# Equinoctial Asymmetry in Solar Quiet Fields along the 120\({}^{\circ}\) E Meridian Chain
###### Abstract  
Equinoctial asymmetry of the range of the solar quiet day variation (Sq) of the horizontal geomagnetic field (H) has been found in some low latitude geomagnetic observatories. This study conducted an investigation of its latitude distribution and the relationship with the solar cycle by using the H field measurements from six observatories along the 120\({}^{\circ}\) E meridian chain in the years 1957-2013. Results illustrate a significant equinoctial asymmetry of the SqH range at all observatories. Three main features were identified. First, the signature of the equinoctial asymmetry of the SqH range is opposite for observatories located at the northern and southern sides of the Northern Hemisphere Sq current focus. It shows larger values around spring than autumn equinox at southern observatories, and the converse is seen at northern observatories. Second, the asymmetry increases with the distance from the Sq current focus, suggesting the stronger sensitivity of the distant observatories than observatories around the focus. The result of linear fitting presents a positive dependence of the asymmetry coefficient on geographic latitude, with a reversal of the asymmetry occurring at 28.1\({}^{\circ}\) N near the focus of the average Sq current. Third, there is no obvious dependence of the equinoctial asymmetry of the SqH range on solar activity, suggesting a possible cause from some regional factors related to the ionospheric dynamo process. solar quiet variation; equinoctial asymmetry; latitudinal distribution 2  
Prior investigations generally combined spring and autumn months to represent the \"equinox season\", as defined by [PERSON] (1874) [25]. However, a few studies differentiated between spring and autumn months. [PERSON] (1950) [26] found that the amplitude of the horizontal (H) component of Sq (SqH) was smaller in September than in March at the Honolulu observatory (HON, 21.3\({}^{\circ}\) N, 201.9\({}^{\circ}\) E). [PERSON] (1963, 1965) [27; 28] compared seasonal variations of the SqH range between the HON, San Juan (SJG, 18.4\({}^{\circ}\) N, 293.9\({}^{\circ}\) E), and Tucson (TUC, 32.2\({}^{\circ}\) N, 249.3\({}^{\circ}\) E) observatories and found a larger maximum in March than in September at HON and SJG, but a reversed situation at TUC. He suggested that it was caused by an anomalous seasonal variation of the large-scale lower-ionospheric circulation. Analyzing data from eighteen observatories located at low-to-high latitudes, [PERSON] et al. (2005) [29] confirmed the significant SqH equinoctial asymmetry at HON and TUC. They suggested that this asymmetry was only weakly correlated with solar activity but was more likely the result of the ionospheric dynamo induced by seasonal asymmetry of lower-thermospheric winds. Recently, similar SqH equinoctial asymmetry was reported by [PERSON] (2014) [30] at an equatorial observatory, Addis Ababa (AAE, 9.0\({}^{\circ}\) N, 38.8\({}^{\circ}\) E). Furthermore, equinoctial asymmetry of the Sq equivalent current also has been found by the method of spherical harmonic analysis. [PERSON] (2002) [31] observed different phases of the Sq equivalent current in March and September and attributed this to the equinoctial asymmetry of tidal winds in the upper atmosphere. [PERSON] et al. (2010) [14] examined the month-to-month variation of the Sq current intensity in East Asia and discovered a spring-autumn asymmetry, with a larger intensity in spring. Obviously, based on few stations in previous ground observations, the understanding of the detailed features of the SqH equinoctial asymmetry is limited in former studies. Its latitudinal distribution and its relationship with the current center and solar activity need to be better clarified. Therefore, the purpose of this study was to investigate the latitudinal distribution of the equinoctial asymmetry of the range of SqH by using hourly observations of the horizontal component of the geomagnetic field, acquired at six geomagnetic observatories located along the 120\({}^{\circ}\) E meridian chain.