http://ir.bdu.edu.et/handle/123456789/1844 2001-01-13T06:41:33Z 2001-01-13T06:41:33Z Cherinet, Assefa http://ir.bdu.edu.et/handle/123456789/16832 2025-08-05T08:07:39Z 2025-06-01T00:00:00Z

Effect of Fresnel First Zone on Trans-ionosphere Propagation of GNSS Signal Cherinet, Assefa The trans-ionospheric propagation of Global Navigation Satellite System (GNSS) signals is significantly influenced by ionospheric irregularities, particularly at equatorial and low-latitude regions. This study investigates the effect of the Fresnel First Zone a fundamental concept in wave diffraction on GNSS signal scintillation. The Fresnel scale defines the spatial dimensions of ionospheric irregularities that most strongly scatter GNSS signals, causing amplitude and phase scintillations that degrade signal quality. Using theoretical models and observational data, including S4 index measurements, the research analyzes how variations in electron density and irregularity drift affect signal propagation. The findings underscore the importance of the Fresnel First Zone in understanding diffractive scattering processes and provide insight into the mitigation of GNSS signal disruptions, especially in space weather-sensitive regions. This work contributes to enhancing GNSS reliability for communication and navigation applications.

2025-06-01T00:00:00Z Shumet, Workie http://ir.bdu.edu.et/handle/123456789/16831 2025-08-05T06:52:09Z 2024-11-01T00:00:00Z

Solar Activity Impact on the Earth’s Ionosphere over East Africa during 2009 - 2019 Shumet, Workie The ionosphere, a vital layer of the Earth’s atmosphere, plays a significant role in influ- encing radio communication, satellite navigation, and space weather prediction, with its behavior being profoundly affected by solar activity, including EUV flux, sunspots, so- lar radio flux, and geomagnetic activity. Understanding the ionospheric response to these solar and geomagnetic activity changes is crucial for space weather research, particularly as these responses vary spatially and temporally, necessitating extensive study to mitigate their impacts on human technologies. This dissertation investigates the impact of solar and geomagnetic activity on the Earth’s ionosphere over East Africa from 2009 to 2019, during solar cycle 24 and it focuses on the medium- and long-term variations in ionospheric Total Electron Content (TEC) using GPS-derived TEC data from eight stations at low/equatorial latitudes; and solar and geomagnetic indices (EUV, F10.7, SSN, Dst, and Kp) observa- tions. We have applied statistical analysis and quadratic fits with solar proxies (EUV, F10.7, and SSN). Daily mean solar and geomagnetic indices and vertical Total-Electron-Content (vTEC) were analyzed using statistical methods and quadratic fits to identify trends, fore- cast vTEC, and describe its daily, monthly, and seasonal variations. The research highlights that while equinoxes exhibit higher vTEC values compared to solstices, the influence of so- lar activity varies significantly across different timescales. Notable findings include peak vTEC values during equinoxes, especially in March, October, and April 2014, and a strong correlation between vTEC and solar parameters, with 45% to 81% of vTEC variations explained by these indices. The variations of vTEC showed positive associations with the solar parameters. The study reveals that EUV flux has the strongest association with vTEC, particularly during solar maxima, while F10.7 serves as a better proxy for EUV flux. In the predictions of vTEC, the maximum deviations and number of errors were observed during solar maxima years compared to solar minima. The quadratic model effectively captures the dependency of vTEC on solar activity, emphasizing the need for continuous monitoring and modeling of ionospheric conditions to mitigate the impacts on technological systems. Insights into ionospheric behavior enhance the understanding of solar activity’s effects on the ionosphere, contributing to improve space weather prediction models and communica- tion systems in the region, and suggesting future research to integrate these findings into global models. v

2024-11-01T00:00:00Z Ambachew, Abeje http://ir.bdu.edu.et/handle/123456789/16826 2025-07-31T12:17:24Z 2024-07-01T00:00:00Z

Spatio-Temporal Variability Of Aerosols Over East Africa-Ethiopia Using Modis Satellite Data Ambachew, Abeje Aerosols are tiny mixtures of liquid-solid particulate matter suspended in the atmosphere that play significant roles in human health and climate dynamics, directly, indirectly, and semi-directly. There have been large spatiotemporal variations in the optical properties of aerosols, clouds, precipitation, and radiation due to environmental and meteorological conditions, industrial and agricultural influences, and other human and natural influences in each ecological functional area. This study was conducted on the spatiotemporal variability of aerosols in sixteen selected stations clustered into four regions over East Africa-Ethiopia using satellite-based data that have not yet been studied for periods 2001–2022. This PhD thesis work reports the spatiotemporal variability of aerosol particles and their optical interactions with the cloud parameters and radiation budget over East Africa, with particular interest in Ethiopia. The study covers sixteen selected stations in East Africa-Ethiopia with neighbouring doughters Eritrea, Djibouti, and South Sudan countries clustered into four regions for the periods of 2001–2022 to obtain detailed information on the spatiotemporal behaviours of aerosol particles and their effects on clouds and radiation budget. The aerosol optical parameters, Ångström exponent AET calculated from the aerosol optical depth AOD, cloud top pressure CTP, cloud top temperature CTT, mean cloud fraction MCF, and atmospheric water vapor AWV were extracted from the Moderate Resolution Imaging Spectroradiometer MODIS satellite data. We collected precipitation PPT data from the Tropical Rainfall Measuring Mission TRMM, and outgoing long-wave radiation OLR flux is collected using clouds and the Earth’s Radiant Energy System CERES satellites. According to the results, there is a significant variation in the daily AOD and AET, with maximum values most likely occurring between August 11 and September 15 for Aqua and between June 22 and July 24 for both Terra and Aqua in the southeast and northeast clusters. The results range from 0.00 to 2.10 and 0.67 to 1.23. The OLR, CTP, and CTT parameters are out of phase with AOD and increase-decrease swings with AET

