Tridimensional distribution of horizontal velocity flows under the solar surface

Abstract

Análisis de las oscilaciones solares con el objeto de estimar la distribución de flujos de velocidades horizontales en función de la profundidad en posición en el disco solar. Se utiliza una técnica de análisis de espectros tridimensionales de las oscilaciones para imágenes de alta resolución espacial que permite obtener información de la distribución de velocidades en función de la frecuencia de los modos y, a partir de aquí, mediante un proceso de inversión, esta distribución se transforma en una dependencia con la profundidad. Los resultados muestran que el campo de velocidades horizontal cambia su orientación en función de la profundidad en forma espiral alrededor de más de 360º. Los flujos presentan coherencia especial en casi todas las posiciones y profundidades desde la superficie hasta unos 20Mm, a partir de la cual la coherencia desaparece. En cuanto a la magnitud de las velocidades es del orden de 100-200 m/s hasta 20Mm de profundidad, aumentando luego a varios cientos. Asimismo se encuentran indicios de la presencia de una celda convectiva toroidal entre el ecuador solar y unos 10º de latitud Norte.

In the past years the accurate measurements of the amplitudes, widths, and frequencies of the solar oscillations have been reflected in good estimations of the frequency splittings, results that have been inverted to infer the rotation rate as a function of depth. In this work we are going to present a method to infer the distribution of horizontal velocity flows underneath the solar surface as a function of depth and position in the solar disk. The new techniques are based in a tridimensional power spectrum of the solar oscillations for high spatial resolution images. Information about the two components of the k number of the modes can be obtained in this way, and the tridimensional aspect of the ridges of the p-modes can tell us about horizontal velocity fields. The data we have used for this purpose are full-disk dopplergrams, obtained with the Magneto-Optical Filter at the Mt. Wilson 60-Foot Tower, with a resolution of about 2.2 arcsec at center of the image, and covering a time period of three days, 3/5 July, 1988. A classic model of a Lorentzian pro le in frequency has been adapted and fi tted to this tridimensional view of the oscillations, where the dependence with the two components of the k number are bound to the components of the horizontal velocity eld. A distribution of the velocity as a function of frequency for p-modes n = 0 to 7, has been inferred, and an inversion process has been developed in order to obtain an estimate of the distribution with depth of these velocities. Finally, we applied all this analysis to 9 di erent sectors of the solar disk around the center, being able then to show, as a last result, a tridimensional distribution of the horizontal velocity flows in the upper convection zone.

The results show that the horizontal velocity fi eld appears to execute a spiral as a funtion of depth. The magnitude of the flows are on the order of 100-200 msð€€€1 in the outer 20 Mm, rising to several hundred msð€€€1 at 50 Mm. The flows present a spatial coherence at almost every position and depth from the surface until about 20 Mm. Below that the spatial coherence dissapears. Three east-west shear layers can be found at almost all longitudes and latitudes, associated in depth with the ionization zones of hidrogen and helium in the outer 20 Mm. Finally, there is an indication of the presence of a toriodal convective roll at about 10 North latitude.