B. Damtie

Characteristics of large‐scale wave structure observed from African and Southeast Asian longitudinal sectors

The spatial large-scale wave structure (LSWS) at the base of F layer is the earliest manifestation of seed perturbation for Rayleigh-Taylor instability, hence, found to play a deterministic role in the development of Equatorial Plasma Bubbles (EPBs). Except for a few case studies, a comprehensive investigation has not been conducted on the characteristics of LSWS because of the complexity involved in detecting the LSWS, particularly, in spatial domain. In this scenario, a comprehensive study is carried out, for the first time, on the spatial and temporal characteristics of LSWS observed in spatial domain over African and Southeast Asian longitudinal sectors during the year 2011. The observations indicate that these wave structures can be detected a few degrees west of E region sunset terminator and found to grow significantly at longitudes past the sunset terminator. The phase fronts of these spatial structures are found to align with the geomagnetic field (B→) lines over a latitudinal belt for at least 5−6° (~500–600 km) centered on dip equator. The zonal wavelengths of these structures are found to vary from 100 to 700 km, which is consistent with the earlier reports, and the EPBs were consistently observed when the amplitudes of LSWS were grown to sufficient strengths. These results would provide better insights on the underlying physical processes involved in excitation of LSWS in terms of important roles being played by E region electrical loading and polarization electric fields induced via spatially varying dynamo current due to neutral wind perturbations associated with atmospheric gravity waves.

Mismatched Filtering of Aperiodic Quadriphase Codes

Matched filtering of quadriphase coded radar signals creates unwanted sidelobes, which may mask important information. This correspondence presents a method of eliminating these sidelobes in aperiodic quadriphase codes. This is done by using a mismatched filter. The signal-to-noise ratio (SNR) at the output of the mismatched filter is less than the SNR at the output of the corresponding matched filter. A thorough computer search has been carried out to find quadriphase code-mismatched filter pairs with minimum possible SNR losses by considering a point target in the presence of white Gaussian noise. The phase patterns of the optimal codes, which were chosen from a total number of 1.466 times 1012 investigated codes, are shown. It turned out that the mismatched filter of a quadriphase Barker code with 15 elements (1, 1, 1, j, j, 1, - j, - j, j, -1, - j, j, 1, -1, 1) has a loss in SNR of only 6.7% when compared with the corresponding matched filter. The mismatched filter of the well known 13-element Barker code, which has a loss in SNR of 4.8%, outperforms this filter. Finally, using numerical experiment it is shown that a randomly selected long code will most likely have very large SNR losses.

Validation of NeQuick TEC data ingestion technique against C/NOFS and EISCAT electron density measurements

This paper investigates a technique to estimate near-real-time electron density structure of the ionosphere. Ground-based GPS receiver total electron content (TEC) at low and high latitudes has been used to assist the NeQuick 2 model. First, we compute model input (effective ionization level) when the modeled slant TEC (sTEC) best fits the measured sTEC by single GPS receiver (reference station). Then we run the model at different locations nearby the reference station and produce the spatial distribution of the density profiles of the ionosphere in the East African region. We investigate the performance of the model, before and after data ingestion in estimating the topside ionosphere density profiles. This is carried out by extracting in situ density from the model at the corresponding location of C/NOFS (Communication/Navigation Outage Forecast System) satellite orbit and comparing the modeled ion density with the in situ ion density observed by Planar Langmuir Probe onboard C/NOFS. It is shown that the performance of the model after data ingestion reproduces the topside ionosphere better up to about 824 km away from the reference station than that before adaptation. Similarly, for high-latitude region, NeQuick 2 adapted to sTEC obtained from high-latitude (Tromso in Norway) GPS receiver and the model used to reproduce parameters measured by European Incoherent Scatter Scientific Association (EISCAT) VHF radar. It is shown that the model after adaptation shows considerable improvement in estimating EISCAT measurements of electron density profile, F2 peak density, and height.

