Coronal Scattering, Absorption and Refraction of Solar Radiobursts

Using a statistical ray. tracing technique, we study the propagation of radio energy from a source radiating in a scattering, refracting and absorbing spherical corona with isotropic irregniarities of refractive index. The treatment itself is described and its domain of validity discussed. It is shown that it can be used to study sources deep enough in the corona to model type m bursts. Reflected rays are taken into account. Computations are carried out for a point source at coronal levels where the index of refraction is 0.38 and 0.85 and for rms relative deviation in electron density e = rms (A N/N) = 1 to 4%. The radius of correlation of the inhomogeneities is taken as = 5 10-8 ( heliocentric distance in Be, IL = 40 km at = 1.1). It is shown that: - The size of the scattered image increases with e, whatever the altitude of the point source, and increases also with the longitude L of the source. It can reach 5' with the values of e used. Asymmetry in the brightness distribution builds up as soon as L exceeds . A tail develops towards the center for intermediate L and e but shifts towards the limb at larger L and s. The shifts in position of the maximum brightness follow the same pattern. - The directivity in the radiated energy distribution is controlled by refractive focussing for s = 1% at both altitudes. For larger sit becomes controlled by scattering effects but the beam width reaches a "saturation" value of (to half maximum) when e is further increased whatever the altitude of the source. The size of the scattered image is then equal to or larger than the observed sizes so that e cannot be further increased. The directivity diagram thus found is not wide enough to account for the large number of type `U's observed on the limb. - The total flux leaving the corona is seriously decreased when scattering is efficient (s> 3). - The dispersion in the times of propagation of the various rays received by an observer builds up a transient response of the corona to a pulsed source. Its duration 6(L) is very short (0.02 to 0.04 s) for L = 0 and e small. It increases with L. When s increases, 6(L) reaches a saturation level of 0.2s whatever the altitude and the longitude of the source. Then the decay time of the transient response would lead to an electron temperature of 2 10 0K if interpreted in terms of collision damping alone. It is concluded that the interpretation of center-limb histograms of occurrence for type I and III bursts on meter waves requires the use of non-spherical models for the electron density distribution and/or the distribution of inhomogeneities. Key words: coronal scattering - refraction - absorption - radio bursts directivity - coronal transient