UNDERSTANDING THE DAY-TO-DAY VARIABILITY OF THE EQUATORIAL ELECTROJET

Abstract

This thesis is divided in three main parts: Parts I, II and III. Part I is a brief review about essential concepts related to Earth's magnetic field and atmosphere, aiming a better understanding of the subsequent Parts II and III. Part II presents a study about the equatorial electrojet (EEJ) ionospheric current. The intensity of the EEJ shows temporal and spatial variability that is not yet fully understood nor accurately modeled. It is known that atmospheric solar tides are among the main drivers of the EEJ variability. Aiming to better understand which atmospheric solar tides are important in the EEJ variability, we developed a new model of the EEJ, namely PCEEJ, which is based on the principal component analysis (PCA) of geomagnetic data. Apart from showing better match to the observed data when compared to the EEJM-2 model, the PCEEJ model allowed us to derive the time series of solar tides amplitudes based on tidal equation fitting. The results indicate that it is possible to obtain a meaningful EEJ spectrum related to solar tides for a relatively short time interval of 70 days. Part III is related to the recovery and processing of the hourly mean values of the geomagnetic field horizontal (H) component measured at the geomagnetic observatory Tatuoca (TTB), from 1957 until 2019. Besides delivering a 60-year long processed data set from the South Atlantic Anomaly region, the work allowed the investigation of long-term changes in the daily variation recorded in TTB due to the influence of secular variation, solar activity, season and lunar phase. Due to the pronounced secular variation in the region and to the long time series, an unusual transition from typical low-latitude solar quiet daily variation to EEJ higher-magnitude daily variation was recorded at TTB data set. This transition impacted several features of the observed daily variation and was compared to simulations performed with the Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIEGCM). The results confirmed that the geomagnetic main field dynamics was the main cause of the observed external field long-term changes.