  We report on the detection (10 \sigma) of 917 events of long-period (3 to 16
hours) intensity pulsations in the 19.5 nm passband of the SOHO Extreme
ultraviolet Imaging Telescope. The data set spans from January 1997 to July
2010, i.e the entire solar cycle 23 and the beginning of cycle 24. The events
can last for up to six days and have relative amplitudes up to 100%. About half
of the events (54%) are found to happen in active regions, and 50% of these
have been visually associated with coronal loops. The remaining 46% are
localized in the quiet Sun. We performed a comprehensive analysis of the
possible instrumental artifacts and we conclude that the observed signal is of
solar origin. We discuss several scenarios which could explain the main
characteristics of the active region events. The long periods and the
amplitudes observed rule out any explanation in terms of magnetohydrodynamic
waves. Thermal nonequilibrium could produce the right periods, but it fails to
explain all the observed properties of coronal loops and the spatial coherence
of the events. We propose that moderate temporal variations of the heating term
in the energy equation, so as to avoid a thermal nonequilibrium state, could be
sufficient to explain those long-period intensity pulsations. The large number
of detections suggests that these pulsations are common in active regions. This
would imply that the measurement of their properties could provide new
constraints on the heating mechanisms of coronal loops.
