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Cresswell-Moorcock K.,University of Otago | Rodger C.J.,University of Otago | Kero A.,University of Oulu | Collier A.B.,SANSA Space Science | And 3 more authors.
Journal of Geophysical Research: Space Physics | Year: 2013

The primary sources of energetic electron precipitation (EEP) which affect altitudes <100 km (>30 keV) are expected to be from the radiation belts and during substorms. EEP from the radiation belts should be restricted to locations between L = 1.5 and 8, while substorm-produced EEP is expected to range from L = 4 to 9.5 during quiet geomagnetic conditions. Therefore, one would not expect any significant D region impact due to electron precipitation at geomagnetic latitudes beyond about L = 10. In this study we report on large unexpectedly high-latitude D region ionization enhancements, detected by an incoherent scatter radar at L ≈ 16, which appear to be caused by electron precipitation from substorms. We go on to reexamine the latitudinal limits of substorm-produced EEP using data from multiple low-Earth orbiting spacecraft, and demonstrate that the precipitation stretches many hundreds of kilometers poleward of the previously suggested limits. We find that a typical substorm will produce significant EEP over the International Geomagnetic Reference Field L shell range L = 4.6 ± 0.2-14.5 ± 1.2, peaking at L = 6-7. However, there is significant variability from event to event; in contrast to the median case, the strongest 25% of substorms have significant EEP in the range spanning L = 4.1 ± 0.1-20.7 ± 2.2, while the weakest 25% of substorms have significant EEP in the range spanning L = 5.5 ± 0.1-10.1 ± 0.7. We also examine the occurrence probability of very large substorms, focusing on those events which appear to be able to disable geostationary satellites when they are located near midnight magnetic local time. On average, these large substorms occur approximately one to six times per year, a significant rate, given the potential impact on satellites. Key Points Unexpectedly high-latitude D region enhancements seen during substorms. A typical substorm will produce significant EEP over the L shell range L = 4.6-15 Satellite-disrupting substorms typically occur one to six times per year. ©2013. American Geophysical Union. All Rights Reserved.

Bilitza D.,George Mason University | Bilitza D.,NASA | Altadill D.,Ramon Llull University | Zhang Y.,Johns Hopkins University | And 5 more authors.
Journal of Space Weather and Space Climate | Year: 2014

The International Reference Ionosphere (IRI) project was established jointly by the Committee on Space Research (COSPAR) and the International Union of Radio Science (URSI) in the late sixties with the goal to develop an international standard for the specification of plasma parameters in the Earth's ionosphere. COSPAR needed such a specification for the evaluation of environmental effects on spacecraft and experiments in space, and URSI for radiowave propagation studies and applications. At the request of COSPAR and URSI, IRI was developed as a data-based model to avoid the uncertainty of theory-based models which are only as good as the evolving theoretical understanding. Being based on most of the available and reliable observations of the ionospheric plasma from the ground and from space, IRI describes monthly averages of electron density, electron temperature, ion temperature, ion composition, and several additional parameters in the altitude range from 60 km to 2000 km. A working group of about 50 international ionospheric experts is in charge of developing and improving the IRI model. Over time as new data became available and new modeling techniques emerged, steadily improved editions of the IRI model have been published. This paper gives a brief history of the IRI project and describes the latest version of the model, IRI-2012. It also briefly discusses efforts to develop a real-time IRI model. The IRI homepage is at © D. Bilitza et al., Published by EDP Sciences 2014.

