Global characteristics of the equinoctial asymmetry in the geomagnetic SqH field

The geomagnetic solar quiet daily variation (Sq) is one of the most fundamental and persistent geomagnetic variations at middle and low latitudes during geomagnetically quiet conditions. Characterized by regular diurnal changes and a sharp variation of approximately tens of nanoteslas near local noon, the Sq field is generated by ionospheric dynamo currents in the E-region at altitudes of 90–150 km, where ionospheric conductivity reaches its peak. As a critical manifestation of ionosphere-magnetosphere coupling and a key component of the near-Earth electromagnetic environment, the Sq field reflects the combined effects of solar radiation, thermospheric neutral winds, ionospheric conductivity, and the structure of the Earth’s main magnetic field. For more than half a century, the horizontal component of the Sq field (SqH) has been observed to exhibit significant equinoctial asymmetry at individual low-latitude and midlatitude stations, meaning that the SqH amplitude differs notably between the spring and autumn equinoxes. However, the global spatial characteristics, long-term stability across solar cycles, hemispheric differences, and underlying physical mechanisms of this asymmetry remain poorly constrained. Previous studies have been limited by sparse station distribution, short data records, or regional focus only, and a unified global understanding is still lacking.

 

This study presents a comprehensive global analysis of the equinoctial asymmetry in SqH amplitude using long-term hourly geomagnetic horizontal component data from 165 observatories distributed between ±60° magnetic latitude, covering the 75-year period from 1947 to 2021. To investigate longitudinal differences, three representative meridian chains are selected: Europe-Africa, Asia-Australia, and North-South America. The equinoctial asymmetry is quantified using the parameter dA, defined as the difference between the average SqH amplitude during the autumn equinox and the spring equinox. A positive dA indicates a stronger SqH amplitude in autumn, while a negative dA indicates a stronger amplitude in spring. This parameter allows systematic evaluation of the asymmetry magnitude, polarity, stability, and spatial distribution across different latitudes, longitudes, and hemispheres.

 

The results confirm that the equinoctial asymmetry of SqH is a global phenomenon observed at nearly all mid-to-low latitude stations. Statistical analysis shows no significant correlation between the polarity of dA and the F10.7 solar flux index, demonstrating that the sign of the asymmetry is independent of the solar activity cycle. The polarity of dA remains highly stable at most stations across multiple solar cycles, while only a small number of stations show frequent year-to-year alternation between positive and negative values. Both dA and the proportion of positive dA values exhibit strong latitudinal, longitudinal, and hemispheric dependencies. Globally, dA values range from −30 nT to 20 nT, with an average amplitude of ±5 nT at low latitudes and ±12 nT at mid-latitudes, indicating that the equinoctial asymmetry is more pronounced in the mid-latitude region.

 

In the Northern Hemisphere, dA shows a clear latitudinal structure: it reaches a peak in the mid-latitude zone, forms a distinct trough near the magnetic equator, and undergoes a systematic sign reversal from negative to positive with increasing latitude. Longitudinal differences are prominent: the peak latitude and sign-reversal latitude of dA vary among the three meridian chains, and the stability of dA is lower in the North-South America chain than in the Europe-Africa and Asia-Australia chains. The Southern Hemisphere displays more complex and variable asymmetry behavior, with mostly negative dA values and weaker temporal stability, which may be related to the influence of the South Atlantic Anomaly, equatorial electrojet effects, and the heterogeneous distribution of the non-dipole magnetic field.

 

We further explore the relationship between dA and the Earth’s main magnetic field using the 13th International Geomagnetic Reference Field (IGRF13) model. No significant association is found between dA and geomagnetic secular variation. However, a statistically significant negative correlation is identified between the proportion of positive dA and the non-dipole vertical magnetic field in the Southern Hemisphere. This result provides robust quantitative evidence that the Earth’s non-dipole magnetic field is the primary factor controlling the longitudinal structure of the SqH equinoctial asymmetry. Local processes, including the equatorial electrojet and regional geomagnetic anomalies, further modulate the spatial pattern and amplitude of the asymmetry in equatorial and low-latitude regions.

 

This study establishes the global distribution, long-term stability, latitudinal and longitudinal dependences, and dominant physical mechanism of the SqH equinoctial asymmetry. It improves our understanding of ionospheric dynamo processes, ionosphere-magnetosphere coupling, and the role of the non-dipole geomagnetic field in shaping upper atmospheric electromagnetic variations. The findings provide essential baseline parameters for high-precision geomagnetic field modeling, ionospheric disturbance quantification, and space weather research related to terrestrial planetary electromagnetic environments.