  As early-type stars with a rotation speed close to their critical velocity,
Be stars experience an event called the Be phenomenon. The material in their
equator is ejected into outside space during the Be phenomenon and forms a
circumstellar disk. The mechanism triggering these events remains poorly
understood, and observations of these events are limited because the duration
of these events ranges from months to years. Long-term epoch photometry in the
infrared bands is expected to be ideal for detecting Be phenomena because the
brightness variation is larger, and the effect of interstellar extinction is
weaker as well. We conducted a systematic search for Be phenomena among Milky
Way OBA stars in the mid-infrared. We examined the brightness and colour
variations of known classical Be stars using the WISE W1 and W2 photometry
bands to quantify their characteristics. Subsequently, we established a set of
criteria to identify similar photometric variations in a large sample of OBA
stars. We found 916 OBA stars that show Be phenomena in the past 13 years, 736
of which are newly discovered. The peak-to-peak variations in magnitude and
colour were found to be correlated, indicating that a decretion disk is common.
The increase in colour was observed to be strongly correlated with the emission
of the H-alpha line, providing further evidence of the association with
circumstellar disks. The brightness variation of a star with Be phenomena can
be up to 1.5 mag, and the colour variations can be up to 0.4 mag. The median
durations for the disk build-up and decay phases are 474 and 524 days,
respectively (durations shorter than 180 days are not sampled). The search for
Be phenomena in the WISE bands greatly enlarges the number of stars showing
disk variation, and it enables multi-band photometry analysis of these events
with the help of current and future optical photometry surveys.
