Research Articles

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https://dir.muni.ac.ug/handle/20.500.12260/61

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Dust ion acoustic double layers in a 4-component dusty plasma
(Springer, 2018-02-22) Habumugisha, Isaac; Anguma, Simon Katrini; Jurua, Edward; Nazziwa, L.
In this paper, we study the Dust ion acoustic (DIA) solitons in an unmagnetised dusty plasma comprising of cold dust particles, electrons that follow Cairns distribution, warm inertial ions, and ion-beams of equal mass, using arbitrary amplitude technique. Our results show that it is possible for both rarefactive (negative) and compressive (positive) DIA solitary waves to coexist. Interestingly, double layers could not limit the existence of solitary waves. These results can therefore help to understand the mechanism for decelerating protons in the accretion flow onto neutron stars in a binary system at radial distances where the effect of magnetic field can be neglected.
New red giant star in the Kepler open cluster NGC 6819
(Elsevier, 2018) Komucyeya, Esther; Abedigambab, O.P.; Jurua, Edward; Anguma, Simon Katrini
A recent study indicated that 39 red giant stars showing solar-like oscillations were discovered in the ﬁeld of Kepleropen cluster NGC 6819. The study was based on photometric distance estimates of 27 stars out of the 39. Using photometric method alone may not be adequate to conﬁrm the membership of these stars. The stars were not previously known in literature to belong to the open cluster NGC 6819. In this study, Kepler data was used to study the membership of the 27 stars. A plot of apparent magnitude as a function of the large frequency se- paration, supplemented with the proper motion and radial velocity values from literature revealed KIC 5112840 to lie on the same plane with the well-known members of the cluster. Echelle diagram was constructed, and the median gravity-mode period spacings (ΔP) calculated for KIC 5112840. A value of ΔP = 66.3 s was obtained, thus placing the red giant star KIC 5112840 on the Red Giant Branch stage of evolution. Our evolutionary status result using the approach in this paper is in agreement with what is in the available literature.
Non-Linear Ion-Acoustic Solitary Waves in Electron-Positron-Ion Plasma with Non-Thermal Electrons.
(Scientific Research Publishing, 2017-05-26) Anguma, Simon Katrini; Habumugisha, Isaac; Nazziwa, L.; Jurua, Edward; Noreen, N.
Ion-acoustic solitary (IAS) waves in electron-positron-ion (e-p-i) plasma have been of interest to many researchers probably due to their relevance in understanding the Universe. However, the study of non-linear ion-acoustic waves in e-p-i plasma with non-thermal electrons has not been adequately studied. A theoretical investigation on non-linear IAS waves in e-p-i plasma comprising of warm inertial adiabatic fluid ions and electrons that are kappa distributed, and Boltzman distributed positron is presented here using the Sagdeev potential technique. It was found that existence domains of finite amplitude IAS waves were confined within the limits of minimum and maximum Mach numbers with varying κ values. For lower values of κ , the amplitude of the solitary electrostatic potential structures increased as the width decreased, while for high values, the potential amplitude decreased as the width of the solitary structure increased.
On the structure of quasi-Keplerian accretion discs surrounding millisecond X-ray pulsars
(Springer, 2020-10-12) Habumugisha, Isaac; Tessema, Solomon B.; Jurua, Edward; Anguma, Simon Katrini
In this study, we investigated the time-independent dynamics (disc structure, forces and torques) of a quasi-Keplerian disc around a millisecond pulsar (MSP) with an internal dynamo. We considered the disc around a MSP to be divided into the inner, middle and outer regions. By assuming that the disc matter flows in a quasi-Keplerian motion, we derived analytical equations for a complete structure (temperature, pressure, surface density, optical depth and magnetic field) of a quasi-Keplerian thin accretion disc, and the pressure gradient force (PGF). In our model, the MSP-disc interaction results into magnetic and material torques, such that for a given dynamo (ϵ) and quasi-Keplerian (ξ) parameter, we obtained enhanced spin-up and spin-down torques for a chosen star spin period. Results obtained reveal that PGF results into episodic torque reversals that contribute to spinning-up or spinning-down of a neutron star, mainly from the inner region. The possibility of a quasi-Keplerian disc is seen and these results can explain the observed spin variations in MSPs like SAX J1808.4-3658 and XTE J1814-338.
