Daniel Okoh - Academia.edu (original) (raw)

Papers by Daniel Okoh

Annales geophysicae, Jul 8, 2024

Social Science Research Network, 2021

Abstract There is increasing interest to understand thermospheric neutral wind and ionized plasma... more Abstract There is increasing interest to understand thermospheric neutral wind and ionized plasma coupling, especially by using empirical observations. This is the first study that reports simultaneous measurements of thermospheric neutral wind and equatorial plasma bubble (EPB) speeds over the African equatorial region. Neutral wind speeds were obtained using a Fabry-Perot interferometer (FPI), while EPB speeds were obtained using an all-sky optical imager. Both instruments are co-located in Abuja (Geographic: 8.99°N, 7.38°E; Geomagnetic: 1.60°S). Data for 11 nights of coincident neutral wind and EPB speed observations were used. The neutral wind speeds were in the range of about −30 to 300 m/s, while EPB speeds were in the range of about 15–400 m/s. The speeds of both neutral winds and EPBs were generally observed to be faster during the early night hours and to slow down towards midnight. In some instances, the neutral wind and EPB speeds were comparable, an observation that is in agreement with full development of the F region dynamo. There were also instances in which the EPBs were faster or slower than the neutral winds. In one of the nights, a southward turning of the IMF-Bz was observed to cause a sudden increase in the EPB speed. For some nights, the neutral winds were faster than the EPBs during earlier hours of the nights, and the disparity reduced during the later hours of the nights. This was attributed to the possibility that the F region dynamo was yet to be fully developed during the earlier hours.

The program is used for the purpose of extracting ionospheric parameters (electron density, total... more The program is used for the purpose of extracting ionospheric parameters (electron density, total electron content (TEC), critical plasma frequency (foF2), peak value of electron density (NmF2), and the height of peak electron density (HmF2)) from ionPrf NetCDF files of the COSMIC (Constellation Observing System for Meteorology Ionosphere and Climate) mission. The data files should have first been downloaded from the site: https://data.cosmic.ucar.edu/gnss-ro (The program at http://doi.org/10.5281/zenodo.4405487 can be useful for bulk downloads). After downloading the files, each file should be uncompressed to a separate folder. Folders of the uncompressed files for the same year should be kept in the same folder that should be named according to the year, e.g. 2020 for data of year 2020. The MATLAB program should be kept in the same folder containing the year folders and run from there. The electron densities will be extracted from the NetCDF files and stored in text files that are contained in a folder named "Edensity" within the same folder from which the program is run. The text files are named in the format: YYYY.DDD where YYYY is the year and DDD is the day of the year. The other ionospheric parameters (TEC, foF2, NmF2, and HmF2) will be extracted from the NetCDF files and stored in text files that are contained in a folder named "TEC_F2parameters" within the same folder from which the program is run. The text files are named according to the year of the data, for example; 2019.txt for year 2019 data.

Journal of Geophysical Research, 2017

The program is a MATLAB program for bulk downloads of specified ionPrf data files from the COSMIC... more The program is a MATLAB program for bulk downloads of specified ionPrf data files from the COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) mission over a specified period of time. Users can specify the start and end years for which they require COSMIC data in lines 1 and 2 respectively of the program. The program is particularly for the cosmic2 ionPrf provisional data, but a slight modification can be done to line 18 of the program to use it for the other data sets. In that case, the path in line 5 of the program can be changed accordingly as follows: Path for cosmic1 postProc data (DOY 121 of 2014 to 2020) is: 'https://data.cosmic.ucar.edu/gnss-ro/cosmic1/postProc/level2/' Path for cosmic1 repro2013 data (DOY 112 of 2006 to DOY 120 of 2014) is: 'https://data.cosmic.ucar.edu/gnss-ro/cosmic1/repro2013/level2/' Path for cosmic2 provisional data (DOY 274 of 2019 to near real time) is: 'https://data.cosmic.ucar.edu/gnss-ro/cosmic2/provisional/spaceWeather/level2/' <br> The downloaded data files appear in the same folder as the program.

