Analysis of the Geomagnetic Component Z Daily Variation Amplitude Based on the China Geomagnetic Network

The daily variation amplitude of geomagnetic component Z is one of the important data products in Geomagnetic Network of China (GNC). It comes from the difference between maximum and minimum of the component Z recorded by the geomagnetic 10 instrument in a day. Based on this data product, the daily variation amplitude of Z is analyzed in the past twelve years (2008-2019), including variation for each month in high and low solar activity years, seasonal variations and comparisons between the stations in Yunnan Province and in southeast China. The study indicates that the ionospheric conductivity mainly contributes to the Z daily variations amplitude in the same month or season changing along with solar activity. But the neutral wind in 15 ionosphere could make the Z daily variations amplitude in equinox months equal to or greater than it in summer solstice months during some solar high activity years. Due to the complicated underground electrical structures in Yunnan province, the conductivity underground acts as an amplifier to make the Z daily variations amplitude increase by about 12% ~41% in Yunnan Province during equinox and summer solstice months. 20

year. Similar to the △ Zm in 2014, the △ Zm in mid-high latitude regions is less than it in the low latitude regions. The difference between these two regions could also change from about 10 to 30 nT in different months. The maximum of △ Zm appears in June with the value about 44.6 nT, and the minimum of △ Zm is about 3.8 nT emerges in December. The seasonal change of △ Zm is same to that in 2014

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(high solar activity year), but the amplitude is much less than it in 2014 for each month in entire.
As the method mentioned in section 2, the Lloyd's season is suitable for analysis the seasonal variation of geomagnetic field.   The contour of △Zs in China for winter solstice months is expressed by Fig. 10. It indicates that the △Zs in winter solstice months is much smaller than it in equinox months and summer solstice months. In most of years, the difference of △Zs between north China and south China is less than 20 nT in winter solstice months. The maximum of △Zs contour is about 35.5 nT appears in 2014 a high solar activity year.

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Nevertheless the maximum of △Zs contour is less than 30 nT in other years. Also, the maximum of △Zs contour in winter solstice months appears in Yunnan province both in high solar activity year and in low solar activity year. This feature will be analyzed in detail in the section 5.
In order to study the overall state of variation △ Zs in China, the average of △ Zs from all the stations of 180 GNC is calculated for each Lloyd's season during the twelve years. Fig. 11  This phenomenon should be arisen from the factors which control the diurnal variation current system Sq, like the conductivity and neutral wind in ionosphere, and will be discussed in section 6. The minimum

Comparisons between stations in Yunnan and southeast regions
As shown in the previous analysis, the maximum of △Zm (or △Zs) contour appears in Yunnan province most of time both in high and low solar activity years, especially during equinox and summer solstice months. In this section, three stations in Yunnan province will be analyzed further. As contrast, three And SHW (Shaowu), QZH (Quanzhou) and XFJ (Xinfengjiang) are stations in southeast China. SHW and QZH are located in Fujian province; XFJ is located in Guangdong province. Table 1 shows the stations location information. The comparison is shown in Fig. 13. It is evident that the △ Zs of all the six stations have larger value in high solar activity years than that in low activity years, whatever the season is. Fig. 13 (a) ~ (c) indicate the equinox months. In pair 1, the △ Zs of YSH is larger than that of SHW with the average error 5.5 nT in high solar activity years and 3.61 nT in low solar activity years. In pair 2, the average errors of △ Zs 215 between CHX and QZH are 1.7 nT and 1.9 nT in high and low solar activity years respectively. These two stations shows very similar variation of △ Zs in twelve years. And the errors of them indicate no obvious distinction between high and low solar activity years. The pair 3 of THJ and XFJ is in the lower latitude. The average errors of △ Zs are much larger than that of the other two pairs, with 9.2 nT and 8.7 nT in high and low solar activity years respectively. Table 2 shows the average errors of △ Zs in three studied by some researchers shows that the current intensity could reach maximum in equinox months (Campbell and Matsushita, 1982;Zhao, 2014). And the Sq current makes the geomagnetic component Z decrease near the noon in the north hemisphere of the Earth. So the geomagnetic component Z daily variation amplitude could also reflect the size of the Sq current. Some researchers consider the Sq current 290 reaching maximum in equinox months may be caused by the neutral wind in ionosphere (Amayenc, 1974;Matsushita and Xu, 1982). The neutral wind is mainly generated by the atmospheric tides which caused by the gravitational tide force and the thermal tide force of the sun (Richmond, 1989(Richmond, , 1995. The strength of the neutral wind in equinox months is higher than it in summer and winter solstice months during the high solar activity years (Campbell and Matsushita, 1982;Yamazaki et al., 2009). The conductivity and besides the influence from conductivity and neutral wind in ionosphere as discussed above, the complicated underground electrical structures in Yunnan province could also make great effect to the internal field caused by the induced current underground. In China, the abnormities of the conductivity 320 underground were discovered along the Coast of Bohai Sea, the eastern part of Gansu Province, the area around Tangshan and Yunnan Province (Chen, 1974;Xu et al., 1978). The abnormities of the conductivity underground not only relate to the composition and temperature of rocks, but also relate to some geophysics phenomenon like the seismic wave low velocity area and geothermal flow anomaly area (Rikitake, 1966). Yunnan Province is located in a special geographical location which has strong 325 tectonic activity (Yuan et al., 2015). Hou and Shi (1984)