Lotz S.,SANSA Space Science | Heilig B.,Geological and Geophysical Institute of Hungary | Sutcliffe P.,SANSA Space Science
Annales Geophysicae | Year: 2015

In this paper we describe the development of two empirical models of Pc3 wave activity observed at a ground station. The models are tasked to predict pulsation intensity at Tihany, Hungary, from the OMNI solar wind data set at 5 min time resolution. One model is based on artificial neural networks and the other on multiple linear regression. Input parameters to the models are iteratively selected from a larger set of candidate inputs. The optimal set of inputs are solar wind speed, interplanetary magnetic field orientation (via cone angle), proton density and solar zenith angle (representing local time). Solar wind measurements are shifted in time with respect to Pc3 data to account for the propagation time of ULF perturbations from upstream of the bow shock. Both models achieve correlation of about 70% between measured and predicted Pc3 wave intensity. The timescales at which the most important solar wind parameters influence pulsation intensity are calculated for the first time. We show that solar wind speed influences pulsation intensity at much longer timescales (about 2 days) than cone angle (about 1 h) © Author(s) 2015. CC Attribution 3.0 License..

Gjesteland T.,University of Bergen | Ostgaard N.,University of Bergen | Collier A.B.,SANSA Space Science | Collier A.B.,University of KwaZulu - Natal | And 3 more authors.
Geophysical Research Letters | Year: 2012

This letter presents a new search algorithm for identifying Terrestrial Gamma ray Flashes (TGFs) in the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) data. The algorithm has been applied to data from the period 2004-2006 and we have found more than twice as many TGFs as previously reported. The new TGFs follow the same geographical and seasonal variations as the previously reported TGFs. The match percentage between the new TGFs and World Wide Lightning Location Network (WWLLN) data is comparable to the RHESSI catalog TGFs. Our results shows that previous searches only identified the most intense events, and that there might be a large population of faint TGFs. Copyright © 2012 by the American Geophysical Union.

Collier A.B.,SANSA Space Science | Collier A.B.,University of KwaZulu - Natal | Collier A.B.,University of Bergen | Gjesteland T.,University of Bergen | Ostgaard N.,University of Bergen
Journal of Geophysical Research: Space Physics | Year: 2011

The times and locations of 972 TGFs detected by Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) between 4 March 2002 and 6 September 2010 were compared to lightning data for the same period. Based on a simple coincidence algorithm, 93 TGFs were uniquely matched to individual lightning discharges. The average delay between the lightning and the associated TGF was -0.77 ms, suggesting that the TGFs occurred prior to the lightning discharge. The majority of the matched lightning occurred within 500 km of the RHESSI sub-satellite point, although a few events were found at larger distances. A comparison of TGF intensity to observation angle for the matched events was found to be consistent with a power law model for the distribution of TGF intensities. Finally it was found that the matched TGFs were predominantly those of lower intensity. Copyright © 2011 by the American Geophysical Union.

Collier A.B.,SANSA Space Science | Collier A.B.,University of KwaZulu - Natal | Burgesser R.E.,National University of Cordoba | Avila E.E.,National University of Cordoba
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2013

The efficiency and stability of global lightning data are not yet sufficient to provide accurate estimates of long term trends in lightning activity. In contrast, regional lightning networks are generally both efficient and stable. Regions with a low level of interannual variability are well suited to the identification of trends in lightning activity. Satellite lightning data are used to identify countries in South America which display only mild interannual variability. These countries are candidates for regional studies of long term trends in lightning activity. These trends can be linked to climate change. © 2012 Elsevier Ltd.

Ostgaard N.,University of Bergen | Gjesteland T.,University of Bergen | Carlson B.E.,University of Bergen | Carlson B.E.,Carthage College | And 5 more authors.
Geophysical Research Letters | Year: 2013

We present the very first simultaneous detection from space of a terrestrial gamma ray flash (TGF) and the optical signal from lightning. By fortuitous coincidence, two independent satellites passed less than 300 km from the thunderstorm system that produced a TGF that lasted 70 μs. Together with two independent measurements of radio emissions, we have an unprecedented coverage of the event. We find that the TGF was produced deep in the thundercloud at the initial stage of an intracloud (IC) lightning before the leader reached the cloud top and extended horizontally. A strong radio pulse was produced by the TGF itself. This is the first time the sequence of radio pulses, TGF, and optical emissions in an IC lightning flash has been identified. ©2013. American Geophysical Union. All Rights Reserved.

