Interpretation of Electron Densities from Corona Brightness

Abstract

The electron density in the corona as determined from observations of coronal brightness will be too high if an appreciable part of the coronal emission is not caused by scattering of electrons. The presence of Fraunhofer lines, and the diminution of polarization with p, the radial distance from the Sun, can both be explained by the existence of a non-electron source of light. It is found that the p" 2 * 6 term in Baumbach's empirical formula can be identified with the non-electron component of the coronal radiation. The change of electron density with p has been tabulated after allowing for the non-electron component. The interpretation is in reasonable agreement with (a) the intensities of coronal Fraunhofer lines, (b) the diffraction theory of non-electron scattering, (c) the change of polarization with p, and {d) the change of polarization with orientation. The distribution of temperature with p is determined from the new electron densities. I. Introduction.-The problem of electron density in the solar corona has been treated by M. Minnaert(i) and S. Baumbach (2), both of whom employ the scattering coefficient of electrons to derive electron densities from the intensity of coronal light. The values published by Baumbach are the ones in common use, because these were obtained from a careful consideration of all the coronal photometric data available. As the coronal details are quite variable there is no need to aim at higher accuracy than that represented by Baumbach's mean. However, Baumbach assumed that all coronal light was due to electron scattering, and if a substantial part of the light from the outer corona comes from another source, as suggested by Grotrian(3), then Baumbach's electron densities in the outer corona will be too high. In the present paper an allowance has been made for the nomelectron source of coronal light, and the distribution of coronal light scattered by electrons alone has been deduced. There are two observations to show that in the light from the corona there is a non-electron component which becomes of greater importance as the distance from the Sun increases. The high temperatures now accepted for the corona broaden the Fraunhofer lines in the light scattered by electrons until it is a continuous spectrum. Hence the Fraunhofer lines visible in the outer corona must be due to sunlight scattered or diffracted by some other medium. Let J F be the intensity of the component that produces Fraunhofer lines, J K the intensity of the continuous light scattered by electrons, and J^. The intensities of the Fraunhofer lines were first used by Grotrian(3) to determine the relative intensity oí J F and J K. In addition, the degree of polarization can be used to show that all the coronal light is not scattered by electrons. As the polarization in the outer corona is less than the theoretical value, it implies thb existence of a non-polarized component, and we see good reason for identifying