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Contribution à l'étude de l'hélium dans la couronne solaire. Observations du télescope spatial EIT

Abstract : Helium is the second most abundant element in the Universe. The understanding of the physicals processes associated with helium as well as the determination of the helium abundance both have implications in various research fields such as cosmology, stellar evolution or the physics of the solar wind. Helioseismology techniques give accurate measurements of the helium abundance in the solar interior, spectroscopic techniques provide diagnostics in the photosphere and in the chromosphere, and in situ measurements in the solar wind at 1 A.U. are carried out with particle detectors. But very few observations of helium exist in the corona and therefore, our knowledge of helium at intermediate distances between th photosphere and the solar wind is essentially based on theoretical studies. The present work is a tentative contribution to help constraint the observational knowledge of helium in the solar corona. The EIT telescope on board the SOHO spacecraft can observe the solar corona up to 2 Rs in an interval of wavelengths in the extreme ultraviolet spectrum including the resonance line of the He+ ion at 30.378 nm. This line being formed in the solar corona by resonant scattering of the chromospheric flux by coronal He+ ions, its intensity is proportional to the number density of He+ ions. Therefore, the observation of this line in the corona can potentially provide interesting diagnostics of the coronal helium. In spite of the contamination by other spectral lines, it seems that a non negligible fraction of the signal recorded by EIT in its 30.4 nm bandpass can be attribuated to the resonance line of He+. Furthermore, a preliminary study seems to show that the observed intensity gradients are anomalously low in the polar regions. The aim of the present work was to investigate further these preliminary results. We first carried out a detailed critical analysis of the characteristics of the EIT instrument in order to confirm that the 30.378 nm line of He+ in the corona can be detected in the 30.4 nm bandpass of EIT. This analysis implies a precise evaluation of several calibration parameters such as the flat-field of the detector, the contamination of the 30.4 nm bandpass and the instrumental stray light level. In order to interpret the intensities measured with EIT, we developed a model of the intensity of the resonance line of He+ in the corona, with the existing models for the Lyman alpha line of neutral hydrogen as a starting point. This model requires as an input some physical parameters such as the electron temperature and electron density, which were independently determined either from previous results or from new observations. The comparisons between the observed intensity and the prediction of the model seem to confirm the results of the preliminary analysis. In the equatorial regions, the intensity gradient of the resonance line of He+ is compatible with the electron density scale height. But at high latitudes in the polar coronal holes, the intensity gradient seems significatively smaller than what is expected from the computations. One can interpret this observation by an accumulation of helium in the polar coronal holes, where the fast solar wind originates. If the coronal ionisation balance computed in the model is valid, this accumulation of He+ could be the signature of an enhanced helium abundance in the corona. Some theoretical models of the corona/solar wind system show that the helium abundance could indeed be 20% or more in the corona, even though it is 10% in the solar interior and 4% in the solar wind. Because helium is four times more massive than hydrogen, it is clear the an enhanced helium abundance in the corona would greatly impact the energy and momentum fluxes in the solar wind. However, further observations, especially with a better spectral resolution and a lower stray light level, are needed to confirm those of EIT.
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Submitted on : Monday, April 18, 2005 - 11:11:18 PM
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Frédéric Auchère. Contribution à l'étude de l'hélium dans la couronne solaire. Observations du télescope spatial EIT. Astrophysique [astro-ph]. Université Pierre et Marie Curie - Paris VI, 2000. Français. ⟨tel-00009055⟩

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