Browsing by Author "Elfagd, Yitagesu (PhD)"
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Item Analysis of Tec Variation During June 21, 2020 Annular Solar Eclipse Over Addis Ababa and Djibouti(Addis Ababa University, 2021-10-10) Sete, Shibre; Elfagd, Yitagesu (PhD)On June 21, 2020, an annular solar eclipse happened. The annular eclipse's central path passed through parts of Central and Eastern Africa. A partial solar eclipse was visible across much of Africa and it traverses Ethiopia and Djibouti. During a solar eclipse the Moon blocks a portion of the solar radiation from reaching the Earth. Following the eclipse a decrease in the ionization is expected due to the obscuration of solar radiation, and thus a decrease in the amount of ionospheric total electron content (TEC). The e_ect of this partial solar eclipse on the ionospheric TEC was studied using two Global Positioning System (GPS) stations found in Addis Ababa and Djibouti. The objective of this thesis is to study the e_ect of the June 21, 2020 annular eclipse on the ionospheric TEC. Because eclipse geometry varies from eclipse to eclipse, it's not always clear whether a di_erence in ionospheric reaction is due to a change in eclipse time of occurrence, obscuration level, geometry, or background conditions such as: di_erent solar and geomagnetic activities, seasons, latitudes and longitudes. Therefore, individual eclipse studies are signi_cant. TEC data taken from the two GPS stations were used to examine the ionospheric behavior during the eclipse. The partial solar eclipse was observed to cause a signi_cant TEC depletion during the eclipse period at both stations. Which indicates that the amount of TEC in the ionosphere was a_ected by the decrease in solar radiation during the eclipse. The TEC measurements during the eclipse period were compared to two days before, two days after the eclipse period and three years of June 21 to make sure that the cause of the TEC depletion was not due to a daily variation in the ionosphere. A maximum TEC reduction of up to 35% were observed compared to the days after and before the eclipse. A time lag of 45 and 15 minutes were found between the maximum obscuration and a maximum TEC depletion over ADIS and DJIG, respectively. After the eclipse ended, the TEC began to return to normal levels and recovered fully after 1 hr over DJIG and 1 hr and 18 minutes over ADIS stations. The TEC reduction shows the partial solar eclipse a_ects the ionospheric TEC and since the ionosphere is mostly governed by the photoionization, the decrease in solar radiation causes the density of the ionosphere to decrease. The eclipse occurred during a period of low solar and magnetic activity. The reference days TEC data and the magnetic activity indicated the eclipse was the cause of the TEC depletion. Further research is needed to study the e_ect of the eclipse in the low latitude region as it can be a_ected by the equatorial ionization anomaly.Item Analysis of the Factors Affecting Ionospheric Total Electron Content in East Africa and Middle East(Addis Ababa University, 2021-11-29) Eshetu, Mulu; Elfagd, Yitagesu (PhD)The low latitude and equatorial ionospheric total electron content (TEC) variations are investigated using dual frequency GPS TEC data taken from 7 GPS stations in which 6 are found in East Africa and one is found in Saudi Arabia in the year of 2015. This GPS TEC data was used to study the diurnal, seasonal, geomagnetic and latitudinal variations of ionospheric TEC at these GPS stations as insu_cient research results of ionospheric variations are available for these areas. The results of the analysis showed that the ionospheric TEC undergoes diurnal, seasonal, geomagnetic (quiet and disturbed day) and latitudinal variations. With regard to the diurnal variation, the TEC value shows minimum at predawn and gradual increase with time of the day attaining a maximum in the afternoon and a gradual decrease after sunset. This is the result of photo ionization of the atmosphere due to the increased intensity of solar radiation during day. On the other hand during night electron loss processes (ion-electron recombination and electron attachment) dominate and the TEC value is decreased. The TEC value in March shows a very rapid decrease after sunset due to the high recombination rate that is proportional to the square of the high electron density in March. Moreover, the night time TEC value shows a secondary minima maxima near dawn which may be due to the Counter Electrojet (CEJ). The TEC value in the solstice months of June and December don't show plateau and secondary maxima. It simply increase till noon and then decreases slowly during afternoon and during night. The seasonal variation is caused by the relative position