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The average behavior of the MF A1 absorption data at night and sunrise times at Calcutta has been described with a view to understanding the maintenance of the nighttime absorbing region. The nighttime variation of absorption which attains a minimum around one hour after local midnight could easily be accounted for in terms of the scattered UV fluxes.

A small decrease in absorption before sunrise is evidenced. The analysis using the partially reflecting Es echo suggests this is caused by a decrease in deviative loss at F-region heights. The onset time for a further increase in absorption agrees very well with that observed from other techniques.