Lifetime of the Hα chromospheric network

Abstract

Time-lapse photographs of the Hα chromospheric network were taken at 3-sec intervals with a$\tfrac{1}{4}$-Å filter centered at Hα + 0.65 Å, where the network appears as a system of dark mottles. A nearly continuous sequence of 62 hr duration was made possible by summertime operation of the telescope at Thule Air Base, Greenland, 10° above the Arctic Circle. A quiet region was followed across the center of the disc. The life history of the network, as determined by measurement of the cross-correlation of selected photographs taken at intervals ranging between 3 sec and 29 hr, shows a rather complicated behavior. Initially the correlation falls off quite rapidly, dropping from 0.97 at 3 sec to 0.60 at about 7 min. Thereafter, the decline becomes increasingly more gradual, reaching a value of 0.24 at 4 hr. From this point the correlation falls off quite slowly. A least-squares fit to that portion of the cross-correlation curve for time differences greater than 4 hr yields a mean (1/e) life of 25.0 ± 1.6 hr and a value for twice the half-life of 38.8 ± 4.0 hr. This rate of decline is believed to be characteristic of the lifetime of the chromospheric network. The rapid initial loss of correlation appears to be due to a redistribution of the features, seen in the red wing of the Hα line, along the network.