of the Earth with the sun which changes the solar zenith angle. The greatest TEC value occurred in the equinoctial month of March and the least occurred during the winter solstice month of December and intermediate TEC value is recorded in summer solstice month of June. The e_ects of geomagnetic storms on TEC values have been also considered in this study. The result reveals that the storm day TEC value is greater than the quiet day TEC value. This is due to the fact that high amount of solar particles reach the earth and there is more ionization during the storm period. The TEC value also vary with latitude. The highest TEC value is obtained for stations that are closer to the geomagnetic equator (MOIU in Kenya) in the southern hemisphere. This may be the result of the occurrence of highly radiation of the sun and the DEBK station in Ethiopia located around EIA crest region is due to the fountain e_ect. The least value of TEC is observed for stations far from the geomagnetic equator (SOLA in Saudi Arabia) which is located beyond the EIA region. Moreover, TEC value of January 3, 2015 (at perihelion) is greater than that of the July 3, 2015 (aphelion), 2015. For the future more research need on the perihelion and aphelion cases of ionospheric TEC.Item Charge Transport Across Metal N-Type Semiconductor Interfaces.(Addis Ababa University, 2018-11-05) Tamrat, Feredelign; Elfagd, Yitagesu (PhD)In this study, we considered the thermionic emission, field emission and tunneling and we derived current density as a functions of different parameters such as, temperature, barrier height, work function, bias voltage and dopant concentration for thermionic emission, field emission and tunneling enhanced electron transport across metal and N-type semiconductor interfaces. The result shows that the thermionic current density is increasing exponentially as a functions of bias voltage, the current density is also increasing as a quadratic functions of temperature and current density is increasing linearly as a functions of dopant concentration. Therefore, we carry out investigation how these factors affect the current density and we solve the current density analytically with a given parameters. The results are plotted using gnuplot.Item Charge Transport Across Metal P-Type Semiconductor Interfaces(Addis Ababa University, 2019-07-03) Mekonnen, Ermias; Elfagd, Yitagesu (PhD)In this study, we considered the thermionic emission, field emission and tunneling and we derived current density as a functions of different parameters such as, temperature, barrier height, work function, bias voltage and dopant concentration for thermionic emission, field emission and tunneling enhanced electron transport across metal and P-type semiconductor interfaces. The result shows that the thermionic current density is increasing exponentially as a functions of bias voltage, the current density is also increasing as a quadratic functions of temperature and current density is increasing linearly as a functions of dopant concentration. Therefore, we carry out investigation how these factors affect the current density and we solve the current density analytically with a given parameters. The results are plotted using gnuplot.Item Climatology of the Lower and Middle Atmosphere(Addis Ababa University, 2022-02-26) Tesfaye, Befikadu; Elfagd, Yitagesu (PhD)Climate change is one of the de_ning issues of our time. It is now more certain than ever based on many lines of evidence that humans are changing the earth's climate. The atmosphere and oceans have warmed, accompanied by the sea level rise, strong decline in arctic sea ice, and other climate related changes. The evidence is clear however, due to the nature of science, not every single detail is ever totally settled or completely certain nor has every pertinent question yet been answered. Scienti_c evidence continues to be gathered around the world and assumptions and _ndings about climate change are continually analyzed and tested. Some areas of active debate and ongoing research include the link between the ocean heat content and the rate of warming estimate of how much warming to expect in the future and the connection between climate change extreme weather events. The focus of this study is aimed to provide a brief over view of climatology at the lower and middle atmosphere, the atmospheric variables the signi_cance of the composition of the atmosphere and their inuence on life on the earth's surface and will extremely deal with the climate system, climate system components, observed climate variability and changes. It also focuses on the e_ect of greenhouse gases, the carbon cycle and atmospheric carbon dioxide, aerosols, their direct and indirect climatic e_ects in the middle and lower atmosphere.Item Coulomb Blockade Oscillations of Conductance in Quantum Dots(Addis Ababa University, 2018-12-04) Bacha, Adisu; Elfagd, Yitagesu (PhD)In this thesis, we studied the Coulomb blockade oscillations of conductance through quantum dots. Quantum dots provide a new challenge to theoretical calculations for their properties using many body methods. We consider a quantum dot, which is weakly coupled by tunneling barriers to two electron reservoirs. Using the constant interaction model between the charge carriers have calculated the conductance G of the quantum dot is defined as G = I V , in the limit V ! 