Influence des perturbations thermiques de l'atmosphère sur la qualité des images solaires

Résumé Le Pic du Midi de Bigorre étant pris comme site expérimental, l'influence sur la qualité des images solaires de deux types de perturbations thermiques de l'atmosphère est examinée, celles-ci étant dues respectivement au sol local et à la convection au sein de l'écoulement incident. Une méthode pyrhéliométrique permettant la détection de la convection en ciel clair est exposée.

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A parameter characterizing the quality of photospheric images has been defined through a three class distribution; it has been estimated over various samples taken during 230 hours of observing time at the Pic du Midi Observatory.Its occurrence is compared to that of two types of atmospheric thermal disturbances, one being created by the local ground and the other brought along by the incident flow and due mainly to convection.The time-derivation of the temperature, which is, under some conditions, well related to the nearby thermal field, may be taken as a characteristic atmospheric parameter, and it is shown that the best quality for solar images occurs only - and then with a fairly good frequency - when its value is slightly positive.A method is described which aims at the detection of convection in the clear atmosphere; as the upward motions are bringing aerosols from the lower layers, a less transparent volume is ascending during the first part of the day and then, its top may intercept the optical axis of the telescope. The diurnal variation of the atmospheric transparency, as observed with a pyrheliometer, shows clearly this occurrence, which may be quite different from one day to another.