Browsing by Author "Hysell, D."
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Item Open Access Measurements of the latitudinal distributions of total electron content during equatorial spread F events(American Geophysical Union, 2001-12) Valladares, C. E.; Basu, S.; Groves, K.; Hagan, M. P.; Hysell, D.; Mazella Jr., A. J.; Sheehan, R. E.We have constructed latitudinal profiles of the total electron content (TEC) using measurements from six GPS receivers conducted during 1998. The TEC profiles have been divided into two groups: One corresponds to days when plumes or equatorial spread F (ESF) develops, and the second group portrays days of no-ESF condition. The presence/absence of ESF is based on the signature of the coherent echoes measured by the Jicamarca Unattended Long-Term Investigation (JULIA) radar and records of scintillations from two sites spaced in latitude. One scintillation station is located near the magnetic equator (Ancon) and the other 12° southward (Antofagasta). The TEC profiles display the typical day-to-day and seasonal variability seen at low latitudes. During the equinoxes, we observed quite often the crests of the anomaly located between 12° and 20° away from the magnetic equator and a trough in-between. The monthly distribution of the appearance of the anomaly and the local time of their appearance are in very good agreement with the reported variability of the upward vertical drifts and the current theory of the equatorial fountain effect. During the equinoxes and the December solstice, the TEC anomaly is observed almost every day, sometimes when there is no ESF activity. Nevertheless, fine inspection of the TEC latitudinal profiles suggests the existence of a close relationship between the temporal evolution of the TEC profiles near sunset and the onset of ESF. We have examined the TEC latitudinal distributions in two different ways. First, we calculated time difference profiles using the distributions corresponding to 1800 and 2000 LT. Second, we used a parameterization of the TEC distributions obtained at 2000 LT. The first method indicates quite drastic increases of the crest values and sharp decreases near the trough during ESF days. In contrast, during days of no ESF there exist almost uniform TEC decreases at all latitudes. The second method displays a preferred high crest/trough ratio (2), small TEC values at the trough, and large latitudinal integrated values during ESF events.Item Open Access Obtención de imágenes de "Spread F" usando multiprocesamiento(Instituto Geofísico del Perú, 2010) Urco, M.; Chau Chong Shing, Jorge Luis; Hysell, D.Diapositivas presentadas en el XIII Encuentro Científico Internacional, (ECI 2010v), realizado del 5 al 8 de enero de 2010 en la ciudad de Lima.Item Open Access The equatorial electrojet radar observations and modeling(Cornell University, 2006-08) Shume, Esayas Belay; Hysell, D.This dissertation describes a theoretical, experimental, and modeling investigation of the equatorial electrojet. We review low latitude ionospheric current models, synthesizing developments from the early times until the present. We then show how to utilize equatorial electrojet irregularities to infer E region electron density and wind profiles from coherent scatter radar experiments. The procedure involves a numerical model of the equatorial ionosphere that relates the vector electric field and current density to the winds. We present electron densities inferred in the equatorial electrojet inferred using a new bistatic radar system installed between Paracas and Jicamarca, Peru. The radar system monitors density profiles using a coherent scatter radar technique that utilizes the Faraday rotation of the scattered signal. Radar measured density profiles are validated by comparing with other electron density measures. A three dimensional electrostatic potential model of the equatorial ionosphere in a magnetic dipole coordinate system is described. The model incorporates realistic ionospheric conductivities, electric fields, winds, and includes anamalous collision effects. The model utilizes bistatic radar measured densities, coherent scatter spectral measurements made at large zenith angles, and electric fields derived from 150 km echo drifts. The model is also constrained by magnetometer records. We next present a technique for extracting zonal winds in the equatorial electrojet from the Doppler shifts of type II radar echoes measured by a narrow beam, obliquely oriented antenna at the Jicamarca Radio Observatory. The wind profiles were retrieved by combining the 3-D model with theory and measurements of type II echo Doppler shifts. The amplitude and phasing of the calculated wind profiles are in general agreement with satellite and rocket-borne wind measurements. We have used height varying type I radar echoes and large-scale electrojet irregularities inferred from interferometric imaging to validate wind profiles estimates derived from type II echoes.Item Open Access Using JULIA long dataset to find preconditioning evidence of ESF in bottom-type layers(Instituto Geofísico del Perú, 2008) Pinedo, H.; Chau Chong Shing, Jorge Luis; Hysell, D.Recently Hysell et al. [2005] has suggested that the periodic structuring observed in the bottomtype (BT) scattering layers might be used to determine the occurrence or not of full-blown equatorial spread F (ESF) on a given day. The seed or precursor waves may be generated by a collisional shear instability. Preliminary observations at Jicamarca and AL TAIR have shown that such structures present wavelengths of the order of tens or hundreds of kilometers. At Jicamarca periodic structures have been observed using in-beam radar imaging techniques, however such observations are limited to few days in the last five years. On the other hand, routine observations using the JULIA system at Jicamarca have been done since 1996 on approximately 100 days per year on average, using interferometric and dual-beam observations with very narrow beams. Since the bottom-type irregularities drift ata relatively constant speed in the westward direction, using the JULIA narrow beams (～1o), the spatial periodicities might be observed as temporal periodicities in typical range-time intensity maps. In this initial work we present the statistics of the observed bottom-type periodicities and the occurrence of ESF plumes as detected with the JULIA system for years 2006-2008. Besides we presenta time occurrence statistics of a weak bottom-type layer that took place previously to bottom-type layers.