Hermanus, South Africa
Hermanus, South Africa

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Bilitza D.,George Mason University | Bilitza D.,NASA | McKinnell L.-A.,Hermanus Magnetic Observatory | Reinisch B.,University of Massachusetts Lowell | Fuller-Rowell T.,University of Colorado at Boulder
Journal of Geodesy | Year: 2011

The international reference ionosphere (IRI) is the internationally recognized and recommended standard for the specification of plasma parameters in Earth's ionosphere. It describes monthly averages of electron density, electron temperature, ion temperature, ion composition, and several additional parameters in the altitude range from 60 to 1,500 km. A joint working group of the Committee on Space Research (COSPAR) and the International Union of Radio Science (URSI) is in charge of developing and improving the IRI model. As requested by COSPAR and URSI, IRI is an empirical model being based on most of the available and reliable data sources for the ionospheric plasma. The paper describes the latest version of the model and reviews efforts towards future improvements, including the development of new global models for the F2 peak density and height, and a new approach to describe the electron density in the topside and plasmasphere. Our emphasis will be on the electron density because it is the IRI parameter most relevant to geodetic techniques and studies. Annual IRI meetings are the main venue for the discussion of IRI activities, future improvements, and additions to the model. A new special IRI task force activity is focusing on the development of a real-time IRI (RT-IRI) by combining data assimilation techniques with the IRI model. A first RT-IRI task force meeting was held in 2009 in Colorado Springs. We will review the outcome of this meeting and the plans for the future. The IRI homepage is at http://www. IRI. gsfc. nasa. gov. © 2010 Springer-Verlag.


McKinnell L.A.,Hermanus Magnetic Observatory | McKinnell L.A.,Rhodes University | Oyeyemi E.O.,University of Lagos
Advances in Space Research | Year: 2010

A new neural network (NN) based global empirical model for the foF2 parameter, which represents the peak ionospheric electron density, has been developed using extended temporal and spatial geophysical relevant inputs. It has been proposed that this new model be considered as a suitable replacement for the International Union of Radio Science (URSI) and International Radio Consultative Committee (CCIR) model options currently used within the International Reference Ionosphere (IRI) model for the purpose of F2 peak electron density predictions. The most recent version of the model has incorporated data from 135 global ionospheric stations including a number of equatorial stations. This paper concentrates on the ability of this new model to predict foF2 for the equatorial sector, an area that has been identified as problematic within the current IRI peak prediction setup. The improvement in the predictions of the foF2 parameter by the new model as compared to the URSI and CCIR model options of the IRI is demonstrated and the requirement for additional foF2 data from the equatorial zone for the purpose of global modeling of foF2 is highlighted in this paper. © 2010 COSPAR. Published by Elsevier Ltd. All rights reserved.


Ackermann E.R.,University of Pretoria | Ackermann E.R.,Council for Scientific and Industrial Research | De Villiers J.P.,University of Pretoria | De Villiers J.P.,Council for Scientific and Industrial Research | Cilliers P.J.,Hermanus Magnetic Observatory
Journal of Geophysical Research: Space Physics | Year: 2011

Two different implementations of Gaussian process (GP) models are proposed to estimate the vertical total electron content (TEC) from dual frequency Global Positioning System (GPS) measurements. The model falseness of GP and neural network models are compared using daily GPS TEC data from Sutherland, South Africa, and it is shown that the proposed GP models exhibit superior model falseness. The GP approach has several advantages over previously developed neural network approaches, which include seamless incorporation of prior knowledge, a theoretically principled method for determining the much smaller number of free model parameters, the provision of estimates of the model uncertainty, and a more intuitive interpretability of the model. Copyright 2011 by the American Geophysical Union.


