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

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1. Introduction

Coronal holes were first observed on X-ray plates by Underwood and Muney (1967), on EUV line spectroheliograms by Reeves and Parkinson (1970) and in white light by Altshuler and Perry (1972). They were then frequently observed during the Skylab Mission (1972-74) in the soft X-ray images and, about in the same years, also at metric wavelengths by Dulk and Sheridan (1974), Chiuderi Drago (1974), Lantos and Avignon (1975).

The aspect of coronal holes depends very much on the atmospheric level at which they are observed: coronal images, taken in all wavelength ranges listed above, show that coronal holes are much darker than the surrounding regions, they don't show up in the spectroheliograms taken in Transition Region lines (Munro and Withbroe, 1972) and they are brighter in the radio maps at mm-wavelengths (Kosugi et al. 1986).

The comparison between X-ray and metric wavelength observations of coronal holes, although very seldom done on the same hole, has evidenced a strong disagreement in the value of the coronal temperature [FORMULA]: all radio observations listed above indicate coronal temperatures of the order of [FORMULA] K, while from X-ray observations electron temperatures larger than one million degrees are inferred. According to recent Yohkoh observations, [FORMULA] K (Hara et al. 1994). The impossibility to reconcile EUV and radio observations of the same equatorial coronal hole, was ascribed in the past to the presence of small scale unresolved inhomogeneities (Chiuderi Drago et al. 1977).

It must be pointed out, however, that recent observations of coronal holes, performed with the SOHO/CDS instrument seem to indicate that also EUV line intensities can be accounted for by assuming a coronal temperature lower than [FORMULA] K (Fludra et al. 1997, 1999a, 1999b; Del Zanna & Bromage 1997, David et al. 1998).

In the present work we compare radio and EUV observations of the same coronal hole in order to determine with a better accuracy the plasma parameters both in the corona and in the Transition Region (TR). To this purpose we have compared the intensity of several EUV lines with the radio emission in the decimetric and metric range of wavelenths, both generated in this portion of the solar atmosphere.

Radio observations of polar coronal holes are difficult because the waves path in the solar corona is strongly bent at metric wavelengths, due to the variable refractive index, so that the inner corona around the limb cannot be observed and what we see in that position is the radiation coming from higher levels (see for instance Kundu, 1965).

Our comparison was therefore done on an equatorial coronal hole, observed on October 18 and 19, 1996 during its central meridian transit. The hole was observed by the CDS and EIT instruments onboard SOHO and by the Radioheliograph of Nançay (France) at four radiofrequencies between 169 and 410 MHz. CDS observations were taken on October 18, while radio observations refer to October 19.

EUV and Radio observations will be described in Sect. 2. The analysis of EUV lines observed by CDS, the method for the determination of the Differential Emission Measure (DEM [FORMULA]) and of the radio brightness temperature are presented in Sect. 3. In Sect. 4 we present the comparison between EUV and radio data, performed using the DEM in the TR up to a certain temperature determined, together with the electron pressure at the basis of the corona, from the best fit of radio data. The best fitting models are then checked by computing the EUV line intensities.

A general discussion on the results is given in Sect. 5. Summary and conclusions are presented in Sect. 6.

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© European Southern Observatory (ESO) 1999

Online publication: July 16, 1999