Applications of spatial altimetry to studies of ocean dynamics in the Gulf of Cadiz

Sea-level records, and their global and local spatio-temporal variations, are related to many oceanographic and geophysical features. The sea surface can also be visualised as the physical representation of the geoid, averaging the sea-surface records over long periods to eliminate the short periods of dynamic perturbation. Over the last ten years, orbit determination improvements have opened up the possibility of using, for ocean dynamic and sea-level variation studies, several altimetric satellites, accurate to the sub-decimetre level (ERS-1, ERS-2, Topex-Poseidon). Of these satellites, Topex-Poseidon makes it possible to measure the sea-level variation over global or regional zones with an accuracy better than 5 cm. This paper presents the results of the sea-surface height measurements from Topex-Poseidon, corrected for wet and dry troposphere range delays, ionosphere delay, sea-state bias, the inverse barometer, loading effects and the oceanic, solid-earth and pole tides. The authors analyse the spatio-temporal stability of the geoid OSU91-a and the mean sea-surface OSUMSS95, comparing the behaviour of the dynamic signal using the two different surfaces along Topex profile 122, which overflies the Atlantic apertures of the Gulf of Cadiz. Long-term analysis has been done over several points located on the Iberian Atlantic continental margin using Topex Poseidon altimeter data collected from October 1992 (cycle 2) to October 1997 (cycle 186). The main findings of the data analysis present a semi-annual variation with peaks in autumn and winter, separated on the order of 20 cm, which could be explained by the seawater temperature seasonal variation and the doubtful use of the inverse barometer model to correct the response of the ocean to changes in atmospheric pressure. Records also show a secular variation in the regional sea level of roughly 3 mm/year, according to historical tide-gauge trends, which could also probably be explained by polar ice melting and a slight warming tendency of the ocean.Los registros de la variación espaciotemporal del nivel del mar están relacionados con un gran número de procesos oceanográficos y geofísicos que afectan a la superficie del océano y que pueden representar físicamente la figura del geoide, al eliminarse las variaciones dinámicas de corto periodo una vez promediados los registros sobre largos periodos de tiempo. En el último decenio los desarrollos técnicos y las mejoras en los formalismos de corrección y cálculo de las órbitas han permitido misiones altimétricas de precisión subdecimétrica, especialmente diseñadas para el estudio de la variación del nivel del mar y la dinámica oceánica global y regional (ERS-1, ERS-2, Topex-Poseidon). Una vez corregidos los observables del altímetro conforme propone la Agencia AVISO, se han comparado las precisiones del geoide OSU91-a con la superficie media OSUMSS95, analizando los valores obtenidos en la señal dinámica en el perfil del Topex n1 122, al sobrevolar la zona del Atlántico ibérico. Los autores presentan los resultados del estudio que han realizado de los datos obtenidos por el satélite Topex-Poseidon entre los ciclos 2 y 186 de sus órbitas (octubre de 1992 y octubre de 1997) en varios puntos del margen continental ibérico. Los principales resultados presentan una variación, del orden de 20 cm, en el nivel del mar en los bajos de Gorringe de tipo semianual, justificable con la variación de la temperatura de las masas de agua superficiales entre otoño e invierno y un comportamiento inadecuado del modelo de barímetro invertido en las zonas de estudio. Una variación del mismo orden se detecta en el golfo de Cádiz, igualmente justificable por la variación estacional de la temperatura del océano en su superficie. Además de la variación semianual se detecta una variación secular del orden de 2 a 3 mm/año, probablemente de naturaleza global, compatible con los resultados obtenidos con mareógrafos clásicos, justificable por la fusión de hielos en los casquetes polares y un ligero aumento secular de la temperatura de las masas de agua del océano en la zona del margen continental ibérico.Instituto Español de Oceanografí

Mapping the secondary star in QQ Vulpeculae

We present high- and medium-resolution phase-resolved far-red spectra of the magnetic cataclysmic variable QQ Vul. The spectra show the Na i doublet absorption features near λ 8190 Å from the cool secondary star, and the lines of He ii, O i, Mg ii, C i, N i, Ca ii and Paschen in emission. Using a Doppler imaging technique, we find that the H i, He ii, C i and O i lines have a narrow component originating near the L1 point and a strong component from the stream, while the Mg ii and Ca ii emission arises solely from the illuminated hemisphere of the red dwarf. We carry out an exhaustive analysis of the emission- and absorption-line velocities and fluxes seen in the QQ Vul spectrum. By simultaneously fitting the radial velocity and flux information we are able to produce surface maps of each line on the secondary star using a technique analogous to the one employed by Davey. The Na i and Mg ii maps show an asymmetric distribution akin to that seen in AM Her. Although the observed velocity semi-amplitudes (K2) of the lines can potentially be corrected for the effects of irradiation, we find that time-dependent changes in the degree of heating on the secondary can lead to large discrepancies in the results, significant enough to give inconsistent values from data taken at different epochs. We discuss the limitations of the surface mapping method as a means of correcting the observed K2. Our results also suggest that the emission features from the red dwarf are likely to be formed at quite high levels of the stellar chromosphere, in some cases probably even beyond the L1 point and inside the Roche lobe of the white dwarf, with the different lines possibly forming at different depths. Using the Na i absorption doublet, we find a velocity semi-amplitude for the secondary star of K2=219±6 km s−1 and a projected rotational velocity of vrot sin i=110±15 km s−1. Thus we estimate the mass ratio to be q=0.54±0.14. Based on the results of the best-fitting surface maps on all the lines, and the nature of the phase-dependent variations of the continuum and lines, we infer a binary inclination of i=65°±7°, and obtain a complete set of binary parameters for QQ Vul. We classify the secondary star as M4V from the TiO band ratios

