Search

Eggshell fragments attributed to large birds have been known from the Palaeogene of southern France for half a century, but reconstructing their original dimensions and identifying the birds that laid the eggs has been fraught with difficulties. On the basis of numerous newly collected specimens and using geometrical calculations, the original size of the thick-shelled eggs is reconstructed, showing that they were slightly larger than ostrich eggs, with a greatest length of 17.8 cm and a mean diameter of 12.0 cm in transversal section. The estimated volume is 1330.4 cm3. The fossil eggs from southern France are thus among the largest known avian eggs, being only surpassed byAepyornisand some moas. Estimated egg mass is about 1.4 kg. On the basis of egg mass, the body mass of the parent bird is estimated at between 135.4 kg and 156.4 kg, assuming that the hatchlings were precocial. These calculations are in good agreement with the dimensions and mass estimates for the Palaeogene giant birdGastornis, a probable anseriform, which lived in Europe at the time the eggs were laid. Other large Early Tertiary birds from Europe (Remiornis,Palaeotis) are too small to have laid these eggs. In all likelihood, the large eggs from the Palaeogene of southern France were laid by gastornithid birds.

The relation between episodically observed in the solar corona faint Hαemission structures and the long lived prominences was studied. Particular consideration was given for cases in which the corresponding prominences had undergone DB process. An MHD interpretation of the phenomenon “emissions froides” (cool emission) is proposed in which an essential role plays the prominence supporting magnetic field.

Broadband linear polarization in the spectra of Ap stars is believed to be due to differential saturation between σ and π Zeeman components in spectral lines. This mechanism has been known for a long time to be the main agent of a similar phenomenon observed in sunspots. Since this phenomenon has been carefully calibrated in the solar case, it can be confidently used to deduce the magnetic field of Ap stars.Given the magnetic configuration of a rotating star, it is possible to deduce the broadband polarization at any phase. Calculations performed for the oblique dipole model show that the resulting polarization diagrams are very sensitive to the values of i (the angle between the rotation axis and the line of sight) and β (the angle between the rotation and magnetic axes). The dependence on i and β is such that the four-fold ambiguity typical of the circular polarization observations ((i,β), (β,i), (π-i,π-β), (π-β,π-i)) can be removed.

Observations of variable broadband linear polarization in magnetic Ap stars (due to the transverse Zeeman effect), when combined with measurements of the mean longitudinal field Bɩ can in some cases allow one to determine the angles i and β (which describe the inclination of the stellar axis of rotation and the obliquity of the magnetic axis to the rotation axis) much more accurately than these angles can be determined from observations of Bɩ alone. Such variable intrinsic linear polarization has been observed for a number of stars; the effect is generally detectable only in cool Ap stars of unusually large field strength. We discuss the data and simple modelling for the stars HD 24712 = HR 1217, HD 137909 = β CrB, and HD 62140 = 49 Cam.

A reanalysis of the prominence magnetic field measurements obtained by Leroy et al. (1983) indicates that only a small fraction of these prominences did not obey the chirality rules they discovered and that Martin et al. (1994) confirmed.

Recent measurements of the polarization of the Hel D3 line (λ5876Å) in solar prominences (Hanle effect) have been used to infer new data about prominence magnetic field (Leroy 1977, Leroy et al. 1977, Sahal-Brèchot et al. 1977). The derived field strengths are in good agreement with previous magnetographic measurements obtained at several observatories in USA (Rust 1972). Using the Hanle effect also gives the direction of the magnetic field, which is a crusial parameter in theories of prominences (Kippenhahn and Schlüter 1957, Anzer 1968, Malville 1976). Some theories require a horizontal oriented magnetic field in the prominences. High quality optical observations, on the other hand, resolve predominantly vertically oriented prominence microstructure (Dunn 1972, Engvold 1976).

The magnetic field may be the most important physical parameter in the explanation of the “prominence” phenomenon and it plays a major role in all the theoretical models. Therefore, it is not amazing that a lot of work has been devoted to measuring the magnetic vector. Good reviews of the present state of our knowledge on this topic have already been given by Rust (1972), Smolkov and Bashkirtsev (1973) and Tandberg-Hanssen (1974). In the following, I shall describe only the problems and results which follow from measurements of the magnetic field in the prominence material itself. Data deduced from measurements of the magnetic field in adjacent layers of the solar atmosphere (chromosphere, corona) will be found in other sections of this Colloquium.

A study of the development in the solar corona of active centers born during the CSSAR period leads to the following remarks: the ‘enhancement’ of coronal emissions seems to take place first within a localized ‘core’ close to the plage, then to extend to a much larger ‘halo’; the core from which all the radiations under study originate does not last much more than the spots, whereas the ‘halo’ is characterized by the major importance of the emission at 5303 Å, and lasts as long as the K 3 plages. These features can be explained by the assumption that the enhancement is inhomogeneous during the first part of its life, then becomes homogeneous after the core has disappeared.