Entity

Time filter

Source Type

South Africa

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. Source


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. Source


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. Source


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 http://IRImodel.org. © D. Bilitza et al., Published by EDP Sciences 2014. Source


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. Source

Discover hidden collaborations