Browsing by Author "Shume, E. B."
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Item Open Access Electron density profiles in the equatorial E region ionosphere derived from a bistatic coherent scatter radar experiment in Perú(American Geophysical Union, 2005-01-14) Shume, E. B.; Hysell, D. L.; Chau Chong Shing, Jorge LuisA new bistatic radar system has been developed at the Jicamarca Radio Observatory. The system is a permanent addition to the facility designed to monitor electron density profiles in the equatorial electrojet region using a coherent scatter radar technique that utilizes the Faraday rotation of the scattered signal. A series of radar experiments has been conducted at Jicamarca since March, 2004. In this report, representative E region electron density profiles of the equatorial ionosphere are presented. We compare our electron density estimates with: theoretical electron densities derived from the Chapman production function, electron densities measured by rocket experiments, the International Reference Ionosphere (IRI‐2001) model, and peak electron density estimates from the Jicamarca Digisonde. Overall, the radar measured density profiles have magnitudes and shapes comparable to the electron density estimators mentioned above. However, the measured and IRI‐2001 model density profiles disagree significantly below the E region peak.Item Restricted Electrostatic plasma turbulence in the topside equatorial F region ionosphere(American Geophysical Union, 2002-10-03) Hysell, D. L.; Shume, E. B.Two‐dimensional, turbulent plasma flows in the topside equatorial F region ionosphere associated with fully developed equatorial spread F are analyzed and simulated numerically. In the inertially dominated flow regime, the governing equations of motion resemble the Navier Stokes equation but are cubicly nonlinear. Large amplitude density irregularities are prerequisite for inertial effects to be important, but when these are present, the third‐order nonlinear effects become significant, mean‐squared velocity and vorticity cease to be conserved by nonlinear mode coupling, and the foundations of the turbulent cascade theory of Kraichnan [1967] are undermined. Nonetheless, one‐dimensional, angle‐averaged velocity spectra computed from simulated flows exhibit similarity ranges with k−5/3 and k−3 power laws, resembling inertial ranges and suggesting turbulent cascades. Invariants of the flow (quantities conserved by nonlinear mode coupling) are found which are generalizations of the quadratic forms of kinetic energy and enstrophy and which are dimensionally equivalent to them. Statistical properties of the flow appear to permit turbulent cascades to arise.Item Open Access Equatorial 150 km echoes and daytime F-region vertical plasma drifts in the Brazilian longitude sector(European Geosciences Union, 2013-10-31) Rodrigues, F. S.; Shume, E. B.; De Paula, E. R.; Milla, MarcoPrevious studies showed that conventional coherent backscatter radar measurements of the Doppler velocity of the so-called 150 km echoes can provide an alternative way of estimating ionospheric vertical plasma drifts during daytime hours (Kudeki and Fawcett, 1993; Chau and Woodman, 2004). Using observations made by a small, lowpower 30 MHz coherent backscatter radar located in the equatorial site of São Luís (2.59◦ S, 44.21◦ W; −2.35◦ dip lat), we were able to detect and monitor the occurrence of 150 km echoes in the Brazilian sector. Using these measurements we estimated the local time variation of daytime vertical ionospheric drifts in the eastern American sector. Here, we present a few interesting cases of 150 km-echoes observations made by the São Luís radar and estimates of the diurnal variation of vertical drifts. These cases exemplify the variability of the vertical drifts in the Brazilian sector. Using same-day 150 km-echoes measurements made at the Jicamarca Radio Observatory in Peru, we also demonstrate the variability of the equatorial vertical drifts across the American sector. In addition to first estimates of the absolute vertical plasma drifts in the eastern American (Brazilian) sector, we also present observations of abnormal drifts detected by the São Luís radar associated with the 2009 major sudden stratospheric warming event.Item Restricted Equatorial zonal electric fields inferred from a 3‐D electrostatic potential model and ground‐based magnetic field measurements(American Geophysical Union, 2009-06-06) Shume, E. B.; De Paula, E. R.; Maus, S.; Hysell, D. L.; Rodrigues, F. S.; Bekele, A.We present a new technique to infer quiet time zonal electric fields in the daytime equatorial ionosphere. The electric field inference technique utilizes a three‐dimensional (3‐D) electrostatic potential model of the low‐latitude ionosphere constrained by ground‐based magnetic field measurements. To test this technique, inferred zonal electric fields for the Peruvian sector in Jicamarca (11.95°S, 283.13°E, 0.6°N dip latitude) were compared with zonal electric field (vertical drift) measurements made by the Jicamarca Incoherent Scatter Radar. The comparison shows a good agreement between the inferred and measured electric fields. An example of electric field estimation for Davao (7.4°N, 125.4°E, 0.58°S dip latitude) in the Philippines sector is also presented in this report. Inferred electric fields for Davao are in good agreement with F region vertical plasma drifts measured by drift sensors onboard the AE‐E and ROCSAT‐1 satellites on that longitude sector. Our results suggest that realistic estimates of quiet time zonal electric fields for the equatorial ionosphere can be obtained from