Diurnal variation of geoelectric field and its tidal response for the stations Xinyi and Malingshan

This paper analyzes the waveform, amplitude, phase, and dominant period of the diurnal variation of geoelectric field observed at the two stations Xinyi of Jiangsu and Malingshan of Shandong, associated with the observation data of the geomagnetic field and geostrain in/near the two stations, and then discussed seasonal effect of diurnal variation amplitude and the effects from the electrical conditions of station site. Furthermore, the mechanism of the diurnal variation is explored. The results showed that the diurnal variation of geoelectric field appear as two obvious undulating changes, in which its main phases generally appear in the time interval close to noon (local time), and the fluctuation of the main phase is approximately the same as that of the diurnal variation of the geomagnetic field, but there is an phase difference between them. However, the fluctuation time of the main phase roughly corresponds to that of the diurnal fluctuation on the partial derivative curves of the geomagnetic field in respect to time. There are significant correlations between the orthogonal components of both the geoelectric field and geomagnetic filed and between the geoelectric field component and parallel geostrain one, although the diurnal variation of geoelectric field is largely different from the diurnal variation of the geostrain. It can also be concluded that the predominant periodic components of the diurnal variations of both geoelectric and geomagnetic fields chiefly are 12 h, 8 h and 24 h, in which the most principal period is 12 h, and the 12 h-and 24 h-period component are just the dominant periods of the two orthogonal components of geostrain. The fluctuations of the diurnal variation of geoelectric field along the same direction is nearly identical for the two stations. However, the range of the diurnal variation between the two orthogonal components of the same station is different in directivity. The range in the lunar new and full moon days is larger than that in the first and last quarter moons, and the range is also larger in the summer than in the winter, which is obviously a seasonal effect. The smaller the conductivity of the underground medium is, the more larger the range is. The above-mentioned appearances of the diurnal variation of geoelectric field at the two stations have both the wide-area and local characteristics. In the end, it is believed that the diurnal variation of geoelectric field is caused by the force from the solar and lunar tides as well as the ionospheric activity induced by the solar wind, and it is also affected by other factors such as the season and electrical conditions of underground medium of station site, etc.