Atmospheric intensity scintillation of stars .1. Statistical distributions and temporal properties

Stellar intensity scintillation in the optical was extensively studied at the astronomical observatory on La Palma (Canary Islands). Photon-counting detectors and digital signal processors recorded temporal auto- and cross-correlation functions, power spectra, and probability distributions. This first paper of a series treats the temporal properties of scintillation, ranging from microseconds to seasons of year. Previous studies, and the mechanisms producing scintillation are reviewed. Atmospheric turbulence causes 'flying shadows' on the ground, and intensity fluctuations occur both because this pattern is carried by winds, and is intrinsically changing. On very short time scales, a break in the correlation functions around 300 mu s may be a signature of an inner scale (similar or equal to 3 mm in the shadow pattern at wind speeds of 10 m s(-1)). On millisecond time scales, the autocorrelation halfwidth decreases for smaller telescope apertures until similar or equal to 5 cm, when the 'flying shadows' become resolved. During any night, time scales and amplitudes evolve on scales of tens of minutes. In good summer conditions, the flying-shadow patterns are sufficiently regular and long-lived to show anti-correlation dips in autocorrelation functions, which in winter are smeared out by apparent wind shear. Recordings of intensity variance together with stellar speckle images suggest some correlation between good (angular) seeing and large scintillation. Near zenith, the temporal statistics (with up to twelfth-order moments measured) is best fitted by a Beta distribution of the second kind (F-distribution), although it is well approximated by log-normal functions, evolving with time.