Browsing by Author "Caton, R. G."
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Item Restricted Equatorial plasma bubbles and L-band scintillations in Africa during solar minimum(European Geosciences Union (EGU), 2012-04-16) Paznukhov, V. V.; Carrano, C. S.; Doherty, P. H.; Groves, K. M.; Caton, R. G.; Valladares, C. E.; Seemala, G. K.; Bridgwood, C. T.; Adeniyi, J.; Amaeshi, L. L. N.; Damtie, B.; D’Ujanga Mutonyi, F.; Ndeda, J. O. H.; Baki, P.; Obrou, O. K.; Okere, B.; Tsidu, G. M.We report on the longitudinal, local time and seasonal occurrence of equatorial plasma bubbles (EPBs) and L band (GPS) scintillations over equatorial Africa. The measurements were made in 2010, as a first step toward establishing the climatology of ionospheric irregularities over Africa. The scintillation intensity is obtained by measuring the standard deviation of normalized GPS signal power. The EPBs are detected using an automated technique, where spectral analysis is used to extract and identify EPB events from the GPS TEC measurements. Overall, the observed seasonal climatology of the EPBs as well as GPS scintillations in equatorial Africa is adequately explained by geometric arguments, i.e., by the alignment of the solar terminator and local geomagnetic field, or STBA hypothesis (Tsunoda, 1985, 2010a). While plasma bubbles and scintillations are primarily observed during equinoctial periods, there are longitudinal differences in their seasonal occurrence statistics. The Atlantic sector has the most intense, longest lasting, and highest scintillation occurrence rate in-season. There is also a pronounced increase in the EPB occurrence rate during the June solstice moving west to east. In Africa, the seasonal occurrence shifts towards boreal summer solstice, with fewer occurrences and shorter durations in equinox seasons. Our results also suggest that the occurrence of plasma bubbles and GPS scintillations over Africa are well correlated, with scintillation intensity depending on depletion depth. A question remains about the possible physical mechanisms responsible for the difference in the occurrence phenomenology of EPBs and GPS scintillations between different regions in equatorial Africa.Item Restricted Signatures of equatorial plasma bubbles in VHF satellite scintillations and equatorial ionograms(American Geophysical Union, 2013-02-22) McNamara, L. F.; Caton, R. G.; Parris, R. T.; Pedersen, T. R.; Thompson, D. C.; Wiens, K. C.; Groves, K. M.Since their discovery in the 1970s, equatorial plasma bubbles (EPBs) have been invoked to explain the propagation of VHF signals on trans‐equatorial circuits at night, and blamed for highly detrimental scintillation of VHF and GHz trans‐ionospheric communications signals in equatorial regions. Over the last four decades, the properties of EPBs have been deduced by multiple techniques such as incoherent scatter radar, 630 nm airglow, depletions in GPS total electron content observations, VHF and GHz scintillations, and HF observations by ionosondes. The initiation and evolution of EPBs have by now been successfully modeled and a good understanding developed of the underlying physics. However, different communities tend to concentrate on a single observing technique, without regard to whether the different techniques provide a consistent physical picture. In contrast, this paper discusses two very different types of observations made on a night‐by‐night basis during the COPEX campaign of late 2002 in Brazil, namely, VHF scintillations and ionograms, and shows that the two methods of observation can provide a consistent interpretation of the properties of EPBs. For example, an EPB seen as an eastward drifting scintillation event can also be seen as an extra ionogram reflection trace that moves closer to and then away from the ionosonde site. The scintillations are attributed to strong gradients across the walls of an EPB, whereas the extra ionogram traces are attributed to oblique reflection of the ionosonde signals from the walls of the EPB.