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

我们已经建立了一个VLBI（甚长基线干涉仪）网络，用于确定木星射电辐射（JDR）源的位置或结构。本研究由福井工业大学和东北大学合作进行。该VLBI系统由福井的芦原站、福岛的饭立站、宫城的Zao站和米山站组成。最大基线长度为540 km（Awara-Yoneyama基线），在20 MHz的观测频率下，空间分辨率约为5角秒。为了解决这一问题，日本东北大学的双频技术小组从1994年开始进行了试验。本研究的目的之一是根据400 km级VLBI观测资料，对双频技术的有效性进行评估。每个观测站安装的观测系统， ...更多信息 五个部分，i。e.前端、主接收机数据记录系统和一些支持设备。前端由可探测频率范围为20 ~ 40 MHz的9单元对数周期天线和前置放大器组成。前置放大器由通带为20 ~ 40 MHz的带通滤波器和低噪声宽带放大器组成，前端接收信号送入由两个接收系统组成的宽带主接收机，实现双频观测。主接收机的频率系统由铯或铷束振荡器的参考钟控制，保证了观测系统的相位稳定性。主接收器的输出直接数字化并存储在每个观测站中。2006年开始观测，但观测系统出现了一些问题，另外2006 - 2007年JDR的活动度很低。因此，第一道曙光出现在2008年3月28日。接收到的JDR事件被归类为与IO相关的辐射，称为“Io-B”。观测是在高信噪比条件下进行的。观测频率设置为22 MHz和22.5 MHz，但是，由于JDR事件的窄带特性，无法进行双频分析。为了考虑接收到的JDR事件的源结构，我们进行了闭合相分析，而不是双频分析。在闭合相位分析中，计算由三条基线检测到的相位之和，由此可以形成三角形。闭合相位的大小仅由源的结构决定，不受电离层效应和仪器误差的影响。接收到的JDR事件的闭合相位值在归一化互相关系数超过0.4的数据点处取零，误差为+/-度，这意味着信噪比超过-4dB。这一高精度是由观测系统的宽频带特性实现的。结果表明，观测的JDR源足够紧凑，可以看作是单点。本研究还表明，闭合相位分析对于消除VLBI观测中的电离层效应和仪器误差也是有用的。在今后的研究中，我们认为有必要对闭合相位和双频分析作为消除VLBI观测中电离层影响的方法的有效性进行进一步的评估。少

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