Duration of equatorial spread-F

A partir d'analyse d'ionogramme obtenus a Kodaikanal (3,5°N) durant 3 ans de fortes activites solaires, on montre que la duree des F diffus equatoriaux est comparable a celle de l'intensification de la region F apres le coucher du soleil de la derive verticale. On observe aussi une correlation positive avec la region F superieure durant les equinoxes les mois d'hiver de l'hemisphere nord

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