1997PASP..109..173D

Stellar intensity scintillation in the optical was extensively studies 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 timescales, a break in the correlation functions around 300 mus may be a signature of an inner scale (approx. 3 mm in the shadow pattern at windspeeds of ms -1). On millisecond timescales, the autocorrelation decreases for smaller telescope apertures until approx. 5 cm, when the 'flying shadows' become resolved. During any night, timescales 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 12:th 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.