Electron density and temperature in the solar corona from multifrequency radio imaging

Context. The 2D images obtained through rotational aperture synthesis with the Nancay Radioheliograph are suitable for quantitative exploitation. First results are presented. Aims. We study the variations of the quiet corona in brightness and size during an 8-year period and derive electron density and temperature in the corona. Methods. Images at 6 frequencies between 150 and 450 MHz for 183 quiet days between 2004 and 2011 were used. Measurements of the brightness temperature T-b beyond the limb allowed coronal density models to be derived in both EW and NS radial directions, with a weak dependence on the electron temperature. The total ranges in the heliocentric distance r are 1.15-1.60 R-circle dot (EW) and 1.0-1.4 R-circle dot (NS). The agreement between results from different frequencies, in the ranges of r where there is overlapping shows the robustness of the method. The electron temperature, in turn, can be derived from the comparison of the observed mean spectra on the disk with those predicted through transfer calculations from the density models derived from limb observations. Results. The widths of the brightness profiles that were averaged yearly have minima at cycle minimum (2008-2009). These minima are more pronounced for EW profiles than for NS ones. The derived yearly-averaged density models along equatorial and polar diameters are consistent with isothermal and hydrostatic models. They are characterized by their density value no extrapolated down to the base of the corona and their scale-height temperature T-H. Changes in no and TH with solar cycle are given for equatorial and polar regions. The kinetic temperature T-e of electrons in the corona (similar to 0.62 MK) is found to be significantly less than T-H (similar to 1.5 MK). This implies an ion temperature T-i similar to 2.2 MK. Conclusions. The yearly-averaged variations of these models are less than the dispersion between models derived from other techniques, such as white light and EUV observations, partly because these two techniques are not time-averaged, and they refer to particular days. The radio models are generally less dense, which is compatible with isothermal hydrostatic equilibrium in their range of heliocentric distances, and they show different behaviors with the solar cycle in the equatorial or polar radial directions. The electron kinetic temperature T-e is substantially less than T-H.