Subtelescopic Inhomogeneities of Electron Density in the Solar Corona

The brightness of the solar corona due to Thomson scattering depends linearly on the electron density, while the brightness due to the Balmer continuum is proportional to its square. As a consequence, information on the distribution of the electron density in the corona can be obtained by comparing the radial profiles of the surface brightness in both continua. This idea was explored for the first time in the solar eclipse of November 03, 1994, in Foz do Iguaçu, PR, Brazil. Pictures of the corona were obtained with interference filters, one centered at 477 nm (Thomson continuum) and another one at 347 nm (Balmer continuum). The second filter also transmits the Thomson continuum through its spectral window, so that the Balmer images contain Thomson contamination. This paper reports on the observational results and presents their preliminary analysis. It was found that in certain radial directions, the normalized profiles of both continua (Thomson and contaminated Balmer) coincide, but in other directions they differ significantly. The non-coincident profiles may only occur if Balmer emission becomes important in relation to the Thomson scattering. A simple calculation shows that in such cases the electron density in the inner corona must exceed the values of standard models by up to ≃ 6.1 × 104 times, maintaining however the total number of electrons along the line of sight in agreement with the prediction of standard models. It is concluded that the corona contains high electron concentration in cloudlets of subtelescopic sizes down to ≃ 106 cm. The varied behavior of the radial profiles of both continua in different radial directions, suggests that the subtelescopic structures might be related to the spatially variable topology of coronal magnetic flux tubes.