Browsing by Author "Mebrahtu, A."
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Item Restricted Comparison of storm time equatorial ionospheric electrodynamics in the African and American sectors(Elsevier, 2010-08-12) Yizengaw, E.; Moldwin, M. B.; Mebrahtu, A.; Damtie, B.; Zesta, E.; Valladares, C. E.; Doherty, P.The characteristics of storm time (corotating interaction regions (CIR)-driven storm that happened on 9 August 2008) equatorial electrojet (EEJ) phenomena and their effect on the ionospheric density structure at two different longitudinal sectors are presented. Equatorial magnetometer data, occultation density profiles from COSMIC and CHAMP LEO satellites, and ground-based GPS TEC are used. We find unusual density reduction around local noon at the same time when we observe the reversal of electrojet current and thus counter-equatorial electrojet (CEJ) signatures. The continuous energy deposition in to high latitudes due to the CIR-driven storm that triggers the E-region dynamo and the penetrating magnetospheric origin electric field is suggested to be responsible for the reversal of equatorial electrojet current flows. We also compare the magnitude and direction of the driving force (E×B drift) in the American and African sectors for the first time. It was found that at the same local time the E×B drift in the American sector is stronger than that of the African sector. Previously, the uneven distribution of ground-based instruments hindered our ability to obtain a global understanding of the dynamics and structure of the ionosphere. The newly deployed ground-based instruments, primarily in the African sector, provide the opportunity to observe the governing equatorial electrodynamics simultaneously with the ionospheric density structures detected by the instrument onboard low-Earth-orbit (LEO) satellites. To our knowledge this is the first simultaneous observation performed in the African sector. This case study may provide additional input that could be used to explain the unique density irregularities that are often seen from in situ satellite observation in the African sector, a region that has been devoid of ground-based instrumentations.Item Restricted Longitudinal differences of ionospheric vertical density distribution and equatorial electrodynamics(American Geophysical Union, 2012-07-19) Yizengaw, E.; Zesta, E.; Moldwin, M. B.; Damtie, B.; Mebrahtu, A.; Valladares, C. E.; Pfaff, R. F.Accurate estimation of global vertical distribution of ionospheric and plasmaspheric density as a function of local time, season, and magnetic activity is required to improve the operation of space‐based navigation and communication systems. The vertical density distribution, especially at low and equatorial latitudes, is governed by the equatorial electrodynamics that produces a vertical driving force. The vertical structure of the equatorial density distribution can be observed by using tomographic reconstruction techniques on ground‐based global positioning system (GPS) total electron content (TEC). Similarly, the vertical drift, which is one of the driving mechanisms that govern equatorial electrodynamics and strongly affect the structure and dynamics of the ionosphere in the low/midlatitude region, can be estimated using ground magnetometer observations. We present tomographically reconstructed density distribution and the corresponding vertical drifts at two different longitudes: the East African and west South American sectors. Chains of GPS stations in the east African and west South American longitudinal sectors, covering the equatorial anomaly region of meridian ∼37°E and 290°E, respectively, are used to reconstruct the vertical density distribution. Similarly, magnetometer sites of African Meridian B‐field Education and Research (AMBER) and INTERMAGNET for the east African sector and South American Meridional B‐field Array (SAMBA) and Low Latitude Ionospheric Sensor Network (LISN) are used to estimate the vertical drift velocity at two distinct longitudes. The comparison between the reconstructed and Jicamarca Incoherent Scatter Radar (ISR) measured density profiles shows excellent agreement, demonstrating the usefulness of tomographic reconstruction technique in providing the vertical density distribution at different longitudes. Similarly, the comparison between magnetometer estimated vertical drift and other independent drift observation, such as from VEFI onboard Communication/Navigation Outage Forecasting System (C/NOFS) satellite and JULIA radar, is equally promising. The observations at different longitudes suggest that the vertical drift velocities and the vertical density distribution have significant longitudinal differences; especially the equatorial anomaly peaks expand to higher latitudes more in American sector than the African sector, indicating that the vertical drift in the American sector is stronger than the African sector.Item Restricted The longitudinal variability of equatorial electrojet and vertical drift velocity in the African and American sectors(European Geosciences Union (EGU), 2014-03-13) Yizengaw, E.; Moldwin, M. B.; Zesta, E.; Biouele, C. M.; Damtie, B.; Mebrahtu, A.; Rabiu, B.; Valladares, C. F.; Stoneback, R.While the formation of equatorial electrojet (EEJ) and its temporal variation is believed to be fairly well understood, the longitudinal variability at all local times is still unknown. This paper presents a case and statistical study of the longitudinal variability of dayside EEJ for all local times using ground-based observations. We found EEJ is stronger in the west American sector and decreases from west to east longitudinal sectors. We also confirm the presence of significant longitudinal difference in the dusk sector pre-reversal drift, using the ion velocity meter (IVM) instrument onboard the C/NOFS satellite, with stronger pre-reversal drift in the west American sector compared to the African sector. Previous satellite observations have shown that the African sector is home to stronger and year-round ionospheric bubbles/irregularities compared to the American and Asian sectors. This study’s results raises the question if the vertical drift, which is believed to be the main cause for the enhancement of Rayleigh–Taylor (RT) instability growth rate, is stronger in the American sector and weaker in the African sector – why are the occurrence and amplitude of equatorial irregularities stronger in the African sector?