# Equinoctial Asymmetry in Solar Quiet Fields along the 120\({}^{\circ}\) E Meridian Chain
## 2 Data and Calculations
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Site** & \begin{tabular}{c} **Geographic** \\ **Coordinates** \\ \end{tabular} & \
\begin{tabular}{c} **Geomagnetic** \\ **Coordinates** \\ \end{tabular} &
\begin{tabular}{c} **Time Coverage** \\ \end{tabular} \\ \hline BMT & 40.3\({}^{\circ}\) N, 116.2\({}^{\circ}\) E & 29.9\({}^{\circ}\), 186.8\({}^{\circ}\) & 1996–2013 \\ LZH & 36.1\({}^{\circ}\) N, 103.9\({}^{\circ}\) E & 25.7\({}^{\circ}\), 175.9\({}^{\circ}\) & 1980, 1986, 1989–1992, 1995, \\ CDP & 31.0\({}^{\circ}\) N, 103.7\({}^{\circ}\) E & 20.6\({}^{\circ}\), 175.7\({}^{\circ}\) & 1995–2002, 2005–2007 \\ WHN & 30.5\({}^{\circ}\) N, 114.6\({}^{\circ}\) E & 20.1\({}^{\circ}\), 185.62\({}^{\circ}\) & 1980, 1995–2002, 2005–2007 \\ GZH & 23.1\({}^{\circ}\) N, 113.3\({}^{\circ}\) E & 12.7\({}^{\circ}\), 184.6\({}^{\circ}\) & 1960–1993, 2003–2009 \\ MUT & 14.4\({}^{\circ}\) N, 121.0\({}^{\circ}\) E & 4.2\({}^{\circ}\), 192.2\({}^{\circ}\) & 1957–1959, 1963–1972 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Coordinates and time coverage of the geomagnetic observatories used in this study. Figure 1: Schematic diagram of the geomagnetic observatories’ locations used in this study, along the 120\({}^{\circ}\) E meridian chain, relative to the Northern Hemisphere solar quiet (Sq) current focus. “GG Lat”. represents the geographic latitude. To calculate SqH, hourly data are first accumulated to obtain daily values. The baseline value derived from the average \"midnight\" values between 23:00 and 02:00 LT and the secular changes calculated with a detrending method [33] are both removed from the daily measurements. Then, the SqH is calculated by averaging the results of the five international quiet days in each month [1], which is derived by GFZ Potsdam from the Kp index. From the daily SqH values, we further derive monthly and yearly averaged SqH values, shown in Figure 3. Marked diurnal and seasonal variations are clearly visible, and the standard deviations (d) as an error value for each hour are also plotted in red bars. In the right panel of Figure 3, the diurnal variations of the yearly averaged SqH of the southern stations (MUT and GZH) show a marked enhancement around 12:00 LT, with a magnitude decreasing closer to the Sq current focus. While, the strong enhancement as seen at noontime at the equatorial station MUT clearly reflects the contribution of equatorial electrojet (EE), which also could produce an impact on low latitude stations and could probably result in the misunderstanding of the characteristics of the Sq focus [11,34] However, curves of the diurnal variations are much more smooth at \"central\" stations (WHN and CDP). Conversely, the diurnal variation pattern of the yearly averaged SqH at northern stations LZH and BMT shows a strong decrease between 09:00 and 12:00 LT.  
Figure 2: **(Top)**: Yearly coverage of the observations used in this study at each observatory. **(Bottom)**: Yearly averages of the 10.7 cm solar radio flux (F107) over the full time period (1957–2013), downloaded from the ftp server: [[ftp://ftp.gfz-potsdam.de/pub/home/obs/Kp_ap_Ap_SN_F107](ftp://ftp.gfz-potsdam.de/pub/home/obs/Kp_ap_Ap_SN_F107)]([ftp://ftp.gfz-potsdam.de/pub/home/obs/Kp_ap_Ap_SN_F107](ftp://ftp.gfz-potsdam.de/pub/home/obs/Kp_ap_Ap_SN_F107)) (accessed on 18 September 2021). In the left panel of Figure 3, average monthly SqH, derived from all available years in each station, are contoured, which shows the peak amplitude of the SqH in the equinoctial months. Additionally, the extreme values appear to be drifting in LT, occurring increasingly late from winter to summer at southern stations but early at northern stations. These combined features can fully correspond to the universal time (UT) variability of the Sq and to its ionospheric dynamo current [2; 33; 35]. Additionally, the SqH amplitude is clearly different between spring and autumn, presenting the asymmetry between the equinoxes.