Variations of Inter-Hemispheric Field-Aligned Currents: Observations From Ground Geomagnetic Measurements

The east-west component of the geomagnetic field (Y-component) at the Trivandrum (TRD) station, an equatorial site, is used to investigate the characteristics of Inter-Hemispheric Field-Aligned Currents (IHFACs) in the Indian sector during solar cycles 21 and 22. The observations reveal an annual variation of IHFAC in the dawn/noon sector with positive/negative peaks around northern summer. Furthermore, it is noted that IHFAC amplitudes are modulated by solar activity, superimposed with seasonal effects, with consistently higher amplitudes during the northern summer months. The dusk-time IHFAC also shows a pattern modulated by seasons and solar cycles, with negative peaks around northern summer only during the low solar active years. The seasonal pattern of IHFAC shows a similar trend from April to September, where dawn/noon IHFAC is south/north directed, and for the remaining months, both are northward directed. The dusk-side IHFAC is mostly south-directed irrespective of the season. Moreover, it is demonstrated that the seasonal pattern of dawn and noon-time IHFAC amplitudes show a strong to moderate correlation with amplitudes of Sq asymmetries between two hemispheres, but dusk-time IHFAC does not follow this trend. The IHFAC directions at TRD follow the Fukushima model for May to September during the dawn-noon sector for 70%-95% of the days and less than 50% of the days for remaining months. The noon-dusk sector shows Fukushima model-type polarity for similar to 60% of days in June-July and less than 50% for remaining months. The global analysis of IHFAC, using monthly averages of hourly values from eight observatories spanning Asia, Africa, and America, reveals spatial differences in amplitude and directions. The directions of noon-time IHFAC in the Indian-East Asian and East-African sectors follow similar trends, and the amplitude variations of noon-time IHFAC can be explained by DE-3 tidal effects. The IHFAC from the South American and West African sectors show intense southward and northward currents compared to other sites, indicating the influence of the South Atlantic Anomaly (SAA). IHFAC features have been identified in the Indian sector using long-term geomagnetic data. The seasonal and solar cycle effects on IHFAC have been evaluated. Further attempts have been made to relate the observed seasonal pattern of IHFAC amplitudes with low and mid-latitude Sq asymmetries between the two hemispheres, using Sq from low and mid-latitude sites of both hemispheres of the Indian sector. It is noted that strong IHFAC in AUG-SEP could be caused by Sq asymmetry in the low latitude sites. The study also tried to look at the IHFAC pattern around the globe using data from different sectors, and significant variations were observed and discussed. Seasonal variation and solar activity dependence of IHFAC in the Indian sector is studied using long -term data The amplitude variations of dawn and noon time IHFAC have been connected to Sq asymmetry from low-mid latitudes in the Indian sector Spatial variations in IHFAC pattern are reported from American, African, Indian, East Asian sectors