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

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6. Summary and conclusions

In this paper we have analysed EUV and radio observations of an equatorial coronal hole at its central meridian transit, on October 18 and 19, 1996.

This analysis has shown the very powerful diagnostic provided by these two types of observations, when combined together. Using the DEM derived from EUV line intensities in the radio transfer equation and comparing the results with the observed radio brightness temperatures, it is possible to derive the parameters of the solar corona.

It has been shown that EUV line intensities can be equally well reproduced by an atmosphere with or without a corona, by properly changing the top temperature in the DEM , while radio data can't.

The best estimate of the coronal electron temperature usually comes from the low frequency observations where [FORMULA] and [FORMULA]. Unfortunately, in our case, due to the low angular resolution of the Nançay Radioheliograph at the 164 MHz, as compared with the hole size, only an upper limit, [FORMULA] can be inferred. Any coronal temperature between 8 and [FORMULA] K seem therefore to fit very well the radio observations. The observed EUV line intensities however put a constraint on the lowest acceptable value of the coronal temperature, which has been estimated [FORMULA] K.

About the same estimate of the coronal temperature has been given by Del Zanna and Bromage 1997, who analysed the CDS line intensies in this same hole, observed two rotations before. Also David et al. 1998 find a temperature profile increasing from [FORMULA] at [FORMULA] to [FORMULA] K at [FORMULA] as a result of analysis of several EUV lines observed by CDS and SUMER above a polar coronal hole. A similar temperature increase, from [FORMULA] to [FORMULA] K has been found above several polar coronal holes by Fludra et al. 1999a,1999b.

All these results agree with the radio brightness temperature derived in this paper. This removes a long-standing discrepancy between the coronal hole temperature required by the radio and EUV observations. However a discrepancy still exist with the Yohkoh X-ray data (Hara et al. 1994 and reference therein).

The estimate of the electron density at the basis of the corona [FORMULA] cm-3 at [FORMULA] K, also derived from the fit of radio data, depends more crucially on the assumption made in deriving the coronal contribution to [FORMULA] (and to the line intensity), namely the hydrostatic equilibrium in the coronal portion of the hole with a scale height [FORMULA]. (The assumptions made on the electron pressure trend in the TR do not seem to influence the results)

It has been shown that the assumption of hydrostatic equilibrium can be safely applied in our case, since the radiation we are considering comes from the very low corona where dynamic effects are negligible.

The assumption of a coronal temperature [FORMULA] can considerably underestimate the real scale height in the corona, thus increasing the estimated value of [FORMULA] in order to get EM and [FORMULA] able to account for radio and EUV observations.

It is in our purposes to check, using the present set of data, coronal hole models which assume a difference between electron and proton temperature, necessary to reconcile the requirement of a very low electron temperature, with the high wind speed observed by Ulysses above coronal holes.

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© European Southern Observatory (ESO) 1999

Online publication: July 16, 1999