A new approach to the study of the Sun's corona and its dynamical processes is possible with radar investigations in the frequency range of about 10-50 MHz. The range of electron densities of the solar corona is such that radio waves at these frequencies can provide diagnostic radar echoes of large scale phenomena such as coronal mass ejections

It is assumed that the two-fold disc-wide symmetry of spirals is caused by density waves, but also the potential of a bar component may have a significant influence on structural properties. The strength of the bar component appears to be anti-correlated with the degree of symmetry of star-forming regions in the spiral arms (Rozas et al., 1998). We

El trabajo se ha centrado en el estudio de los procesos que acompañan a la emergencia de mancaas en la superficie del sol. Para ello se ha utilizado el método de seguimiento de trazas y el espectroscópico. El primero de ellos permite determinar los movimientos transversales de las estructuras, mientras que el seguno permite medir los movimientos a

We present the results of two studies of the inner Galaxy : on the one hand, a maximum likelihood fitting method using the Besançon model of Galaxy to constrain the density laws of the outer bulge and the holed disc; on the other hand, the comparison of Besançon simulations with infrared data on fields near longitude 20 27 degrees to verify the

Evolutionary synthesis models have been used to study the physical properties of unresolved populations in a wide range of scenarios. Unfortunately, their self-consistency is difficult to test and there are some theoretical open questions without an answer: (1) The change of the homology relations assumed in the computation of isochrones due to the

We present a Monte Carlo model to compute cometary dust tail brightnesses. The model, anologous in many ways to that developed by Fulle (1989, Astron. Astrophys. 217, 283), computes the keplerian orbit of each grain released from the nucleus surface in the Sun gravity field reduced by the radiation pressure. It can be used to derive the time