# Equinoctial Asymmetry in Solar Quiet Fields along the 120\({}^{\circ}\) E Meridian Chain
## 4 Discussion  
[PERSON] (1963, 1965) [27; 28] reported opposite signs of the SqH equinoctial asymmetry at HON and TUC and suggested that the meridional shift of the Sq current focus could cause this asymmetry. For the month-to-month changes of the Sq current focus position, [PERSON] (1973) [17] examined the meridional movement of Sq foci during the solar maximum of 1958 and found month-to-month shifts with latitude for both the northern and southern Sq foci. [PERSON] (1973) [18] detected that the northern focus shifts southwards during autumn months, then northwards in winter. He suggested that this seasonal movement might be caused by the semiannual variation of the electrojet amplitude, induced by different seasonal changes at equatorial and middle-high latitudes. [PERSON] et al. (2017) [43] also found a drift toward lower latitudes of the Sq focus from March to September.  
However, in terms of ionospheric dynamic processes, equinoctial asymmetry of Sq variations shows a strong relationship with the geomagnetic field, ionospheric conductivity, and tidal wind fields [31; 44]. The correlation of the asymmetry with seasonal variations of lower-thermospheric winds and lower-ionospheric conductivity was discussed in numerous studies [27; 28; 29; 30]. Obvious equinoctial asymmetry of electron and ion temperatures, ionospheric plasma density, and plasma drifts were also reported [45; 46; 47; 48; 49] and might be related to the marked equinoctial asymmetry of thermospheric winds [47; 50]. Additionally, the asymmetry of vertical plasma drifts--mainly driven by E and F regions' neutral wind dynamo processes--has been observed and simulated [51; 52; 53]. [PERSON] et al. (2012) [53] suggested that equinoctial asymmetry of the migrating semidiurnal tide and semiannual oscillation of the migrating diurnal tide in the tropical mesosphere-lower thermosphere region might be the principal drivers of the vertical plasma drift asymmetry.  
In this study, we have shown that the SqH equinoctial asymmetry does not depend on solar activity (Figure 6). This implies a possible connection with regional factors caused by the ionospheric dynamo process. As is known, the Sq variation and the equivalent ionospheric current both show a strong longitudinal dependence [54; 55; 56]. Simulating the combined effects of the geomagnetic axis tilt and the Sq current magnetic anomaly, [PERSON] et al. [55] suggested that longitudinal dependence of the Sq current is mainly caused by the geomagnetic field. This result was confirmed by [PERSON] et al. [57]. Moreover, [PERSON] et al. [53] analyzed the effect of the geomagnetic field on longitudinal variations of ionospheric vertical plasma drifts. They simulated ionospheric vertical plasma drifts driven by the migrating diurnal tide at 600 km around the equinoxes. Their results showed  
Figure 7: Dependence of the averaged equinoctial asymmetry AA and averaged asymmetry coefficient on geographic latitude. Red bars represent the SEM. Green curve in the right panel is the linear fit on the asymmetry coefficient, with fitting parameters indicated in the corner.  
a significantly weaker longitudinal asymmetry in a coaxial central dipole field than in a realistic geomagnetic field, suggesting the importance of the geomagnetic field in the ionospheric dynamo process.  
In summary, the SqH equinoctial asymmetry is a complex phenomenon with multiple causes, such as the latitudinal movement of the Sq current focus, lower thermospheric winds, and the ionospheric dynamo process. In this study, we found that the sign of the equinoctial asymmetry is generally stable at a given observatory--positive north of the Sq current focus and negative at southern observatories.