Gjesteland T.,University of Bergen | Ostgaard N.,University of Bergen | Collier A.B.,SANSA Space Science | Collier A.B.,University of KwaZulu - Natal | And 3 more authors.
Journal of Geophysical Research: Space Physics | Year: 2011

Terrestrial gamma ray flashes (TGFs) are bremsstrahlung emissions from relativistic electrons accelerated in electric fields associated with thunder storms, with photon energies up to at least 40 MeV, which sets the lowest estimate of the total potential of 40 MV. The electric field that produces TGFs will be reflected by the initial angular distribution of the TGF emission. Here we present the first constraints on the TGF emission cone based on accurately geolocated TGFs. The source lightning discharges associated with TGFs detected by RHESSI are determined from the Atmospheric Weather Electromagnetic System for Observation, Modeling, and Education (AWESOME) network and the World Wide Lightning Location Network (WWLLN). The distribution of the observation angles for 106 TGFs are compared to Monte Carlo simulations. We find that TGF emissions within a half angle >30° are consistent with the distributions of observation angle derived from the networks. In addition, 36 events occurring before 2006 are used for spectral analysis. The energy spectra are binned according to observation angle. The result is a significant softening of the TGF energy spectrum for large (>40°) observation angles, which is consistent with a TGF emission half angle (<40°). The softening is due to Compton scattering which reduces the photon energies. Copyright 2011 by the American Geophysical Union.

De Villiers J.S.,SANSA Space Science | Cilliers P.J.,SANSA Space Science
Annales Geophysicae | Year: 2014

This research focuses on the inversion of geomagnetic variation field measurement to obtain source currents in the ionosphere. During a geomagnetic disturbance, the ionospheric currents create magnetic field variations that induce geoelectric fields, which drive geomagnetically induced currents (GIC) in power systems. These GIC may disturb the operation of power systems and cause damage to grounded power transformers. The geoelectric fields at any location of interest can be determined from the source currents in the ionosphere through a solution of the forward problem. Line currents running east-west along given surface position are postulated to exist at a certain height above the Earth's surface. This physical arrangement results in the fields on the ground having the magnetic north and down components, and the electric east component. Ionospheric currents are modelled by inverting Fourier integrals (over the wavenumber) of elementary geomagnetic fields using the Levenberg-Marquardt technique. The output parameters of the inversion model are the current strength, height and surface position of the ionospheric current system. A ground conductivity structure with five layers from Quebec, Canada, based on the Layered-Earth model is used to obtain the complex skin depth at a given angular frequency. This paper presents preliminary and inversion results based on these structures and simulated geomagnetic fields. The results show some interesting features in the frequency domain. Model parameters obtained through inversion are within 2% of simulated values. This technique has applications for modelling the currents of electrojets at the equator and auroral regions, as well as currents in the magnetosphere. © 2014 Author(s).

Collier A.B.,SANSA Space Science | Collier A.B.,University of KwaZulu - Natal | Hughes A.R.W.,University of KwaZulu - Natal
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2011

The idea that tropical lightning activity follows the motion of the Intertropical Convergence Zone (ITCZ) is based on anecdotal evidence, or at best indirect measurements. Definitive observations of lightning from satellite instruments are used here to demonstrate that the seasonal motion of peak lightning over tropical Africa has the same phase as the migration of the ITCZ but that there is no precise space-time coincidence. It is also shown that over shorter time scales the tropical band of lightning activity does not simply follow the Sun nor does it adhere to the ITCZ. What emerges is a complex pattern of behaviour which, though clearly influenced by solar declination and the ITCZ, is also determined by the underlying terrain and humidity. It is sometimes considered that lightning in the tropics would be a good locator of the ITCZ, it is shown here that this is not the case. © 2011 Elsevier Ltd.

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