come on March 28 in 2008. The received JDR event is categorized to Io-related radiation which is called “Io-B". The observation was carried out under the condition of high Signal to Noise ratio because of its strung intensity. The observation frequencies were set to 22 MHz and 22.5 MHz, however, the dual frequency analyses could not be carried out because of the narrow band characteristics of the JDR event. For considering the source structure of the received JDR event, we have carried out the closure phase analyses instead of the dual frequency analyses. In the closure phase analyses, the sum of phases detected by three baselines by which triangles can be made is calculated. The value of closure phase is decided only by the source structure and is free from the ionospheric effects and the instrumental errors. The closure phase value of the received JDR event takes zero with error of +/- degrees at the data points whose normalized cross correlation coefficients are over 0.4, which means the Signal to Noise ratio is over -4dB. This high accuracy can be achieved by the wideband character of the observation system This result shows that the source of observed JDR is compact enough to consider as single point.In this study, it has been showed that the closure phase analyses are also useful for eliminating the ionospheric effects and the instrumental errors in the VLBI observation for decameter waves. For the future study, it is considered that more investigation is necessary for evaluating the validity of the closure phase and the dual frequency analyses as a method of eliminating the ionospheric effects on the VLBI observations. Less