Otley-based UFO Magazine wrote to the RAF in 1998 to tell officers about sightings of two unidentified bright lights on February 2 that year.
The letter said hundreds of people had reported seeing “two large white spheres” moving slowly over towns and cities in West Yorkshire.
It said the lights were seen north of Manchester at 5.15pm, west of Bradford at 5.25pm, in Dewsbury at 5.30pm and in Leeds between 5.30pm and 5.45pm.
It said: “Several observers state a low humming noise emanated from the lights, whilst around 90 per cent of the witnesses reported no shape which could have indicated an aeroplane or perhaps an airship.”
The newly-released files also contain details of a sighting in the sky over Heckmondwike on September 13, 2003.
The sighting of two “very dull yellow” circular shapes was reported by a senior curator from the Royal Armouries in Leeds