the 3‐D potential model whenever observatory magnetic field measurements are available.Item Restricted Flux tube analysis of L‐band ionospheric scintillation(American Geophysical Union, 2013-04-23) Shume, E. B.; Mannucci, A. J.; Butala, M. D.; Pi, X.; Valladares, C. E.This manuscript presents magnetic flux tube analysis of L‐band signal scintillation in the nighttime equatorial and low‐latitude ionosphere. Residues of the scintillation index urn:x wiley:jgra:media:jgra50285:jgra50285-math-0001 estimated from the L‐band signals received from Geostationary Earth Orbit (GEO) satellites are employed in the analysis. The urn:x wiley:jgra:media:jgra50285:jgra50285-math-0002 estimates have been shown to be associated with simultaneous GPS VTEC variations derived from JPL's GIPSY‐GIM package. We have applied the wavelet decomposition technique simultaneously on the urn:x-wiley:jgra:media:jgra50285:jgra50285-math-0003 time series in a flux tube over the equatorial and low‐latitude regions. The technique decomposes the urn:x-wiley:jgra:media:jgra50285:jgra50285-math-0004 signal to identify the dominant mode of variabilities and the temporal variations of scintillation‐producing irregularities in the context of a flux tube. Statistically significant regions of the wavelet power spectra considered in our study have mainly shown that (a) dominant plasma irregularities associated with urn:x-wiley:jgra:media:jgra50285:jgra50285-math-0005 variabilities in a flux tube have periods of about 4 to 15 minutes (horizontal irregularity scales of about 24 to 90 km). These periods match short period gravity waves, (b) scintillation‐producing irregularities are anisotropic along the flux tube and in the east‐west direction, and (c) the occurrences of scintillation‐producing irregularities along the flux tube indicate that the entire flux tube became unstable. However, plasma instability occurrences were not simultaneous in most cases along the flux tube, there were time delays of various orders. Understanding the attributes of L‐band scintillation‐producing irregularities could be important for developing measures to mitigate L‐band signal degradation.Item Restricted Longitudinal variation in Global Navigation Satellite Systems TEC and topside ion density over South American sector associated with the four‐peaked wave structures(American Geophysical Union, 2013-12-10) Nogueira, P. A. B.; Abdu, M. A.; Souza, J. R.; Bailey, G. J.; Batista, I. S.; Shume, E. B.; Denardini, C. M.Recent observations of the low‐latitude ionospheric electron density revealed a four‐peaked longitudinal structure in the equatorial ionization anomaly when plotted at a constant‐local‐time frame. It was proposed that neutral wind‐driven E region dynamo electric fields due to nonmigrating tidal modes are responsible for this pattern. We examine the four‐peaked structure in the observed topside ion density and its manifestation as longitudinal structures in total electron content (TEC) over South America. The strong longitudinal variation in TEC characterized by larger value over Brazilian eastern longitude sector as compared to that over the Peruvian western longitude is modeled using the Sheffield University plasmasphere‐ionosphere model (SUPIM) aiming to identify the control factors responsible for the longitude variation. We found that the SUPIM runs using as input the existing standard models of vertical drift, and thermospheric winds do not explain the TEC longitudinal structure. Realistic values of these control parameters were generated based on the strong vertical drift longitudinal variation as determined from magnetometer and Digisonde data and appropriately adjusted winds (horizontal wind model). These realistic vertical drifts together with the modified thermospheric wind, when used as input to the SUPIM, are found to satisfactorily explain the longitudinal differences in the TEC and topside ion density (Ni) over South America. The study shows that the TEC in the whole latitude distribution is larger over the east coast than over the west coast of South America and that the vertical drift and thermospheirc winds control the longitudinal four wave structure in the TEC and Ni.Item Restricted Variabilities of the equatorial electrojet in Brazil and Perú(American Geophysical Union, 2010-06-16) Shume, E. B.; Denardini, C. M.; De Paula, E. R.; Trivedi, N. B.This report presents seasonal and longitudinal variabilities of the equatorial electrojet in the east (Brazil, São Luís: 2.3° S; 315.8° E; 0.5° S dip latitude) and west (Jicamarca, Perú: 11.95° S; 283.13° E; 0.6° N dip latitude) coasts of the continent of South America. Ground‐based magnetic field perturbation measurements ΔH for solar maximum (2001/2002) and solar minimum (2006/2007) conditions from the two equatorial stations (São Luís and Jicamarca) have been used for the study. The ΔH signal which is a measure of the strength of the equatorial electrojet is spectrally analyzed using wavelet analysis. The results of our analysis show that (1) the equatorial electrojet has maxima around equinoxes in Jicamarca, Perú but it has a prominent maximum during Southern Hemisphere summer (centered about December/January) in São Luís, Brazil. The observed seasonal behavior of the equatorial electrojet in São Luís is highly likely due to the large magnetic declination angle (about 20° west) there. (2) The equatorial electrojet is stronger in the west coast (Jicamarca) compared to the east coast (São Luís), irrespective of solar activity condition. (3) The magnitude of the equatorial electrojet is more variable with season and solar cycle over São Luís than over Jicamarca.