Planet population synthesis driven by pebble accretion in cluster environments

The evolution of protoplanetary discs embedded in stellar clusters depends on the age and the stellar density in which they are embedded. Stellar clusters of young age and high stellar surface density destroy protoplanetary discs by external photoevaporation and stellar encounters. Here we consider the effect of background heating from newly formed stellar clusters on the structure of protoplanetary discs and how it affects the formation of planets in these discs. Our planet formation model is build on the core accretion scenario including pebble accretion. We synthesize planet populations that we compare to observations. The giant planets in our simulations migrate over large distances due to the fast type-II migration regime induced by a high disc viscosity ($\alpha=5.4 \times 10^{-3}$). Cold Jupiters (r>1 AU) originate preferably from the outer disc, while hot Jupiters (r<0.1 AU) preferably form in the inner disc. We find that the formation of gas giants via pebble accretion is in agreement with the metallicity correlation, meaning that more gas giants are formed at larger metallicity. However, our synthetic population of isolated stars host a significant amount of giant planets even at low metallicity, in contradiction to observations where giant planets are preferably found around high metallicity stars, indicating that pebble accretion is very efficient in the standard pebble accretion framework. On the other hand, discs around stars embedded in cluster environments hardly form any giant planets at low metallicity in agreement with observations, where these changes originate from the increased temperature in the outer parts of the disc, which prolongs the core accretion time-scale of the planet. We therefore conclude that the outer disc structure and the planet's formation location determines the giant planet occurrence rate and the formation efficiency of cold and hot Jupiters.Comment: 12 pages, accepted for publication in MNRA

Effects of space weather on the ionosphere: A case study of geomagnetic storms during 17-28 February 2014

26-37This study focused on the effects of space weather on the ionosphere during geomagnetic storms for the period 17-28 February 2014 over the African low, latitude region. The Global Positioning System (GPS) data, derived from dual frequency receivers installed along the African low, latitude were analyzed to get Total Electron Content (TEC) and this was used to study the response of the ionosphere to the geomagnetic storms. Positive and negative ionospheric storm effects were observed during the period of study. These geomagnetic storm effects were discussed in terms of the Prompt Penetration Electric Field (PPEF) storm induced wind lifting effect and Disturbance Dynamo (DD) electric field. Although these storms occurred during the same period, their impacts and associated features on the ionosphere varied due to different contributing factors to their driving mechanisms. A shift in equatorial VTEC enhancement from one GPS station to another was observed, showing a longitudinal dependence of the ionospheric response to the geomagnetic storms and this was attributed to composition changes. In addition, Rate of Change of TEC Index (ROTI) was used to examine the occurrence of ionospheric irregularities. Out of all the storms studied, the storms on 19 and 20 February 2014, inhibited the occurrence of ionospheric irregularities, while the remaining storms triggered ionospheric irregularities. The generation of post-sunset irregularities was attributed to the Rayleigh Taylor Instability mechanism. A longitudinal dependence of the enhancement/inhibition of ionospheric irregularities was also observed

Determination of Natural Radioactivity Levels due to Mine Tailings from Selected Mines in Southwestern Uganda

The aim of this study was to determine the natural radioactivity levels of primordial radionuclides in soil mine tailings from selected mines in Southwestern Uganda. This was achieved by analyzing thirty six samples of soil mine tailings from three mining sites using NaI(Tl) gamma ray spectrometer. The specific activity concentrations of 238U (Uranium), 232Th (Thorium) and 40K (Potassium-40) in the samples were computed. The specific activity concentrations varied from 35.5 to 147.0 Bq kg-1 for 238U, 119.3 to 376.7 Bq kg-1 for 232Th and 141.0 to 1658.5 Bq kg-1 for 40K. The outdoor absorbed dose rates in air 1.0 m above the ground level were determined. The mean absorbed dose rates for Mashonga Gold Mine, Kikagati Tin Mine and Butare Iron Ore mine were 181.2±66.8 nGy h-1, 167.2±43.0 nGy h-1 and 191.6±29.6 nGy h-1 respectively which are more than three times the world wide average value of 59 nGy h-1. The mean outdoor annual effective dose rates for the three mines were 0.37±0.14 mSv y-1, 0.34±0.09 mSv y-1 and 0.39±0.06 mSv y-1 respectively which are more than five times the world average value of 0.07 mSv y-1. Thus the mine tailings (soil) from these areas must not be used as major building material to minimize radiological hazards. Key words: Activity concentration, Gamma index, Soil mine tailing