0. We have shown the effect of the charging and thermal energy, and level spacing on the conductance. Since transport through quantum dot proceeds by tunneling through its discrete electronic states, it will be clear that for a small V a net current can flow only for certain values of the gate voltage (if _E >> kBT). We found an analytical expression for the conductance of a single electron transistor in the regime when temperature, level spacing, and charging energy of a dot are all of the same order. In addition, we consider the model of equidistant energy levels in a dot in the sequential tunneling approximation. For _ = 0.3, the normalized conductance is highly oscillating with the largest amplitude and the normalized conductance becomes less oscillatory as _ increases and in our case the least oscillation occurs for _ = 0.8 which is consistent with the fact that the conductance becomes constant at high temperature. To carried out the study we used a thermal energy for different level of spacing energy. In this work we have done the numerical computation using fortran program and plotted the result using gnuplot. We also studied the Coulomb blockade oscillations of conductance at high and low temperatures using the variation of level spacing and thermal energy. At low temperature there are sharp Coulomb blockade peak and for high temperatures kBT >> Ec, _E, Coulomb blockade is lifted and small oscillations of the conductance can be observed.Item Physics of Cloud and Precipitation(Addis Ababa University, 2022-04-11) Kuchi, Nigatu; Elfagd, Yitagesu (PhD)In this project we present a review of the physics of clouds and precipitation in the atmosphere. Thunderstorms are a global phenomenon in the mid-latitudes and tropics. They form where and whenever the ingredients for their formation come together: instability, moisture and lift. Especially upon interaction with vertical wind shear, they may develop in to wellorganized systems that produce hazards such as large hail, severe winds, heavy precipitation, and tornadoes. We must look in to the processes by which the clouds are formed and precipitation is produced in order to the meaning of clouds they related to weather. We will see how clouds are classified and, named and what kinds of precipitation certain types of clouds produce. And also in this project we study how to clouds form and precipitation develop in the atmosphere must be saturated with moisture. As a cold air passes over warm water, rapid evaporation takes places and the saturation is quickly reached. Lifting of air, and the resulting adiabatic expansion, is the most important cooling method. The lifting may be accomplished by thermal orographic or frontal action. It produces most of the clouds and precipitation.Item Solar Radiative Transfer and Global Climate Modelling(Addis Ababa University, 2021-12-10) Dagnaw, Habtam; Elfagd, Yitagesu (PhD)In this project we have studied Solar radiative transfer and global climate modelling. We consider a parallel beam of electromagnetic radiation incident on a slab of thickness ds. As this radiation passes through the atmosphere it interacts and is being absorbed by the atmospheric gases, the continental lands and oceans and large water bodies. In this project work, we describe the optical depth or optical thickness, the intensity or spectral radiance, absorption, reflection, scattering of particles by applying Beer’s law or Lambert’s law. The intensity of the incident radiation decreases exponentially as a function of solar zenith angle, fraction of absorbers in the atmosphere, the scattering coefficient, scale height, altitude and density of the atmosphere. The absorption and emission of radiation by the atmospheric gases causes climate changes. More over the major climate system components (atmosphere, land surface, ocean, and sea ice), and their interactions are described using the basic equations that govern the atmosphere. These can be formulated as a set of seven equations with seven unknowns such as the three velocity components (u, v, w), the pressure p, the temperature T, the specific humidity q and the density _. These equations, solved numerically