Lithospheric magnetic mapping of the northern Caribbean region

A more complete crustal perspective of the northernmost part of the Venezuela Basin is provided by data from an international public database processed with new geomagnetic models (CM4) together with data from aeromagnetic surveys and from a recent marine cruise in areas offshore Puerto Rico and the Dominican Republic. The magnetic anomaly map set off three main domains: the North Atlantic plate, a narrow zone extending from the Dominican Republic to Puerto Rico-Virgin Islands, and the North Caribbean Plate. We focused mainly on the latter, applying the Euler deconvolution algorithm in the Venezuela Basin. Shallow and middle depth sources (located between 2km and 12km) are inferred to be the dominant sources (85%), while deep sources are located mostly in the Venezuela Basin, and scarcely appear in the Beata and Aves Ridges. We performed 2D+1/2 modeling on an eastwest transect, using Bouguer gravity and magnetic anomaly data to derive a more detailed crustal description of the northern part of the Venezuela Basin. The model suggests that in the Caribbean crust, long wavelengths (>200km) are controlled by variations in crustal thickness, whereas the intermediate ones (50-100km) are mainly controlled by variations in their magnetic properties. It also suggests the presence of a local anomalous body located in the east-west transect, between 487km and 560km, displaying a remarkable positive magnetization contrast (0.3 SI) with regard to the rest of the basin. This body shows a slight increase in density (3010kg/m3), and extends from the bottom of the UCL (Underplate Cumulate Layer) to a depth of 7.5km. We correlate it with the volcanic wedge reported by Driscoll and Diebold (1998)

HST and ground-based eclipse observations of V2051 Ophiuchi: Binary parameters

We report on high-speed eclipse photometry of the dwarf nova V2051 Oph while it was in a low brightness state, at B ~ 16.2 mag. In comparison to the average IUE spectra, the ultraviolet continuum and emission lines appear reduced by factors of, respectively, ~4 and ~5. Flickering activity is mostly suppressed and the lightcurve shows the eclipse of a compact white dwarf at disc centre which contributes ~60 per cent of the total light at 3900--4300 A. We use measurements of contact phases in the eclipse lightcurve to derive the binary geometry and to estimate masses and relevant dimensions. We find a mass ratio of q= 0.19+/-0.03 and an inclination of i= 83+/-2 degrees. The masses of the component stars are M_1 = 0.78+/-0.06 M_dot and M_2 = 0.15+/-0.03 M_dot. Our photometric model predicts K_1 = 83+/-12 km/s and K_2= 435+/-11 km/s. The predicted value of K_1 is in accordance with the velocity amplitude obtained from the emission lines after a correction for asymmetric line emission in the disc is made (Watts et al. 1986). The secondary of V2051 Oph is significantly more massive than the secondaries of the other ultra-short period dwarf novae. V2051 Oph is probably a relatively young system, whose secondary star had not enough time to evolve out of thermal equilibrium.Comment: 16 pages, 7 postscript figures, coded using MNRAS latex style. To appear in Monthly Notices of the Royal Astronomical Society. Revised version with changes in section 4.3. For related papers and files see ftp://fsc01.fsc.ufsc.br/pub/bap and http://www.fsc.ufsc.br/~astr

Aplicación de métodos de compresión de cabeceras TCP/IP como optimización de las comunicaciones móviles

El protocolo TCP (Transmission Control Protocol) está implementado para actuar en redes fija s, donde las incidencias en la comunicación pueden interpreta rse como problemas de con ges tión. En entornos móviles, en cambio, los problemas son debidos a una alta tasa de error en bit (BER); la utilización del protocolo TCP en este tipo de entornos afecta negativamente a la efici encia de la transmisión. En este artíc ulo se presenta una solución pa ra optimizar la comunicación TCP: la compresión de cabeceras; concretamente se expone la necesidad de un algoritmo especifico para comunicaciones móviles y se valora la propuesta de compresión de cabeceras de la lETF .Peer Reviewe