Phase diagram for the co-adsorption of O and OH on Pt(100) and Pt(111) as determined by DFT

The Langmuir adsorption isotherm is often used to model molecular adsorption on catalyst surfaces. The model assumes that adsorption occurs on a homogenous energy surface at specific localized sites with no lateral interactions between adsorbents. This simplification causes some concerns when considering adsorption at higher coverages as species have been observed to have a maximum coverage less than one monolayer (ML), such as O and OH on platinum (Pt) surfaces for use in direct methane to methanol synthesis. It has been suggested that the maximum coverages are due to repulsive lateral interactions which limit coverages on Pt to 0.50 ML and 0.75 ML for O and OH respectively, weakening the Langmuir assumption. For reactions sensitive to coverage it is useful to have a model representation of these interactions and the obtainable coverages. This would require determining the effect these interactions have on obtainable coverages and whether possible hydrogen bonding could allow for co-adsorption to fully saturate Pt catalysts. Thus, this study focuses on the coverage of Pt surfaces with O, OH and co-adsorbed O/OH adsorbents as a function of temperature and partial pressure with particular interest given to full coverage conditions. To determine the obtainable coverages on the dominant Pt surfaces, namely Pt(100) and Pt(111), a Density Functional Theory (DFT) study was done using a GGA-PBE and GGA-optB88 model utilising VASP. The coverages were modelled on a p(2x2) Pt cell which could model 0.25, 0.50, 0.75 and 1.00 ML. The relative Gibbs free energies were then calculated for all adsorbent combinations on the surface with oxygen and water as the gas phase reference. The change in Gibbs free energy upon adsorption was calculated across a chemical potential range of -0.22 eV, corresponding to the critical point for O2 (-118.6 °C, 50.06 bar), up to -3.5 eV. These chemical potentials were then related to specific temperatures and partial pressures. It was found that only full coverage with OH was achievable on Pt(111). In contrast, Pt(100) yielded several full coverage combinations. The generation of these phase diagrams showed a trend of increasing lateral interactions that prevent full coverage with a single O adsorbent species. As shown, by co-adsorbing OH it could be possible to achieve higher coverages through attractive lateral interactions. This weakens the lateral interaction assumption used in the Langmuir model and indicates the possibility of low temperature direct methane to methanol synthesis, around 80 °C, due to the formation of a fully saturated Pt surface

GPS TEC and ionosonde TEC over Grahamstown, South Africa: first comparisons

The Grahamstown, South Africa (33.3°S, 26.5°E) ionospheric field station operates a UMass Lowell digital pulse ionospheric sounder (Digisonde) and an Ashtech geodetic grade dual frequency GPS receiver. The GPS receiver is owned by Chief Directorate Surveys and Mapping (CDSM) in Cape Town, forms part of the national TrigNet network and was installed in February 2005. The sampling rates of the GPS receiver and Digisonde were set to 1 s and 15 min, respectively. Data from four continuous months, March–June 2005 inclusive, were considered in this initial investigation. Data available from the Grahamstown GPS receiver was limited, and, therefore, only these 4 months have been considered. Total Electron Content (TEC) values were determined from GPS measurements obtained from satellites passing near vertical (within an 80° elevation) to the station. TEC values were obtained from ionograms recorded at times within 5 min of the near vertical GPS measurement. The GPS derived TEC values are referred to as GTEC and the ionogram derived TEC values as ITEC. Comparisons of GTEC and ITEC values are presented in this paper. The differential clock biases of the GPS satellites and receivers are taken into account. The plasmaspheric contribution to the TEC can be inferred from the results, and confirm findings obtained by other groups. This paper describes the groundwork for a procedure that will allow the validation of GPS derived ionospheric information with ionosonde data. This work will be of interest to the International Reference Ionosphere (IRI) community since GPS receivers are becoming recognised as another source for ionospheric information

Application of neural networks to South African GPS TEC modelling

The propagation of radio signals in the Earth’s atmosphere is dominantly affected by the ionosphere due to its dispersive nature. Global Positioning System (GPS) data provides relevant information that leads to the derivation of total electron content (TEC) which can be considered as the ionosphere’s measure of ionisation. This paper presents part of a feasibility study for the development of a Neural Network (NN) based model for the prediction of South African GPS derived TEC. The South African GPS receiver network is operated and maintained by the Chief Directorate Surveys and Mapping (CDSM) in Cape Town, South Africa. Vertical total electron content (VTEC) was calculated for four GPS receiver stations using the Adjusted Spherical Harmonic (ASHA) model. Factors that influence TEC were then identified and used to derive input parameters for the NN. The well established factors used are seasonal variation, diurnal variation, solar activity and magnetic activity. Comparison of diurnal predicted TEC values from both the NN model and the International Reference Ionosphere (IRI-2001) with GPS TEC revealed that the IRI provides more accurate predictions than the NN model during the spring equinoxes. However, on average the NN model predicts GPS TEC more accurately than the IRI model over the GPS locations considered within South Africa

