Solar cycle variation of the radiance and the global electron density of the solar corona Original Research Article

The question of the global activity of the solar corona, as a function of time and distance from the center of the Sun, is considered by analyzing SOHO/LASCO-C2 daily polarized images over four years, i.e., from the minimum to the rising phase of the 23rd solar cycle. After proper correction for instrumental polarization, they are combined to produce maps of the polarized radiance pB and of the radiance of the K-corona (assuming a model for the K-corona polarization pK). The electron density is obtained by inversion of the pB images assuming local spherical symmetry. Synoptic maps are created at four distances between 2.7 and 5.5 solar radii. In order to conveniently quantify their temporal variation, the polarized radiance, the radiance and the electron density are integrated first globally and then in zones of different latitudes to separate the equatorial and the polar regions. These integrals remained constant in 1996 and then progressively increased with the rising activity of the Sun. However the polar and equatorial regions exhibited different behaviours as the former remained stable over a much longer time interval and experienced a larger increase than the latter.