Sutcliffe P.R.,Hermanus Magnetic Observatory | Sutcliffe P.R.,University of KwaZulu - Natal | Luhr H.,Helmholtz Center Potsdam
Journal of Geophysical Research: Space Physics | Year: 2010

Pi2 pulsations are typically observed at night and often during local daytime at low-latitude ground stations. Their daytime occurrence is most common and clearest near the dip equator; however, daytime Pi2s are regularly observed at low-latitude ground stations away from the dip equator. In contrast, a study utilizing satellite data at low latitudes within the plasmasphere could find no evidence for dayside Pi2s. Consequently, in this investigation we used high-resolution vector magnetometer data from the CHAMP satellite to carry out a search for daytime Pi2 pulsations in the F region ionosphere. We confirm that nighttime Pi2s are observed simultaneously on CHAMP and at low-latitude ground stations and that their characteristics are consistent with a cavity mode resonance. However, our comparisons of daytime Pi2 pulsation events identified in ground station data with data from CHAMP clearly demonstrate that Pi2s in the dayside hemisphere differ from their nighttime counterparts. Specifically, we find no convincing evidence of their occurrence in CHAMP data. Our results suggest that the generation mechanism for daytime Pi2s may be similar to the mechanisms that other authors have proposed to explain sudden impulses and DP2 current systems. We propose a model in which the electric field imprinted by the field-aligned currents connected to the substorm current wedge is distributed globally in the atmospheric waveguide. Toroidal currents flowing on the dayside between ionosphere and ground generate magnetic fields that are confined to the waveguide. As a consequence, no Pi2-related magnetic signals can be detected in the topside ionosphere. Copyright 2010 by the American Geophysical Union.


Collier A.B.,Hermanus Magnetic Observatory | Collier A.B.,University of KwaZulu - Natal | Hughes A.R.W.,University of KwaZulu - Natal
Journal of Geophysical Research: Atmospheres | Year: 2011

Concise rainfall, temperature, and humidity climatologies are available for most countries, reflecting the regular annual variation of these quantities, usually with monthly resolution. Although numerous surveys of global and regional lightning activity exist, no comparable national climatologies of lightning activity are available. Annual lightning flash rate patterns for most African countries are derived here using 10 years of data from two satellite lightning detectors. These patterns can be represented accurately by annual and semiannual sinusoidal components superimposed on a mean level. The highest-average lightning activity (47.8 km-2 yr-1) occurs in the Democratic Republic of the Congo, while the lowest (0.2 km-2 yr-1) is found in Egypt. Very little lightning is observed in the Sahara, while a consistently high intensity is found throughout the tropics. There is a clear reversal in the phase of the annual component across the equator. Within the tropics the amplitude of the semiannual variation is a significant fraction of that of the annual variation but becomes less important in the subtropics. Copyright 2011 by the American Geophysical Union.


Sutcliffe P.R.,Hermanus Magnetic Observatory | Sutcliffe P.R.,University of KwaZulu - Natal
Geophysical Research Letters | Year: 2010

Pi2 pulsations are impulsive, damped oscillations associated with magnetospheric substorm onsets and intensifications. We show that Pi2 activity, with different characteristics, occurs during non-substorm intervals. Examples showing the nature of the association between low-latitude Pi2s and "tail reconnection during IMF-northward, non-substorm intervals" or TRINNIs are presented. These events are thought to be related to reconnection at a more distant neutral line than that associated with substorms and that maps to the poleward auroral oval boundary. In particular, using a new method of Pi2 identification, we for the first time show that modulated quasi continuous activity in the Pi2 band occurs regularly (∼40% of the time) at low latitudes during IMF-northward, non-substorm intervals. Copyright © 2010 by the American Geophysical Union.


Collier A.B.,Hermanus Magnetic Observatory | Collier A.B.,University of KwaZulu - Natal | Lichtenberger J.,Etvs University | Clilverd M.A.,British Antarctic Survey | And 2 more authors.
Journal of Geophysical Research: Space Physics | Year: 2011

The accepted mechanism for whistler generation implicitly assumes that the causative lightning stroke occurs within reasonable proximity to the conjugate foot point of the guiding magnetic field line and that nighttime whistlers are prevalent because of low transionospheric attenuation. However, these assumptions are not necessarily valid. In this study we consider whistler observations from Rothera, a station on the Antarctic Peninsula, and contrast their occurrence with global lightning activity from the World Wide Lightning Location Network. The correlation of one-hop whistlers observed at Rothera with global lightning yields a few regions of significant positive correlation. The most probable source region was found over the Gulf Stream, displaced slightly equatorward from the conjugate point. The proximity of the source region to the conjugate point is in accord with the broadly accepted whistler production mechanism. However, there is an unexpected bias toward oceanic lightning rather than the nearby continental lightning. The relationship between the diurnal pattern of the Rothera whistlers and the conjugate lightning exhibits anomalous features which have yet to be resolved: the peak whistler rate occurs when it is daytime at both the source and the receiver and when source lightning activity is at its lowest. As a result, we propose that preferential whistler-wave amplification in the morning sector is a possible cause of the high whistler occurrence, although this does not account for the bias toward oceanic lightning. Copyright 2011 by the American Geophysical Union.