VizieR Online Data Catalog, Nov 1, 2017

Effects of space weather are usually related to disturbances in the ionosphere such as change in ... more Effects of space weather are usually related to disturbances in the ionosphere such as change in total electron content (TEC), which is an important descriptive quantity for the ionosphere of the earth. This has an important effect on the Global Navigation Satellite System (GNSS) as it causes delay in range measurements. The knowledge of this TEC change is therefore very significant to various GNSS users. This work presents the variation of TEC over the equatorial station of Abuja, Nigeria (Geographic: 7.49 °E, 9.03 °N, Altitude 533.60 m; Geomagnetic: 79.49 °E, 1.60 °S) using GNSS data collected over an year 2013 (a year of moderate solar activity), and is one of the first studies of TEC variation over Abuja, Nigeria done using data from the GNSS. The results obtained from this work show significant diurnal and seasonal TEC variations. It was specifically observed that the TEC values were relatively higher during the equinoxes than the solstices, but the high values of TEC were also observed during the December solstice. The highest TEC values were recorded in the month of March, which is an equinox month. Keywords: TEC, GNSS, Abuja, equatorial, ionosphere Cite this Article Okoh and Oliha. A study of total electron content variations over Abuja (Nigeria) using the GNSS. Research and Reviews: Journal of Space Science and Technology. 2015; 4(3): 1–6p.

AGU Fall Meeting Abstracts, Dec 1, 2017

Doing graphics with MATLAB can be interesting, the results look quite appealing, and a generality... more Doing graphics with MATLAB can be interesting, the results look quite appealing, and a generality of users prefer it for its robustness and portability: Graphics in MATLAB can be tweaked to take different desired looks, and can be easily exported into various other applications. This book is a very modest 1-Chapter guide on how to use MATLAB to develop 2-D static graphics. Lots of examples and illustrations have been used to enhance readers understanding of the subject.

Proceedings of SPIE, Jul 16, 2010

ABSTRACT Generally, panels of radio telescopes are mainly shaped in trapezoid and each is support... more ABSTRACT Generally, panels of radio telescopes are mainly shaped in trapezoid and each is supported/positioned by four adjustors beneath its vertexes. Such configuration of panel supporting system is essentially hyper-static, and the panel is overconstrained from a kinematic point of view. When the panel is to be adjusted and/or actuated, it will suffer stress from its adjusters and hence its shape is to be distorted. This situation is not desirable for high precision panels, such as glass based panels especially used for sub-millimeter and shorter wavelength telescopes with active optics/active panel technology. This paper began with a general overview of panel patterns and panel supports of existing radio telescopes. Thereby, we proposed a preferable master-slave active surface concept for triangular and/or hexagonal panel pattern. In addition, we carry out panel error sensitivity analysis for all the 6 degrees of freedom (DOF) of a panel to identify what DOFs are most sensitive for an active surface. And afterwards, based on the error sensitivity analysis, we suggested an innovative parallel-series concept hexapod well fitted for an active panel to correct for all of its 6 rigid errors. A demonstration active surface using the master-slave concept and the hexapod manifested a great save in cost, where only 486 precision actuators are needed for 438 panels, which is 37% of those actuators needed by classic segmented mirror active optics. Further, we put forward a swaying-arm based design concept for the related connecting joints between panels, which ensures that all the panels attached on to it free from over-constraints when they are positioned and/or actuated. Principle and performance of the swaying-arm connecting mechanism are elaborated before a practical cablemesh based prototype active surface is presented with comprehensive finite element analysis and simulation.

Research in Astronomy and Astrophysics, May 25, 2011

A 30-m TeraHertz (THz) radio telescope is proposed to operate at 200 µm with an active primary su... more A 30-m TeraHertz (THz) radio telescope is proposed to operate at 200 µm with an active primary surface. This paper presents sensitivity analysis of active surface panel positioning errors with optical performance in terms of the Strehl ratio. Based on Ruze's surface error theory and using a Monte Carlo simulation, the effects of six rigid panel positioning errors, such as piston, tip, tilt, radial, azimuthal and twist displacements, were directly derived. The optical performance of the telescope was then evaluated using the standard Strehl ratio. We graphically illustrated the various panel error effects by presenting simulations of complete ensembles of full reflector surface errors for the six different rigid panel positioning errors. Study of the panel error sensitivity analysis revealed that the piston error and tilt/tip errors are dominant while the other rigid errors are much less important. Furthermore, as indicated by the results, we conceived of an alternative Master-Slave Concept-based (MSC-based) active surface by implementating a special Series-Parallel Concept-based (SPC-based) hexapod as the active panel support mechanism. A new 30-m active reflector based on the two concepts was demonstrated to achieve correction for all the six rigid panel positioning errors in an economically feasible way.

