Intra-, Inter- Seasonal and Inter-Hemispheric Dynamical Changes in TEC in the Maximum and Minimum Years of Solar Cycle 24 at the Southern Low Latitude Station Cocos Islands Vis-A-Vis the Northern Magnetically Conjugate Station Dibrugarh

Abstract

The long-term (2009–2019) TEC variation over a southern low latitude station Cocos Islands (12.2°S, 96.8°E) lying in edge of the Equatorial Ionization Anomaly (EIA) region and around the WN4 peak longitude are investigated and compared with its magnetic conjugate station Dibrugarh (27.5°N, 95°E). The causative mechanisms for the observed intra- and inter-seasonal variations are explored via the study of EEJ, neutral O/N₂ ratio, hmF2, meridional winds and IRI simulations. The month-to-month variation of the mean TEC showed a tendency of peaking in November (early summer) during 2009/2018 but in March (autumn) during 2014. In contrast, the seasonal maximum was always in March (spring equinox) over Dibrugarh. The hemispheric asymmetry was significant during the solar minimum period. The intra-seasonal variation of the asymmetry showed a relation to the position of the sub-solar point and solar activity. The equatorial anomaly was estimated by comparing it with equatorial TEC. The anomaly affected the stations only during respective summer of 2009/2018 whereas during 2014 the anomaly affected both stations from February to April. The monthly mean TEC over Cocos was well correlated with EEJ only during 2014. In contrast, the TEC was correlated with hmF2 in both 2009 and 2014. The IRI-2020 mostly overestimated TEC with the exception of March equinox of 2014 when the EIA effect was strongest over the stations. The dynamics of TEC variation over Cocos seemed to vary from solar minimum to maximum, changing from being photo-chemically driven to electric field driven with increase in solar activity.