Electrons in the solar corona

Abstract

During the eclipse of 15 February 1961, photometric measurements were obtained for the inner corona till r/Ro = 2.5 at hnperia, Italy (Figure 1), and for the outer corona between r/Ro = 2.0 and 7.0 at Hvar, Jugoslavia (Plate I). Morphologic pictures of the streamers were taken at Observatoire de Haute-Provence, France, with the technique of the gradient compensation (Plate II). K-corona- meter observations were daily recorded at Meudon and Pic-du-Midi Observatories, France, from 9 to 22 February 1961 (Figures 9 and 10). These combined techniques give accurate localization, shape and photometry of the streamers. Four of them lie near the limb-plane and the mathematical approach of Van de Hulst applies to derive the electron-density repartition within these streamers (Figures 13, 15 and 16a). Compared with the pre- viously published electron-density determinations on 10 corona-streamers (Figure 16b), the new determinations correspond to a particularly large variety of streamers. Local increases of electron- densities are found at the neck of helmet-shaped streamers, with an almost local zero gradient and are discussed. A research program was initiated in the decade of the 50's by the Laboratory 'Physique du Syst6me Solaire', Observatoire de Paris, Meudon (France) to refine our knowledge about the electron densities and their variations in coronal streamers and condensa- tions. The electrons are detected by their Thompson scattering of the solar light in the optical range. Observational techniques were: the K-coronameter photoelectric polarimeter to survey the inner parts of the corona, the balloon-borne externally occulted corono- graph to study the streamers up to 5 solar radii from the center of the Sun, and the eclipse white light photometry. The present Part ( refers to the solar eclipse of 15 February 1961. Measurements of the number of electrons by cm 3 within 4 coronal streamers are given. Typical electron density models for coronal streamers are derived.