Development in Astronomy and Space Science in Africa

The development of astronomy and space science in Africa has grown significantly over the past few years. These advancements make the United Nations Sustainable Development Goals more achievable, and open up the possibility of new beneficial collaborations.Comment: Paper published in Nature Astronomy. Figures 1 and 2 are included in the published version, that can be seen at https://rdcu.be/2oE

Ionospheric irregularities and scintillations : a direct comparison of in situ density observations with ground-based L-band receivers

Ionospheric irregularities can affect satellite communication and navigation by causing scintillations of radio signals. The scintillations are routinely measured using ground-based networks of receivers. This study presents observations of ionospheric irregularities by Langmuir probes on the Swarm satellites. They are compared with amplitude scintillation events recorded by the Global Positioning System-Scintillation Network and Decision Aid (GPS-SCINDA) receiver installed in Mbarara (Lat: 0.6 degrees S, Lon: 30.8 degrees E, Mag. lat: 10.2 degrees S). The study covers the years from 2014 to 2018 when both data sets were available. It was found that the ground-based amplitude scintillations were enhanced when Swarm registered ionospheric irregularities for a large number of passes. The number of matching observations was greater for Swarm A and C which orbited at lower altitudes compared to Swarm B. However, some counterexamples, i.e., cases when in situ electron density fluctuations were not associated with any observed L-band amplitude scintillation and vice versa, were also found. Therefore, mismatches between observed irregularity structures and scintillations can occur just over a few minutes and within distances of a few tens of kilometers. The amplitude scintillation strength, characterized by the S4 index was estimated from the electron density data using the well-known phase screen model for weak scattering. The derived amplitude scintillation was on average lower for Swarm B than for A and C and less in accordance with the observed range. Irregularities at an altitude of about 450 km contribute strongly to scintillations in the L-band, while irregularities at about 510-km altitude contribute significantly less. We infer that in situ density fluctuations observed on passes over or near Mbarara may be used to indicate the risk that ionospheric radio wave scintillations occur at that site

Traits of sub-kilometre F-region irregularities as seen with the Swarm satellites

During the night, in the F-region, equatorial ionospheric irregularities manifest as plasma depletions observed by satellites, and they may cause radio signals to fluctuate. In this study, the distribution characteristics of ionospheric F-region irregularities in the low latitudes were investigated using 16 Hz electron density observations made by a faceplate which is a component of the electric field instrument (EFI) onboard Swarm satellites of the European Space Agency (ESA). The study covers the period from October 2014 to October 2018 when the 16 Hz electron density data were available. For comparison, both the absolute (dN(e)) and relative (dN(e)/N-e) density perturbations were used to quantify the level of ionospheric irregularities. The two methods generally reproduced the local-time (LT), seasonal and longitudinal distribution of equatorial ionospheric irregularities as shown in earlier studies, demonstrating the ability of Swarm 16 Hz electron density data. A difference between the two methods was observed based on the latitudinal distribution of ionospheric irregularities where (dNe) showed a symmetrical distribution about the magnetic equator, while dN(e)/N-e showed a magnetic-equator-centred Gaussian distribution. High values of dNe and dN(e)/N-e were observed in spatial bins with steep gradients of electron density from a longitudinal and seasonal perspective. The response of ionospheric irregularities to geomagnetic and solar activities was also investigated using Kp index and solar radio flux index (F10.7), respectively. The reliance of dN(e)/N-e on solar and magnetic activity showed little distinction in the correlation between equatorial and off-equatorial latitudes, whereas dNe showed significant differences. With regard to seasonal and longitudinal distribution, high dNe and dN(e)/N-e values were often found during quiet magnetic periods compared to magnetically disturbed periods. The dNe increased approximately linearly from low to moderate solar activity. Using the high-resolution faceplate data, we were able to identify ionospheric irregularities on the scale of only a few hundred of metres