It may be caused by some high level very thin kind of clouds. The clouds are made up of ice crystals due to the high attitude at the upper layers of troposphere, which is always below freezing. Like you fly in a commercial liner at as high as 41,000ft.....it's like -40+ deg C. For eg, cirrus clouds. If the ice crystals form a very thin uniform layer, this will cause halos round the sun as each crystal refracts the light acting as a prism. Even the moon can have this effect.
Yours is not exactly a true halo but similar effect lar.
