Showing papers by "Cesar E. Valladares published in 2012"

Equatorial plasma bubbles and L-band scintillations in Africa during solar minimum

Abstract: . 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.

The Low‐Latitude Ionosphere Sensor Network: Initial results

Abstract: [1] The Low-Latitude Ionospheric Sensor Network (LISN) is a distributed observatory designed to nowcast the state and dynamics of the low-latitude ionosphere and to develop forecasts of the electric fields, densities, and equatorial spread F over the South American continent. The LISN observatory consists of three different types of instruments: GPS receivers, fluxgate magnetometers, and vertical incidence pulsed ionospheric radar (VIPIR) ionosondes. This report provides a succinct summary of recent observations obtained using the LISN GPS receivers and complemented with measurements from other instruments and GPS receivers that operate in South America. More specifically, the following are shown here: (1) observations of total electron content (TEC) enhancements that occur near local midnight, (2) maps of TEC perturbations associated with the passage of traveling ionospheric disturbances over South America, and (3) statistics of TEC depletions for 2 years of low solar activity. Near-midnight TEC enhancements consist of sudden increases in TEC that occur after sunset at low latitudes on 30% of the days. These TEC enhancements last for several hours and can have amplitudes between 1 and 50 TEC units. On 11–12 March 2011 the largest TEC enhancement was observed in South America at times when the Jicamarca incoherent scatter radar operated and observed peak densities above 106 el/cc at 300 km altitude. It is suggested that a combination of zonal electric fields and meridional neutral winds are able to redistribute the plasma along the field lines and create regions of enhanced TEC. Maps of TEC perturbations associated with the passage of gravity waves (GWs) over South America have been used to measure the phase velocity and direction of propagation of GWs. The large number of GPS receivers over South America has allowed us to record bubble events for every day during 2008 and 2009. It was found that the number of TEC depletion detections varies with a periodicity of 28 days. It is mentioned how these new observations and the installation of the last four VIPIR ionosondes will lead to new discoveries in the near future.

Longitudinal Differences of Ionospheric Vertical Density Distribution and Equatorial Electrodynamics

Abstract: Accurate estimation of global vertical distribution of ionospheric and plasmaspheric density as a function of local time, season, and magnetic activity is required to improve the operation of space-based navigation and communication systems. The vertical density distribution, especially at low and equatorial latitudes, is governed by the equatorial electrodynamics that produces a vertical driving force. The vertical structure of the equatorial density distribution can be observed by using tomographic reconstruction techniques on ground-based global positioning system (GPS) total electron content (TEC). Similarly, the vertical drift, which is one of the driving mechanisms that govern equatorial electrodynamics and strongly affect the structure and dynamics of the ionosphere in the low/midlatitude region, can be estimated using ground magnetometer observations. We present tomographically reconstructed density distribution and the corresponding vertical drifts at two different longitudes: the East African and west South American sectors. Chains of GPS stations in the east African and west South American longitudinal sectors, covering the equatorial anomaly region of meridian approx. 37 deg and 290 deg E, respectively, are used to reconstruct the vertical density distribution. Similarly, magnetometer sites of African Meridian B-field Education and Research (AMBER) and INTERMAGNET for the east African sector and South American Meridional B-field Array (SAMBA) and Low Latitude Ionospheric Sensor Network (LISN) are used to estimate the vertical drift velocity at two distinct longitudes. The comparison between the reconstructed and Jicamarca Incoherent Scatter Radar (ISR) measured density profiles shows excellent agreement, demonstrating the usefulness of tomographic reconstruction technique in providing the vertical density distribution at different longitudes. Similarly, the comparison between magnetometer estimated vertical drift and other independent drift observation, such as from VEFI onboard Communication/Navigation Outage Forecasting System (C/NOFS) satellite and JULIA radar, is equally promising. The observations at different longitudes suggest that the vertical drift velocities and the vertical density distribution have significant longitudinal differences; especially the equatorial anomaly peaks expand to higher latitudes more in American sector than the African sector, indicating that the vertical drift in the American sector is stronger than the African sector.

Impacts of ionospheric scintillations on GPS receivers intended for equatorial aviation applications

Abstract: [1] 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.

Latitudinal and Local Time Variation of Ionospheric Turbulence Parameters during the Conjugate Point Equatorial Experiment in Brazil

Abstract: Previous authors have reported on the morphology of GPS scintillations and irregularity zonal drift during the 2002 Conjugate Point Equatorial Experiment (COPEX) in Brazil. In this paper, we characterize the turbulent ionospheric medium that produced these scintillations. Using 10 Hz GPS carrier-to-noise measurements at Boa Vista (2.9°N, 60.7°W), Alta Floresta (9.9°S, 56.1°W), and Campo Grande (20.5°S, 54.7°W), we report on the variation of turbulent intensity, phase spectral index, and irregularity zonal drift as a function of latitude and local time for the evening of 1-2 November 2002. The method of analysis is new and, unlike analytical theories of scintillation based on the Born or Rytov approximations, it is valid when the scintillation index saturates due to multiple-scatter effects. Our principal findings are that (1) the strength of turbulence tended to be largest near the crests of the equatorial anomaly and at early postsunset local times, (2) the turbulent intensity was generally stronger and lasted two hours longer at Campo Grande than at Boa Vista, (3) the phase spectral index was similar at the three stations but increased from 2.5 to 4.5 with local time, and (4) our estimates of zonal irregularity drift are consistent with those provided by the spaced-receiver technique.

Measurement of the Characteristics of TIDs Using Small and Regional Networks of GPS Receivers during the Campaign of 17–30 July of 2008

Abstract: This scientific report presents the results of a dedicated experiment that was conducted within the framework of the Low-latitude ionospheric Sensor Network (LISN) observatory to measure the characteristics of medium-scale (hundreds of km) Traveling Ionospheric Disturbances (TIDs) as they transit through the low-latitude ionosphere A small array of 3 GPS receivers separated by 4-5 km placed in a triangular configuration was installed near Huancayo in Peru possessing several characteristics of a radio-interferometer During the campaign days, 17–30 July 2008, TIDs were observed daily On July 20, 2008 between 22 and 24 UT several TIDs moved across the small array of GPS receivers with a velocity near 130 m/s, were directed northward and had wavelengths close to 450 km Other GPS receivers that were operating hundreds of km away from Huancayo show also similar TEC traces and provide a phase velocity equal to 150 m/s This value was measured using the GPS at Piura, Cuzco and Huancayo Based on this positive result, we conclude that small and/or regional arrays of GPS receivers can be used at low latitudes to study the role that gravity waves may have on seeding plasma bubbles