2024-07-01T00:00:00Z Zewdie, Yayeh http://ir.bdu.edu.et/handle/123456789/16819 2025-07-31T06:21:07Z 2025-02-01T00:00:00Z

Study of The Interplay Between Superconductivity and Antiferromagnetism In Nd2-Xcexcu𝐎𝟒−𝛅 Superconductor Zewdie, Yayeh The current PhD research work focuses on the study of the interplay between antiferromagnetism and superconductivity in electron doped cupper based Nd2-xCexCuO4−δ (NCCO) superconductor. Within this system, we have considered the effect of layer coupling, electron doping and annealing, and magnetic field order on superconducting transition temperature (TC). By formulating the system Hamiltonian and by using the retarded double time temperature dependent Green’s function technique, the TC of Nd2−xCe𝑥CuO4 supercponductor is computed quantitatively by considering interlayer interactions as independent variables. Furthermore, we have obtained the mathematical expression for the temperature dependence of superconducting order parameter. The corresponding phase diagrams are also plotted from which we have observed the enhancement of TC for n ≤ 4. Moreover, we have studied the dependence of TC on electron doping, oxygen reduction and magnetic field order in NCCO superconductor by employing the Green’s function technique and Tikhonov regularization method. We have obtained the mathematical expression for the parameters of TC and antiferromagnetic transition temperature (TN) as a function of doping and magnetic field order for NCCO system. By using Tikhonov regularization method, we have determined the quantitative inter-connection of oxygen loses, δ and Ce (x). Furthermore, we have demonstrated that, when an external magnetic field is applied, TC is suppressed whereas TN is enhanced. Moreover, we have calculated the quantitative values of TC as a function of doping and found that, the critical temperature of NCCO occurs in the range of x between 0.05 and 0.27 with TC,max ≈ 25.63 K at x ≈ 0.157, δ~0.02. The electron doping (n type) superconducting phase appears upon suppressing (or degrading) the antiferromagnetic order at about, x = 0.05 − 0.12. This shows the occurance of strong interplay between antiferromagnetism and superconductivity. Furthermore, in this research work, we have investigated the influence of doping, angle and temperature on upper critical magnetic field in layered NCCO superconductor with two band model. By employing the Ginzburg-Landau free energy density functional theory (GL theory), we have calculated the doping and temperature dependence of upper critical magnetic field (HC2), Ginzburg Landau coherence length (ξGL) and Ginzburg Landau penetration depth (ʎGL) based on the configurations of the applied magnetic field parallel to both in plane and out of plane for NCCO superconductor. By using some plausible experimental values for the expression of HC2, ʎGL and ξGL, we have plotted the correspondimg phase diagrams. At optimal doping levels of xopt = 15, xv ||ab ≈ 72.5 T δopt ≈ 0.02, the in plane and out of plane upper critical fields are generated as HC2 and HC2 ||c ≈ 8.05 T respectively. Similarly, for x = 0.12, HC2 of the system becomes, HC2 65.15 T and HC2 ||ab ≈ ||c ≈ 5.05 T which are lower than the values of upper critical magnetic field that are calculated at optimally doping level for NCCO since at optimal doping, antiferromagnetism is suppressed completely resulting in the enhancement of superconducting state so that causes HC2 to be increased. These values are sufficient to suppress the superconductivity because within the under doped regime, an applied magnetic field enhances magnetic order (antiferromagnetism) that could be suppressed by increasing the electron doping reduction annealing. This shows the interplay between magnetism and superconductivity. Moreover, the result shows that the upper critical magnetic field along the ab-plane is greater than the upper critical magnetic field along the out of plane. Our results are in consistence with the previous work. The angle dependent upper critical magnetic fields are also calculated according to their geometry and depict similar trends. Likewise, we have calculated the saturated values of coherence length along the in plane and out of plane to be 𝜁𝐺𝐿 𝑎𝑏~ 6.52 nm and 𝜁𝐺𝐿 𝑐 ~ 0.73 respectively at optimal doping. The theoretical calculations indicate that, the coherence length along the c-axis is less than that of the ab-plane and both decrease as a function of the applied magnetic field. Furthermore, the inter-plane penetration depth, ʎGL ∥c and in plane penetration depth, ʎGL ∥ab are calculated for optimally doped and non-optimally doped Nd2−xCexCuO4−δ superconductor and show significant variations. Similarl to the dome-like shape of the doping dependence of the superconducting transition temperature, both ʎGL ||ab and ʎGL ∥c increase with increasing doping in the under doped regime and become very large near TC at optimal doping ( x ≈15,δ~0.02 ). Finally, we have plotted the phase diagrams for Ginzburg Landau characteristic parameters, K, in plane, KGL ab and inter-plane, KGL c from which the large values indicate that, Nd2−xCexCuO4−δ is a type II superconductor. The results we obtained in this research work are in broad agreement with the experimental findings

2025-02-01T00:00:00Z