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Astron. Astrophys. 348, 261-270 (1999)

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5. Discussion

The obtained results indicate that radio data cannot be fitted without the presence of a corona with a slowly decreasing density and that the crucial parameters in the derivation of the radio spectrum are the DEM in the TR and the parameters ([FORMULA]) at the bottom of the corona.

It must be pointed out however, that the coronal contribution to the radio brightness temperature (and also to coronal line intensities) depends on the temperature and on the emission measure, [FORMULA] rather than on the electron density itself. In our case the electron density at the basis of the corona was derived from the best fit of radio data having assumed the corona in hydrostatic equilibrium and getting therefore: [FORMULA] with the scale height given by [FORMULA]. The resulting Emission Measure is [FORMULA] cm-5, in agreement with Yohkoh determinations ([FORMULA]) according to Hara et al. 1994.

The assumption of hydrostatic equilibrium in coronal holes could appear in conflict with the very well established origin of the fast Solar Wind in these coronal features. However, UVCS observations (Kohl et al. 1998) have shown that outflow velocities are negligible below 1.5 [FORMULA] where no difference is noticed in the electron density profiles between a static and a dynamic coronal model. Similar results have been obtained, using LASCO data (Sheeley et al. 1997, below 2 [FORMULA]).

Since the coronal contribution to the radio brightness temperature at the considered frequencies, as well as that to the EUV line intensities, comes from the deepest layers of the solar corona ([FORMULA]), the assumption of hydrostatic equilibrium is fully justified.

On the contrary, the consequences of having assumed the temperature derived from the fit of radio data to determine the coronal scale height are important. In fact the radio emission of the quiet corona is due to thermal bremstrahlung and the corresponding temperature is the electron temperature, while that entering the scale height is an average plasma temperature.

Recent SOHO and Ulysses observations seem to indicate that the proton and the electron kinetic temperatures are quite different, mostly in coronal holes, the latter being much lower than the former. According to Fludra et al. 1999b, the average density profile of several polar coronal holes suggest that [FORMULA].

If [FORMULA], the scale height of the coronal plasma, which must be neutral, depends on the average coronal temperature and is therefore larger than that determined assuming [FORMULA], thus increasing the emission measure, the radio optical depth and hence the [FORMULA]. In order to find again a good fit of radio observations we should either decrease [FORMULA] or decrease [FORMULA] proportionally to [FORMULA]. It has been shown in Fig. 6 that in the former case, it would not be possible to get the correct intensity of EUV lines formed at [FORMULA] K, while, in the latter one, no variation would be noticed, since the coronal contribution to the line intensity also depends on [FORMULA]. In order to have the same EM with [FORMULA] we should decrease the electron density [FORMULA] to [FORMULA] according to:


Assuming a ratio [FORMULA] between the electron density derived from the fit of radio data and the values measured from [FORMULA], a proton temperature [FORMULA] is found, providing an average coronal temperature [FORMULA] K. This value seems too large and its overestimation is probably due to one of the reasons mentioned in Sect. 4.1.

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© European Southern Observatory (ESO) 1999

Online publication: July 16, 1999