# Equinoctial Asymmetry in Solar Quiet Fields along the 120\({}^{\circ}\) E Meridian Chain
## 2 Data and Calculations  
In this study, we used hourly measurements of the horizontal component of the geomagnetic field from six geomagnetic observatories located in the Northern Hemisphere along the 120\({}^{\circ}\) E meridian chain. Five are in China--Beijing Ming Tombs (BMT), Lanzhou (LZH), Chengdu (CDP), Wuhan (WHN), and Guangzhou (GZH)--and one--Muntlinupa (MUT)--is in the Philippines. The five Chinese observatories are member stations of the Chinese meridian project, more information on which can be found on the website: \"[[https://data.meridianproject.ac.cn/about-us/](https://data.meridianproject.ac.cn/about-us/)]([https://data.meridianproject.ac.cn/about-us/](https://data.meridianproject.ac.cn/about-us/))\" (accessed on 18 September 2021). The MUT station is part of the 210 magnetic meridian chain, listed in the catalogue of stations on \"[[https://stdb2.isee.nagoya-u.ac.jp/mm210/station.html](https://stdb2.isee.nagoya-u.ac.jp/mm210/station.html)]([https://stdb2.isee.nagoya-u.ac.jp/mm210/station.html](https://stdb2.isee.nagoya-u.ac.jp/mm210/station.html))\" (accessed on 18 September 2021). Here, the data of these stations are downloaded from the World Data Center for Geomagnetism (Edinburgh) on site: \"ftp:nmh.ac.uk/\" (accessed on 18 September 2021). As declared by WDC (Edinburgh), the geomagnetic data are collected from operating observatories and INTERMAGNET (International Real-time Magnetic Observatory Network), and keep the data policy of IAGA (International Association of Geomagnetism and Aeronomy) to publish filtered one-minute data and one-hour data. As known from the introduction of the observatories on WDC (Edinburgh) and INTERMAGNET, the instrumental system including vector fluxgate magnetometer and scalar magnetometer is equipped to obtain the absolute observed values of the magnetic field. Theses six observatories are distributed over about 25\({}^{\circ}\) in geographic latitude (14-40\({}^{\circ}\) N) with an interval of 5-8\({}^{\circ}\), except between CDP and WHN. Their coordinatesare listed in Table 1. Figure 1 shows their positions relative to the Northern Hemisphere Sq current system. This Sq current diagram describes an average ionospheric equivalent current, which is derived by the commonly used spherical harmonic analysis technology and by using the data of the yearly averaged solar quiet day variation of observatories in geographical longitude from 90 to 130 degrees in 2000--hence, with no focusing on its daily and monthly variation. CDP and WHN are near the focus of the Sq current. BMT and LZH will hereafter be referred to as \"northern stations\", while GZH and MUT will be referred to as \"southern stations\". Figure 2 shows the variation of solar radio flux at 10.7 cm (F107) in solar flux units (1 sfu = 10\({}^{-22}\) W m\({}^{-2}\) Hz\({}^{-1}\)) [32] and the yearly coverage of the geomagnetic field measurements used in this study. As we can see, the full dataset covers five solar cycles from 1957 to 2013. None of the observatories were operational during the complete period; however, time coverage is sufficient for investigating the latitudinal distribution of the SqH equinoctial asymmetry because of the good temporal overlap between the northern stations (1995-2007) and between both southern stations (1963-1972). These common periods are used to compare observatories on the same side of the Sq current focus.