Abstract
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 ⊙ (EW) and 1.0–1.4 R ⊙ (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 n 0 extrapolated down to the base of the corona and their scale-height temperature T H . Changes in n 0 and T H with solar cycle are given for equatorial and polar regions. The kinetic temperature T e of electrons in the corona (~0.62 MK) is found to be significantly less than T H (~1.5 MK). This implies an ion temperature T i ~ 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 .
Highlights
Electron density and temperature are basic variables for describing the solar corona and the physical processes that occur there
We examine the consistency between the value accepted for Te (620 kK or 1.5 Mk) in deriving density models from brightness measurements beyond the limb, and the mean spectra observed on the disk
The 2D images of the quiet Sun obtained through rotational synthesis with the Nançay radioheliograph (NRH) are suitable for quantitative measurements
Summary
Electron density and temperature are basic variables for describing the solar corona and the physical processes that occur there. Coronal density measurements relied only on white light observations. Eclipse observations allowed the density to be derived down to low heliocentric distances, e.g. 1.05 times the solar radius (R ) but were restricted to eclipse times. Ground-based coronographic observations allowed continuous time coverage but were restricted to larger heliocentric distances (typically >1.2 R ). Since the launch of Skylab (May 1973) and, more, recently of SoHO (Dec. 1995), extreme-UV (EUV) and soft X-rays observations were used to derive coronal densities, with the following limitations: i) they are generally limited to heliocentric distances
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