OAlib, 2015

In this paper, presentation of specific features of TEC behaviour as a possible precursor of Orkn... more In this paper, presentation of specific features of TEC behaviour as a possible precursor of Orkney earthquake of August 5th, 2014 (M5.5) was made. To achieve this, TEC data from Suth station and GIM TEC maps were used. The period understudied (prior to and after the earthquake) experienced quiet geomagnetic situation which made the analysis much easier. A characteristic anomaly was noticed from 4 days before the earthquake and it was found out that the daytime TEC increases significantly and consistently relative to the background condition up to the day before the earthquake. To reveal the spatial dimensions of seismo-ionospheric anomaly longitudinally, the maps from IONEX database were used. The ionosphere, experiencing a modification as an increase of electron concentration over a large area, was revealed.

AGU Fall Meeting Abstracts, Dec 1, 2018

Frontiers in Astronomy and Space Sciences

We investigate the occurrence of ionospheric irregularities in Nigeria during 2014, using the Glo... more We investigate the occurrence of ionospheric irregularities in Nigeria during 2014, using the Global Navigation Satellite System (GNSS) total electron content (TEC) rate of change index (ROTI). We categorized days with |Dst| < 30 nT as quiet days and days with |Dst| ≥ 50 nT as disturbed days, during both the daytime and nighttime periods. Our results reveal significant latitudinal differences in both quiet and disturbed conditions. The quiet-time observations indicate that irregularities were consistent across all the stations. The occurrence was strong and consistent at CLBR station in Calabar, Nigeria, which is located further away from the geomagnetic equator (and closer to the southern anomaly crest) than the other receivers. TEC variability is greatly influenced by the geographical location of the Earth’s magnetic field, while its spatial patterns of variability are modulated by the Earth’s diurnal rotation. Near the September equinox, the occurrence of irregularities was in...

Journal of Geophysical Research: Space Physics, 2009

In this study, we statistically analyze the latitudinal dependence of F2-layer peak electron dens... more In this study, we statistically analyze the latitudinal dependence of F2-layer peak electron densities (NmF2) and total electron content (TEC) responses to solar eclipses by using the ionosonde observations during 15 eclipse events from 1973 to 2006 and the GPS TEC observations during six solar eclipse events from 1999 to 2006. We carried out a model study on the latitudinal dependence of eclipse effects on the ionosphere by running a theoretical ionospheric model with the total eclipse occurring at 13 latitudes from 0°N to 60°N at intervals of 5°. Both the observations and simulations show that the NmF2 and TEC responses have the same latitudinal dependence: the eclipse effects on NmF2 and TEC are smaller at low latitudes than at middle latitudes; at the middle latitudes (>40°), the eclipse effect decreases with increasing latitude. The simulations show that the smaller NmF2 responses at low latitudes are mainly because of much higher heights of hmF2 at low latitudes and electron density response decreasing rapidly with increasing height. For the eclipse effects at the middle latitudes (>40°), the simulations show that the smaller NmF2 or TEC response at higher latitude is mainly ascribed to the larger downward diffusion flux induced by the larger dip angle at this region, which can partly make up for the plasma loss and alleviate the depression of electron density in the F region. The simulated results show that there is an overall decrease in electron temperature throughout the entire height range at the middle latitude, but for the low latitudes the eclipse effect on electron temperature is much smaller at high heights, which is mainly because of the much smaller reduction of photoelectron production rate at its conjugate low heights where only a partial eclipse with small eclipse magnitude occurs.

