Typical image associated with the claim:
What’s is this beam?
Kyoto University Disaster Prevention Research Institute has used Lidar in ash cloud observations since 2014. Lidar is a method for determining ranges (variable distance) by targeting an object with a laser and measuring the time for the reflected light to return to the receiver.
The research institute is using two Lidars for observations as shown in the image below: SVO and KUR. Image credit: MDPI.
Note: there are also videos and photographs of something flying in the sky near the volcano. Many posts claim that these videos and photos shows UFOs, possibly related to DEW. These claims are also false: they are just meteorites / fireballs observed near Sakurajima. One example time-lapse video:
Big thanks to Scott / UfoOfInterest who worked with me on this fact check. See e.g.