Onset of linear instability in a complex plasma with cairns distributed electrons
(Scientific Research Publishing, 2016-03-07) Habumugisha, Isaac; Anguma, Simon Katrini; Jurua, Edward; Noreen, N.
A rigorous theoretical investigation of linear dust ion acoustic (DIA) solitary waves in an unmagnetized complex plasma consisting of ion and ion beam fluids, nonthermal electrons that are Cairns distributed and immobile dust particles were undertaken. It was found out that, for large beam speeds, three stable modes propagated as solitary waves in the beam plasma. These were the “Fast”, “Slow” and “Ion-acoustic” modes. For two stream instability to occur between ion and ion beam, it is shown that......
Planetary core formation via multispecies pebble accretion
(Oxford University Press, 2021-12-02) Andama, Geoffrey; Ndugu, Nelson; Anguma, Simon Katrini; Jurua, Edward
In the general classical picture of pebble-based core growth, planetary cores grow by accretion of single pebble species. The growing planet may reach the so-called pebble isolation mass, at which it induces a pressure bump that blocks inward drifting pebbles exterior to its orbit, thereby stalling core growth by pebble accretion. In recent hydrodynamic simulations, pebble filtration by the pressure bump depends on several parameters including core mass, disc structure, turbulent viscosity and pebble size. We have investigated how accretion of multiple, instead of single, pebble species affects core growth rates, and how the dependence of pebble isolation mass on turbulent viscosity and pebble size sets the final core masses. We performed numerical simulations in a viscous one-dimensional disc, where maximal grain sizes were regulated by grain growth, fragmentation and drift limits. We confirm that core growth rates and final core masses are sensitive to three key parameters: the threshold velocity at which pebbles fragment on collision, the turbulent viscosity and the distribution of pebble species, which yield a diversity of planetary cores. With accretion of multiple pebble species, planetary cores can grow very fast, reaching over 30–40 ME in mass. Potential cores of cold gas giants were able to form from embryos initially implanted as far as 50 au. Our results suggest that accretion of multispecies pebbles could explain: the estimated 25–45 ME heavy element abundance inside Jupiter’s core; the massive cores of extrasolar planets; the disc rings and gaps at wider orbits; and the early and rapid formation of planetary bodies.
Spatial distribution of unique biological communities and their control over surface reflectivity of the stanley glacier, Uganda
(Frontiers, 2022-02-17) Uetake, Jun; Samyn, Denis; Anguma, Simon Katrini; Takeuchi, Nozomu
Diverse microbes have been revealed to live in glaciers worldwide, but only a few biological studies were dedicated to glaciers in tropical Africa. These glaciers are shrinking rapidly and are expected to disappear shortly. In this study, we carried out biological and glaciological field observations on Stanley Glacier, the largest remaining glacier in the Rwenzori Mountains, Uganda, Africa. Microbial aggregates ranging from micrometer to centimeter in size were found on the glacier surface and contained moss and various types of Chlorophyta, among which a new endemic species of green alga. Concentrations of total impurities on the glacier surface, including microbial aggregates, varied spatially and decreased as altitude increased. The large microbial aggregates (larger than 4 cm in diameter) were found only at the glacier surface near the terminus and side margins, where the surface was less frequently covered with snow. It is also shown that the total organic matter on the glacier surface is determined by the timing of snow cover, which affects the quantity of solar radiation reaching the glacier ice surface. Furthermore, the total impurity content was negatively correlated with surface reflectivity, revealing their potential role in albedo reduction at the glacier surface through positive feedback between enhanced meltwater and increased biological growth.