Temporal and Spatial Climate Variability and Trends Over Abay (Blue Nile) River Basin

The spatial and temporal variabilities of climate in Abay (Blue Nile) river basin from 1979 to 2014 have been studied using both station and satellite based observations. Rotated empirical orthogonal function (REOF), regression and wavelet analyses were used to investigate the trend, frequency and intra-annual variability of climate over the Abay (Blue Nile ) river basin. High variability of rainfall has occurred over the western regions during spring and southern regions during summer seasons from 1979–2014. The results have shown that, the variability over the regions is dominated by inter-decadal signals, which is similar periodic variability with large-scale circulation. Different conditions of atmospheric and oceanic anomalies are responsible for the observed negative/positive anomaly years in the two seasons. For instance, negative anomaly years during spring season are significantly associated with the negative phase of El Nino Southern Oscillation (ENSO), the weakening of the Arabian High and the subtropical westerly jet streams whereas during summer they are linked to the positive ENSO phase, a weakening of the upper level jet streams, and a lower level moisture influx from the Atlantic and Indian oceans. Un-even heating of equatorial Pacific, Atlantic and Indian Ocean significantly affects extreme anomaly years during both seasons via altering the rain producing large-scale circulations over Abay river basin.

Study of TEC fluctuation via stochastic models and Bayesian inversion

We propose stochastic processes to be used to model the total electron content (TEC) observation. Based on this, we model the rate of change of TEC (ROT) variation during ionospheric quiet conditions with stationary processes. During ionospheric disturbed conditions, for example, when irregularity in ionospheric electron density distribution occurs, stationarity assumption over long time periods is no longer valid. In these cases, we make the parameter estimation for short time scales, during which we can assume stationarity. We show the relationship between the new method and commonly used TEC characterization parameters ROT and the ROT Index (ROTI). We construct our parametric model within the framework of Bayesian statistical inverse problems and hence give the solution as an a posteriori probability distribution. Bayesian framework allows us to model measurement errors systematically. Similarly, we mitigate variation of TEC due to factors which are not of ionospheric origin, like due to the motion of satellites relative to the receiver, by incorporating a priori knowledge in the Bayesian model. In practical computations, we draw the so-called maximum a posteriori estimates, which are our ROT and ROTI estimates, from the posterior distribution. Because the algorithm allows to estimate ROTI at each observation time, the estimator does not depend on the period of time for ROTI computation. We verify the method by analyzing TEC data recorded by GPS receiver located in Ethiopia (11.6°N, 37.4°E). The results indicate that the TEC fluctuations caused by the ionospheric irregularity can be effectively detected and quantified from the estimated ROT and ROTI values.

Research Needs in the Lake Tana Basin Social-Ecological System

This chapter synthesizes the research needs presented in the book and proposes actions for addressing them. It highlights the socioeconomic and biophysical characteristics, development, production and consumption patterns, challenges and threats and management approaches that were discussed in the preceding chapters and discusses the priorities for future Lake Tana basin research that were raised. The analysis shows that the basin has many unique features, and great potential for sustainable development, especially in water resources. The lake and basin provide multiple benefits to the local community, the region and downstream riparian countries. Many of the problematic trends in the region result from an imbalance between resource production and consumption patterns, and limited or poorly implemented conservation measures. The many and complex challenges and threats in the basin are caused, in part, by the lack of an integrated basin plan and poor collaborative mechanisms to share data and find sustainable solutions. The available data and information about the basin is limited. It is not well-organized, accessible, or used well for the basin’s resource management. The identification of major research gaps and priorities for future research highlighted in this book, as well as the systems framework for integration and collaboration, provide a basis for further research and policy decisions to promote sustainability in the region.