Ngwira C.M.,Rhodes University | Ngwira C.M.,Hermanus Magnetic Observatory | McKinnell L.-A.,Rhodes University | McKinnell L.-A.,Hermanus Magnetic Observatory | Cilliers P.J.,Hermanus Magnetic Observatory
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2010

Radio wave scintillation due to the presence of ionospheric irregularities can cause fading and phase variation of L-band navigation signals such as those used by the Global Positioning System (GPS). The high-latitude regions experience increased scintillation events under geomagnetically disturbed conditions, particularly during solar maximum. This paper presents phase scintillation measurements observed at the South African Antarctic polar research station during solar minimum conditions. In addition, a multi-instrument approach is shown in which the scintillation events are associated with auroral electron precipitation. Furthermore, it is shown that external energy injection into the ionosphere is an important factor in the development of irregularities producing scintillation. © 2010 Elsevier Ltd.


Habarulema J.B.,Hermanus Magnetic Observatory | Habarulema J.B.,Rhodes University | McKinnell L.-A.,Hermanus Magnetic Observatory | McKinnell L.-A.,Rhodes University | Opperman B.D.L.,Hermanus Magnetic Observatory
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2010

This paper presents the results from a study designed to investigate the ability of a newly developed neural network (NN) based model to follow total electron content (TEC) dynamics over the Southern African region. The investigation is carried out by comparing results from the NN model with actual TEC data derived from Global Positioning System (GPS) observations and TEC values predicted by the International Reference Ionosphere (IRI-2007) model during magnetic storm periods over Southern Africa. The magnetic storm conditions chosen for the study presented in this paper occurred during the periods 16-21 April 2002, 1-6 October 2002, and 28 October-01 November 2003. A total of six South African GPS stations were used for the validation of the two models during these periods. A statistical analysis of the comparison between the actual TEC behaviour and that predicted by the two models is shown. In addition, ionosonde measurements from the South African Louisvale (28.5°S, 21.2°E) station, located close to one of the validation GPS stations used, are also considered during the Halloween storm period of 28-31 October 2003. The generalisation of TEC behaviour by the NN model is demonstrated by producing predicted TEC maps during magnetic storm periods over South Africa. Presented results demonstrate the ability of NNs in predicting TEC variability over South Africa during magnetically disturbed conditions, and highlight areas for improvement. © 2010 Elsevier Ltd.


Habarulema J.B.,Hermanus Magnetic Observatory | Habarulema J.B.,Rhodes University | McKinnell L.-A.,Hermanus Magnetic Observatory | McKinnell L.-A.,Rhodes University | Opperman B.D.L.,Hermanus Magnetic Observatory
Journal of Geophysical Research: Space Physics | Year: 2011

In this paper, the potential extrapolation capabilities and limitations of artificial neural networks (ANNs) are investigated. This is primarily done by generating total electron content (TEC) predictions using the regional southern Africa total electron content prediction (SATECP) model based on the Global Positioning System (GPS) data and ANNs with the aid of multiple inputs intended to enable the software to learn and correlate the relationship between their variations and the target parameter, TEC. TEC values are predicted over regions that were not covered in the model's development, although it is difficult to validate their accuracy in some cases. The SATECP model is also used to forecast hourly TEC variability 1 year ahead in order to assess the forecasting capability of ANNs in generalizing TEC patterns. The developed SATECP model has also been independently validated by ionosonde data and TEC values derived from the adapted University of New Brunswick Ionospheric Mapping Technique (UNB-IMT) over southern Africa. From the comparison of prediction results with actual GPS data, it is observed that ANNs extrapolate relatively well during quiet periods while the accuracy is low during geomagnetically disturbed conditions. However, ANNs correctly identify both positive and negative storm effects observed in GPS TEC data analyzed within the input space. Copyright 2011 by the American Geophysical Union.

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