Spatio-Temporal Variation in the Concentration of Inhalable Particulate Matter (PM10) in Uganda.
(MDPI AG, 2019-05-17) Onyango, Silver; Parks, Beth; Anguma, Simon Katrini; Meng, Qingyu
Long-term particulate matter (PM10) measurements were conducted during the period January 2016 to September 2017 at three sites in Uganda (Mbarara, Kyebando, and Rubindi) representing a wide range of urbanization. Spatial, temporal and diurnal variations are assessed in this paper. Particulate matter (PM10) samples were collected for 24-h periods on PTFE filters using a calibrated pump and analyzed gravimetrically to determine the average density. Particulate levels were monitored simultaneously using a light scattering instrument to acquire real time data from which diurnal variations were assessed. The PM10 levels averaged over the sampling period at Mbarara, Kyebando, and Rubindi were 5.8, 8.4, and 6.5 times higher than the WHO annual air quality guideline of 20 µg·m−3, and values exceeded the 24-h mean PM10 guideline of 50 µg·m−3 on 83, 100, and 86% of the sampling days. Higher concentrations were observed during dry seasons at all sites. Seasonal differences were statistically significant at Rubindi and Kyebando. Bimodal peaks were observed in the diurnal analysis with higher morning peaks at Mbarara and Kyebando, which points to the impact of traffic sources, while the higher evening peak at Rubindi points to the influence of dust suspension, roadside cooking and open-air waste burning. Long-term measurement showed unhealthy ambient air in all three locations tested in Uganda, with significant spatial and seasonal differences.
The structure of a Quasi-Keplerian accretion disk around magnetized stars
(The American Astronomical Society, 2018-06-04) Habumugisha, Isaac; Jurua, Edward; Tessema, Solomon B; Anguma, Simon Katrini
In this paper, we present the complete structure of a quasi-Keplerian thin accretion disk with an internal dynamo around a magnetized neutron star. We assume a full quasi-Keplerian disk with the azimuthal velocity deviating from the Keplerian fashion by a factor of ξ (0 < ξ < 2). In our approach, we vertically integrate the radial component of the momentum equation to obtain the radial pressure gradient equation for a thin quasi-Keplerian accretion disk. Our results show that, at large radial distance, the accretion disk behaves in a Keplerian fashion. However, close to the neutron star, pressure gradient force (PGF) largely modiﬁes the disk structure, resulting into sudden dynamical changes in the accretion disk. The corotation radius is shifted inward (outward) for ξ > 1 (for ξ < 1), and the position of the inner edge with respect to the new corotation radius is also relocated accordingly, as compared to the Keplerian model. The resulting PGF torque couples with viscous torque (when ξ < 1) to provide a spin-down torque and a spin-up torque (when ξ > 1) while in the advective state. Therefore, neglecting the PGF, as has been the case in previous models, is a glaring omission. Our result has the potential to explain the observable dynamic consequences of accretion disks around magnetized neutron stars.
Validation of the atmospheric boundary layer height estimated from the MODIS atmospheric profile data at an equatorial site
(MDPI, 2020-08-26) Onyango, Silver; Anguma, Simon Katrini; Andima, Geodfrey; Parks, Beth
The atmospheric boundary layer height is important for constraining air pollution and meteorological models. This study attempted to validate the MODIS-estimated atmospheric boundary layer height (ABLH), and variation in the ABLH in Uganda was evaluated. The ABLH was estimated from MODIS data using the mixing ratio profile gradient method and compared to the ABLH estimated from radiosonde data using three different methods. Unlike in studies in other regions of the world, correlations between ABLH estimated using MODIS and radiosonde data were weak, implying limited usefulness of MODIS data for determining ABLH. However, the diurnal variation in MODIS-derived ABLH and particulate matter (PM10) was consistent with the expected inverse relationship between PM10 mass concentration and ABLH, and the mean MODIS-derived ABLH values were significantly lower during wet seasons than dry seasons, as expected.

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