Browsing by Author "Groves, K. M."
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Item Restricted C/NOFS satellite observations of equatorial ionospheric plasma structures supported by multiple ground‐based diagnostics in October 2008(American Geophysical Union, 2011-10-28) Nishioka, M.; Basu, Su.; Basu, S.; Valladares, C. E.; Sheehan, R. E.; Roddy, P. A.; Groves, K. M.In early October 2008, the C/NOFS satellite orbited near the magnetic equator at its perigee altitude of ∼400 km at dusk in the Peruvian sector. This provided an ideal opportunity for a comparison, under the current very low solar flux condition, of equatorial ionospheric disturbances observed with the Communication/Navigation Outage Forecasting System (C/NOFS) in situ measurements and ground‐based observations available near Jicamarca Observatory. The primary objective was the comparison of plasma density disturbances measured by a Planar Langmuir Probe (PLP) instrument on the C/NOFS satellite with VHF scintillation activity at Ancon near Jicamarca for this period. Here we discuss in detail two extreme cases: one in which severe in situ disturbances were accompanied by mild scintillation on a particular day, namely, 10 October while there was little in situ disturbance with strong scintillation on 5 October. This apparent contradiction was diagnosed further by a latitudinal ground‐based GPS network at Peruvian longitudes, a Digisonde, and the incoherent scatter radar (ISR) at Jicamarca. The crucial distinction was provided by the behavior of the equatorial ionization anomaly (EIA). The EIA was well‐developed on the day having severe in situ disturbances (10 Oct). This led to lower equatorial plasma density and total electron content (TEC) at the equator and consequently reduced the scintillations detected at Ancon. On the other hand, on the day with severe scintillations (5 Oct), the EIA was not so well developed as on 10 October, leading to relatively higher equatorial plasma density and TEC. Consequently the severe scintillations at Ancon were likely caused by ionospheric structure located below the altitude of C/NOFS. The NRL SAMI2 model was utilized to gain a greater understanding of the role of neutral winds and electric fields in reproducing the TEC as a function of latitude for both classes of irregularities. Spectral studies with high resolution in situ PLP data were also performed. The power law spectra within the plasma bubbles showed two slopes: the low frequency slope being ∼−5/3 and the high frequency ∼−5 with a break around λ = 70 m. This particular type of two‐slope spectra may be related to the extremely low solar activity and its impact on ion composition and temperature.Item Restricted Effect of magnetic activity on the dynamics of equatorial F region irregularities(American Geophysical Union, 2002) Bhattacharyya, A.; Basu, S.; Groves, K. M.; Valladares, C. E.; Sheehan, R.Two different aspects of the effect of magnetic activity on the dynamics of equatorial spread F (ESF) irregularities are studied here using spaced receiver scintillation observations. The first one deals with the question of how magnetic activity affects the generation of ESF irregularities. For this, a parameter designated the “random velocity,” which is a measure of random changes in the irregularity drift velocity, is evaluated from the data. In past studies, this parameter has been found to have large values in the early phase of evolution of ESF irregularities during the postsunset period, with a steep decline to a low value by 22 LT. This behavior is attributed to the decline in the height of the F region. Therefore, a sudden increase in the “random velocity” in the postmidnight period is attributed to an increase in the height of the F region due to the ionospheric zonal electric field turning from westward to eastward due to the effect of magnetic activity, which may also generate fresh irregularities that produce the observed scintillations. This idea has been used to suggest that for two of the magnetically active days considered in the present study the irregularities may be freshly generated in the postmidnight period. The second aspect is the identification of geomagnetically disturbed plasma drifts, which is generally possible only after 22 LT, when the estimated irregularity drift velocities are close to that of the background plasma. The pattern of the estimated drift after 22 LT (3 UT) is found to be well defined for magnetically quiet days with scintillations during a period of a month. This allows the identification of a superimposed westward perturbation in the drift, produced by a disturbance dynamo due to magnetic activity, for all the three events studied here. On 19 February and 1 March 1999, the eastward drift velocities show an identical decrease of about 50 m/s from the undisturbed drift at 0440 UT. On 1 March, the decay phase of the storm sets in later, and the eastward velocity continues to decrease until 0530 UT, turning westward with a maximum decrease of about 80 m/s from the undisturbed drift. On 22 October 1999, which was more disturbed than these two days, the westward perturbation was larger, causing the drift velocity to turn westward around 5 UT and a decrease of nearly 150 m/s from the quiet time drift at 8 UT. The results are in broad agreement with some of the recent empirical models of the evolution, with storm time, of equatorial disturbance dynamo electric fields.