This work examines the performance of four ionospheric models for estimating Total Electron Conte... more This work examines the performance of four ionospheric models for estimating Total Electron Content (TEC) over Nigeria. The observed Vertical TEC values retrieved from five GPS stations located between geographic latitudes 4.80° and 12.47°N were analyzed and compared with corresponding values obtained from the International Reference Ionosphere (IRI-01corrected option), the IRI-Plasmasphere 2017, NeQuick-2 and Nigerian Total Electron Content (NIGTEC) models. NIGTEC model used in this work is a neural network based model developed at the Nigerian Centre for Atmospheric Research as a regional model, while the IRI-01cor, IRI-Plas 2017 and NeQuick are well-known global ionospheric models. We evaluated TEC from the four models at hourly levels for all the days of the year 2012 (sunspot number Rz = 84.4). TEC directly derived from the five GPS stations under consideration was also evaluated for all the days of the year. The paper also considered the performance of the models under geomagnetically quiet condition (Ap ≤ 5). The NIGTEC model has shown better agreement with the observed VTEC when compared with the IRI-01cor, NEQUICK and IRI-Plas 2017 models especially during the pre-Sunrise period. With mean values of RMSE as 3.378 TECU and 3.8403 TECU for all days and geomagnetically quiet condition respectively, NIGTEC returned the lowest RMSE values at all conditions. The modelled TEC obtained from all the four models, at both geomagnetically disturbed and quiet times, follow the diurnal pattern of the observed GPS-TEC with variations in magnitudes. The NIGTEC model performed better than IRI-01cor, NEQUICK and IRI-Plas 2017 even when only geomagnetically quiet condition was considered. The NeQuick and NIGTEC model correctly captured the annual distribution pattern of the observed GPS-TEC across the stations. This distribution was over-smoothed by the IRI and IRI-Plas models.

Advances in Space Research, Nov 1, 2013

This paper presents the development of a Total Electron Content (TEC) map for the Nigerian ionosp... more This paper presents the development of a Total Electron Content (TEC) map for the Nigerian ionosphere. In this work, TEC measurements obtained from the AFRL-SCINDA GPS (Air Force Research Laboratory-Scintillation Network Decision Aid, Global Positioning System) equipment installed at Nsukka (6.87°N, 7.38°E) are used to adapt the International Reference Ionosphere (IRI) model for the Nigerian Ionosphere. The map is being developed as a computer program (implemented in the MATLAB programming language) that shows spatial and temporal representations of TEC for the Nigerian ionosphere. The method is aimed at showing how the IRI model can be used to estimate VTEC over wide areas by incorporating GPS measurements. This method is validated by using GPS VTEC data collected from a station in Ilorin (8.50°N, 4.55°E).

Journal of Atmospheric and Solar-Terrestrial Physics, Nov 1, 2017

In Africa, we assessed the performance of all the three options of International Reference Ionosp... more In Africa, we assessed the performance of all the three options of International Reference Ionosphere 2012, IRI-2012 (i.e. IRI-2001, IRI-2001COR and IRI-NeQuick), NeQuick-2 and IRI-Plas 2015 models prior to and during 2009 sudden stratospheric warming (SSW) event to predict equatorial ionization anomaly (EIA) crest locations and their magnitudes using total electron content (TEC) from experimental records of Global Positioning System (GPS). We confirmed that the IRI-Plas 2015 that appeared as the best compared to all of the models as regard prediction of the EIA crest locations in the northern hemisphere of Africa is due to discontinuities in the GPS data between~8 N and 22 N. As regard the predictions of EIA crest magnitudes and the location of EIA crests in the southern hemisphere of Africa, they are not present in all the models. The NeQuick-2 model does not have the capability to predict either the EIA crest location in the northern or southern hemisphere. The SSW effect on the low latitude was able to modify a single EIA crest to pre-noon and post noon EIA crests in the northern hemisphere during the SSW peak phase and significantly reduced the GPS TEC magnitudes over the hemispheres as well. These SSW effects and delays of plasma transportation to higher latitudes in GPS TEC were absent in all the models. For future improvements of IRI-2012, NeQuick-2 and IRI-Plas 2015 models, SSW conditions should be included in order to characterize the effect of lower atmosphere on the ionosphere. The EIA trough modeling is only present in IRI-2001COR and IRI-2001NeQuick options. In the middle latitude, all the model could not predict the location of highest TEC magnitudes found at RBAY (Richardsbay, South Africa).