Introduction: Regional Challenges and Policy Questions

The Lake Tana Basin is the headwater catchment of the Upper Blue Nile River. This highly dynamic region is experiencing significant population, economic, ecosystem, environment and social changes, raising concerns about sustainable development at regional, national and international levels. About 85% of the approximately three million people that live in the basin currently work in the agricultural sector, but factors such as population growth, environmental degradation, and rising education levels are driving migration from rural to urban areas and putting pressure on urban infrastructure and economy. The strong links between the region’s human and environmental conditions mean that developing the region sustainably requires an understanding of its social and ecological characteristics as well as a framework for examining how the social system and the ecological system interact. This chapter describes key policy challenges in the region that motivated this book and lays out the structure of the sections and chapters that follow.

Scintillation of GNSS signals at equatorial latitudes

A particular threat to global navigation satellite systems (GNSS) are small scale ionospheric disturbances. These can lead to fluctuations of the received satellite signal, so called signal scintillations. Strong scintillations can lead to a loss of lock between satellite and receiver. All GNSS signals are affected by this phenomenon. The influence of the short scale disturbances on the different GNSS signals is expected to be different for each signal, since the signals are transmitted by different carrier frequencies and are constructed in different ways.

Temporal solar irradiance variability analysis using neural networks

A feed-forward neural network which can account for nonlinear relationship was used to model total solar irradiance (TSI). A single layer feed-forward neural network with Levenbergmarquardt back-propagation algorithm have been implemented for modeling daily total solar irradiance from daily photometric sunspot index, and core-to-wing ratio of Mg II index data. In order to obtain the optimum neural network for TSI modeling, the root mean square error (RMSE) and mean absolute error (MAE) have been taken into account. The modeled and measured TSI have the correlation coefficient of about R=0.97. The neural networks (NNs) model output indicates that reconstructed TSI from solar proxies (photometric sunspot index and Mg II) can explain 94% of the variance of TSI. This modeled TSI using NNs further strengthens the view that surface magnetism indeed plays a dominant role in modulating solar irradiance.

Observations of ULF wave related equatorial electrojet and density fluctuations

Global magnetospheric Ultra Low Frequency (ULF) pulsations with frequencies in the Pc 4–5 range (f = 1.0–8 mHz) have been observed for decades in space and on Earth. ULF pulsations contribute to magnetospheric particle transport and diffusion and play an important role in magnetospheric dynamics. However, only a few studies have been performed on ionospheric observations of ULF wave-related perturbations in the vicinity of the equatorial region. In this paper we report on Pc5 wave related electric field and thus vertical drift velocity oscillations at the equator as observed by ground magnetometers and radar. We show that the magnetometer estimated equatorial ExB drift oscillate with the same frequency as ULF Pc5 waves, creating significant ionospheric density fluctuations. For independent confirmation of the vertical drift velocity fluctuation, we used JULIA 150 km radar drift velocities and found similar fluctuation with the period of 8–10 minutes. We also show ionospheric density fluctuations during the period when we observed ULF wave activities. All these demonstrate that the Pc5 wave can penetrate to the equatorial ionosphere and modulate the equatorial electrodynamics. Finally, in order to detect the ULF activities both on the ground and in space, we use ground-based magnetometer data from African Meridian B-field Education and Research (AMBER) and the South American Meridional B-field Array (SAMBA). From space, we use magnetic field observations from the GOES 12 and the Communication/Navigation Outage and Forecast System (C/NOFS) satellites. Using the WIND spacecraft as the upstream solar wind monitor, we present direct evidence that solar wind number density and ram pressure fluctuations observed far upstream from the terrestrial magnetosphere are the main drivers of ULF wave activity inside the magnetosphere. Finally, we show that the ULF waves in the same frequency range are observed in the magnetosphere by the geosynchronous GOES spacecraft, in the ionosphere by the equatorial C/NOFS satellite, and on the ground by ground-based magnetometers, indicating that the magnetospheric origin ULF wave can penetrate to the ground equatorial region and modulate the equatorial electrodynamics.