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 Open Access Equatorial scintillation and systems support(American Geophysical Union, 1997-09) Groves, K. M.; Basu, S.; Weber, E. J.; Smitham, M.; Kuenzler, H.; Valladares, C. E.; Sheehan, R.; MacKenzie, E.; Secan, J. A.; Ning, P.; McNeill, W. J.; Moonan, D. W.; Kendra, M. J.The need to nowcast and forecast scintillation for the support of operational systems has been recently identified by the interagency National Space Weather Program. This issue is addressed in the present paper in the context of nighttime irregularities in the equatorial ionosphere that cause intense amplitude and phase scintillations of satellite signals in the VHF/UHF range of frequencies and impact satellite communication, Global Positioning System navigation, and radar systems. Multistation and multifrequency satellite scintillation observations have been used to show that even though equatorial scintillations vary in accordance with the solar cycle, the extreme day-to-day variability of unknown origin modulates the scintillation occurrence during all phases of the solar cycle. It is shown that although equatorial scintillation events often show correlation with magnetic activity, the major component of scintillation is observed during magnetically quiet periods. In view of the day-to-day variability of the occurrence and intensity of scintillating regions, their latitude extent, and their zonal motion, a regional specification and short-term forecast system based on real-time measurements has been developed. This system, named the Scintillation Network Decision Aid, consists of two latitudinally dispersed stations, each of which uses spaced antenna scintillation receiving systems to monitor 250-MHz transmissions from two longitudinally separated geostationary satellites. The scintillation index and zonal irregularity drift are processed on-line and are retrieved by a remote operator on the Internet. At the operator terminal the data are combined with an empirical plasma bubble model to generate three-dimensional maps of irregularity structures and two-dimensional outage maps for the region.Item Restricted Impacts of ionospheric scintillations on GPS receivers intended for equatorial aviation applications(American Geophysical Union, 2012-07-25) Akala, A. O.; Doherty, P. H.; Carrano, C. S.; Valladares, C. E.; Groves, K. M.This study examines the impacts of ionospheric scintillations on GPS receivers that are intended for equatorial or transequatorial aviation applications. We analyzed GPS data that were acquired at Ascension Island during the Air Force Research Laboratory (AFRL) campaign of the solar maximum year of 2002. Strong scintillations impacted the receiver‐satellite geometry, leading to poor dilution of precisions and positioning accuracy. In addition, deep signal fades (>20 dB‐Hz), leading to navigation outages were observed during most of the nights of the campaign. Under quiescent conditions, the C/No of satellites fluctuated slowly between 50 dB‐Hz and 35 dB‐Hz baselines for both L1 (1.5754 GHz) and L2 (1.2276 GHz) signals, depending on the satellite's elevation angle. The satellite's elevation angle and the effective scan velocity of the satellite's ionospheric penetration point (IPP) with respect to the magnetic field and plasma drift influenced the rate of fading of satellite signals.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.Item Restricted Specification of the occurrence of equatorial ionospheric scintillations during the main phase of large magnetic storms within solar cycle 23(American Geophysical Union, 2010-10-05) Basu, S.; Basu, Su.; MacKenzie, E.; Bridgwood, C.; Valladares, C. E.; Groves, K. M.; Carrano, C.Satellite communication and navigation systems operating at low latitudes suffer outages due to ionospheric scintillations during large magnetic storms that are not currently specified by any model. This paper describes and demonstrates how in the framework of an eastward electric field penetration from high to low latitudes at dusk during the main phase of a large storm, for which the rate of change of Dst ≤ −50 nT/h and the Dst minimum index ≤ −100 nT, it is possible to specify the longitude interval within the low‐latitude ionosphere where scintillations and plasma bubbles are most likely to occur. It is known that the eastward prompt penetration electric field becomes enhanced near sunset due to the day‐to‐night conductivity gradient. Such enhanced eastward electric fields generally set off the Rayleigh‐Taylor plasma instability at F region heights and cause the formation of plasma bubbles and irregularities of electron density that give rise to scintillations of satellite signals. We first discuss two individual magnetic storms that satisfy the criterion of large magnetic storms mentioned above and for which the onsets of the main phase are about 15 h apart. We show that the dusk sectors corresponding to these two storms are such that irregularities and scintillations were observed in the Atlantic‐Peruvian longitude sector for one storm and in the Pacific sector for the other. We then present a statistical study with 30 large magnetic storms during solar cycle 23 which satisfy the two criteria of large magnetic storms and we attempt to specify the longitude interval of irregularity and scintillation occurrence during the main phase of such storms. We have tracked globally the occurrence of equatorial scintillations during magnetic storms by the use of scintillation observations made by the Air Force Research Laboratory's Scintillation Network Decision Aid (SCINDA) network and the DMSP satellite in situ measurements of plasma bubbles at 840 km. The statistical study reveals that during large magnetic storms, scintillations and plasma bubbles occur over a specific longitude sector for which the local dusk corresponds to the time interval of the main phase of storms. The magnetic storm induced scintillations may enhance the general seasonal/longitudinal pattern of quiet time scintillations at the station but may also occur where it is least expected in accordance with climatology. The storm time response of the equatorial ionosphere discussed in this paper will be implemented in the SCINDA algorithm to enhance its capability to specify scintillations during large magnetic storms.