Development of a regional GPS-based ionospheric TEC model for South Africa

Advances in South African space physics research and related disciplines require better spatial and time resolution ionospheric information than was previously possible with the existing ionosonde network. A GPS-based, variable degree adjusted spherical harmonic (ASHA) model was developed for near real-time regional ionospheric total electron content (TEC) mapping over South Africa. Slant TEC values along oblique GPS signal paths are quantified from a network of GPS receivers and converted to vertical TEC by means of the single layer mapping function. The ASHA model coefficients and GPS differential biases are estimated from vertical TEC at the ionospheric pierce points and used to interpolate TEC at any location within the region of interest. Diurnal TEC variations with one minute time resolution and time-varying 2D regional TEC maps are constructed. In order to validate the ASHA method, simulations with an IRI ionosphere were performed, while the ASHA results from actual data were compared with two independent GPS-based methodologies and measured ionosonde data

Observations from SANSA’s geomagnetic network during the Saint Patrick’s Day storm of 17–18 March 2015

Geomagnetic storms are space weather events that result in a temporary disturbance of the earth’s magnetosphere caused by a solar wind that interacts with the earth’s magnetic field. We examined more closely how some southern African magnetic observatories responded to the Saint Patrick’s Day storm using local K-indices. We show how this network of observatories may be utilised to model induced electric field, which is useful for the monitoring of geomagnetically induced anomalous currents capable of damaging power distribution infrastructure. We show an example of the correlation between a modelled induced electric field and measured geomagnetically induced currents in southern Africa. The data show that there are differences between global and local indices, which vary with the phases of the storm. We show the latitude dependence of geomagnetic activity and demonstrate that the direction of the variation is different for the X and Y components. Significance: • The importance of ground-based data in space weather studies is demonstrated. • We show how SANSA’s geomagnetic network may be utilised to model induced electric field, which is useful for the monitoring of geomagnetically induced anomalous currents capable of damaging power distribution infrastructure. Open data set:  http://www.intermagnet.org/data-donnee/download-eng.ph

Exploring South Africa’s southern frontier : a 20-year vision for polar research through the South African National Antarctic Programme

Abstract: Please refer to full text to view abstrac

Investigation of the Physical Processes Involved in GNSS Amplitude Scintillations at High Latitude: A Case Study

The storm onset on 7 September 2017, triggered several variations in the ionospheric electron density, causing severe phase fluctuations at polar latitudes in both hemispheres. In addition, although quite rare at high latitudes, clear amplitude scintillations were recorded by two Global Navigation Satellite System receivers during the main phase of the storm. This work attempted to investigate the physical mechanisms triggering the observed amplitude scintillations, with the aim of identifying the conditions favoring such events. We investigated the ionospheric background and other conditions that prevailed when the irregularities formed and moved, following a multi-observations approach. Specifically, we combined information from scintillation parameters and recorded by multi-constellation (GPS, GLONASS and Galileo) receivers located at Concordia station (75.10°S, 123.35°E) and SANAE IV base (71.67°S, 2.84°W), with measurements acquired by the Special Sensor Ultraviolet Spectrographic Imager on board the Defense Meteorological Satellite Program satellites, the Super Dual Auroral Radar Network, the Swarm constellation and ground-based magnetometers. Besides confirming the high degree of complexity of the ionospheric dynamics, our multi-instrument observation identified the physical conditions that likely favor the occurrence of amplitude scintillations at high latitudes. Results suggest that the necessary conditions for the observation of this type of scintillation in high-latitude regions are high levels of ionization and a strong variability of plasma dynamics. Both of these conditions are typically featured during high solar activity

Development of a regional GPS-based ionospheric TEC model for South Africa

Advances in South African space physics research and related disciplines require better spatial and time resolution ionospheric information than was previously possible with the existing ionosonde network. A GPS-based, variable degree adjusted spherical harmonic (ASHA) model was developed for near real-time regional ionospheric total electron content (TEC) mapping over South Africa. Slant TEC values along oblique GPS signal paths are quantified from a network of GPS receivers and converted to vertical TEC by means of the single layer mapping function. The ASHA model coefficients and GPS differential biases are estimated from vertical TEC at the ionospheric pierce points and used to interpolate TEC at any location within the region of interest. Diurnal TEC variations with one minute time resolution and time-varying 2D regional TEC maps are constructed. In order to validate the ASHA method, simulations with an IRI ionosphere were performed, while the ASHA results from actual data were compared with two independent GPS-based methodologies and measured ionosonde data