Navegando por Autor "Agyei-Yeboah, Ebenezer"

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    Ionospheric disturbances over the American and African sectors due to the 2019 major Sudden Stratospheric Warming (SSW 2019), under low solar activity conditions
    (Elsevier) Vieira, Francisco; Fagundes, Paulo Roberto; Pillat, Valdir Gil; Agyei-Yeboah, Ebenezer; Venkatesh, Kavutarapu; Arcanjo, Mateus de Oliveira
    Sudden Stratospheric Warming (SSW) is one of the most spectacular atmospheric large-scale phenomena, which takes place at high latitudes during winter months and is more frequent in the Arctic region than in the Antarctic region. SSWs can change the vertical, latitudinal, and longitudinal distributions of the neutral atmosphere and its dynamics, which in turn affects the ionospheric electrodynamic processes. Simultaneous inferred VTEC from GPS networks over the American and African sectors are used to investigate the ionospheric response due to the SSW 2019 from DOY 356 to DOY 20 (December 22, 2018–January 20, 2019). This study investigates the VTEC and EIA diurnal and day-to-day responses in the American and African sectors during the SSW. It is noted that the VTEC decreased on most of the days at several latitude regions. However, it is also noted that the VTEC increased on some days and in some latitude regions, particularly during the SSW temperature peak. The EIA exhibits significant changes in its shape, intensity, and symmetry during the SSW. This study using simultaneous observations over American and African sectors covering a large geographical extent demonstrates the similarities and differences in ionospheric response to the SSW 2019 event over different regions.
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    Multi-instrument study of various aspects of equatorialionospheric irregularities over American and African sectors
    (2022-08-02) Fagundes, Paulo Roberto; Pillat, Valdir Gil; Ojeda González, Arian; Silva, Igo Paulino; Batista, Inez Staciarini; Agyei-Yeboah, Ebenezer; Tardelli, Alexandre; São José dos Campos
    This thesis is devoted to the study of ionospheric irregularities observed over Brazil and Africa using different instruments and observational techniques. First, the occurrence of ionospheric irregularities like plasma bubbles, blobs, and Spread-F (typical and atypical have been investigated using all-sky imaging systems and ionosondes located at Araguatins, Jataí, and São José dos Campos. Afterward, GPS-TEC data from 33 receiver stations located in different parts of Brazil and Africa were used in the investigation of ROT (rate of TEC change) phase fluctuations during the Antarctic Sudden Stratospheric Warming (SSW). Electron density measurements from the Swarm constellation have also been employed to aid in the analysis and discussion of some of the results obtained from ionosonde and ASI. The thesis focuses on two main studies. One study was dedicated to exploring the observation of the simultaneous occurrence of ionospheric plasma bubbles and blobs using an all-sky imaging system and the observation of atypical Spread-F on ionograms. Based on these observations, a novel methodology is proposed for the observation and studies of ionospheric plasma blobs using ionosondes. The proposed methodology employed a colocated all-sky imager and ionosonde at Araguatins to observe simultaneously occurring plasma bubbles and blobs and then the corresponding ionograms were analyzed. The characteristics of the echoes on the ionograms were compared with the observations from the OI 630 nm nightglow images from the all-sky imager for when there were no irregularities and when bubbles and/or blobs were present. The results showed that in the presence of bubbles and blobs, Spread-Fs with high-frequency echoes were drawn on the ionograms at almost the same time as the images were taken. The results also showed that the generation of plasma blobs could be influenced by the development of bubbles and that the equatorial ionization anomaly may play a major role in their development. The other focus of the thesis is the study of the effects of the 2019 minor Antarctic SSW event on the generation/suppression of ionospheric irregularities. The critical frequency obtained from the three ionosonde stations at Araguatins, Jataí, and São José dos Campos was used to monitor the ionospheric irregularities during the different phases of the SSW event. Spread-F (Brazil) and ROT phase fluctuations (Brazil, Africa) occurrence at different latitudes during different phases of the event were analyzed. Irregularity occurrence from a similar period in 2018 when no SSW event was recorded in the Antarctic region was also analyzed. The results showed a general decrease in occurrence rates during the peak phase of the event compared to the other phases in 2019. The results also showed a decrease in occurrence rate from the peak phase in 2018 (no SSW) to the peak phase in 2019 (SSW).
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    Occurrence of Ionospheric irregularities over Brazil and Africa during the 2019 Antarctic minor sudden stratospheric warming
    (Elsevier) Agyei-Yeboah, Ebenezer; Fagundes, Paulo Roberto; Tardelli, Alexandre; Pillat, Valdir Gil; Vieira, Francisco; Arcanjo, Mateus de Oliveira
    The influence of sudden stratospheric warming (SSW) on the ionosphere and ionospheric irregularities has been studied extensively over the years. However, majority of these investigations have been conducted using warming events originating from the northern hemisphere. Only a few studies have been done on ionospheric variations due to the Antarctic SSW events and to the best of our knowledge, there have not been any studies on southern hemisphere SSW and the occurrence of ionospheric irregularities. In this study, the occurrence of ionospheric irregularities during the 2019 minor Southern hemisphere (SH)/Antarctic SSW is investigated. The event occurs in a relatively calm solar and geomagnetic activity period which makes it possible to identify the effects of SSW on the occurrence of irregularities. Three ionosondes located in different latitudinal regions in Brazil as well as a network of ground-based GPS receiver stations located in both Brazil and Africa were used for this undertaking. Complimentary data from the same ionosonde stations using the same months from 2017 and 2018 were also used. On average more Spread-F was observed in 2019 than in 2017 or 2018 at all stations. ROT observations showed more occurrence in the Brazil sectors followed by West Africa and thenEast Africa. It was observed that the occurrence frequency decreased between 8% and 46 % from the pre-SSW phase to ascending/peak phases and from 2018 to 2019 for the peak phase.