by computers, are used to produce our weather forecasts. While there are details about these equations which are uncertain (for example, how we parameterize processes smaller than the grid size of the models), the equations for the most part are accepted as fact. The radiation that passes through the atmosphere is absorbed by the lands and ocean surfaces and is re-emitted as infrared back into the atmosphere so as to heat it and this causes climatic changes.Item Thermodynamic Properties of Strongly Degenerate Fermi Gas(Addis Ababa University, 2018-10-05) Ayenew, Ayalew; Elfagd, Yitagesu (PhD)In this project we have studied thermodynamic properties of strongly degenerate Fermi gases. Using the basic law of thermodynamic we derived Grand potential ( ), number of fermions (N), Heat capacity (Cv), pressure (p), Energy (E), Entropy (S), Chemical potential (μ), Magnetization (M) as a functions of temperature (T) and magnetic field (H). We have also the concept of pauli paramagnetism and Landau diamagnetism in degenerate fermi gas (Electron gas). Generally, we can get the result from the derived equation the plot of these parameters as temperature increase the grand potential is large negative. As temperature increases the chemical potential decreases, the pressure increases as the temperature increases, the pressure decreases as magnetic field increases at low temperature. The total energy versus magnetic field graph shows that the energy increases rapidly when magnetic field is less than 1500 T. The density of state has discontinuous behavior and has a real and positive value. We also observed that as the total energy, grand potential and pressure depend on density of states and thus show similar behavior.Item Thermodynamics and Quantum Properties of Ultra-Relativistic Degenerate Electron Gas in White Dwarf and Neutron Stars(Addis Ababa University, 2022-01-07) Ali, Ibrahim; Elfagd, Yitagesu (PhD)We study the problem of the relativistic free electron gas at arbitrary degenerate electron gas. The speci_c heat at constant volume and particle number cv and the speci_c heat at constant pressure and particle number cp are calculated. The equation of state is also studied. Non degenerate and degenerate electron limits are considered. We generalize the formulas obtained in the non-relativistic and ultra-relativistic degenerate electron gas. Neutron stars are much denser than white dwarf stars, which, once again, causes the core of the stars to collapse. The compression of neutrons in the contracting core, however, creates a neutron degenerate pressure. This pressure, analogous to the electron degenerate pressure in white dwarf stars, combats the gravitational collapse of the star. If, however, the neutron star is too massive (more than three solar masses), the neutron degenerate pressure fails and the neutron star collapses into a black hole. We now see that the role of both the neutron degenerate pressure, and the electron degenerate pressure is crucial to the maintained stability of a star The energy corresponding to this momentum, called the Fermi energy which we will discuss in the next section will also increase. So, with a decreasing volume and an increasing particle momentum, we can say that a pressure formed inside of the core of the star, and continues to increase as long as the volume continues to increase, and that there are degenerate neutron energy states. Now that we know where the pressure comes from, we can _nally derive a mathematical expression for the neutron degeneracy pressure by non-relativistic neutrons inside of a neutron star.Item Thermodynamics of Charge Enhanced Nucleation and Cloud Droplets Formation and Growth:(Addis Ababa University, 2020-12-12) Yibeltal, Gizachew; Elfagd, Yitagesu (PhD)In this study we considered thermodynamics of the atmosphere,under this We have seen moist air humidity variables mean molecular weight of moist air and other quantities and we derived the virtual temperature Tvrt.We also considered surface tension and nucleation of cloud droplets. we studied homogeneous and heterogeneous nucleation as well as droplet growth and derived the change in droplet radius rd. The result shows that droplet radius grows as the square root of time and how the droplet radius reduces in time through evaporation. In addition to this, we considered thermodynamics of charge enhanced nucleation formation and growth, the source of charge droplet which are,charge separation by the differential motion of drops of different sizes, or the always-present electric current between ionosphere and the Earths surface, which can charge up individual droplets at cloud boundaries. The result shows increasing the charge Q also increases the amount of work done against the electrostatic potential.