|Contrail Shadows from below and above -
Upper image by by Doug Short of Anchorage, Alaska.
Lower image over Greece by Chris Gerekos.
A weak 22° halo is apparently crossed in the upper image by by a contrail and two shadows.
The contrail is above two thin layers of cirrus and casting its shadow on them. One or both of the layers contains large enough and optically good enough crystals to form the halo.
In Chris Gerekos's lower image the contrail shadow is clearly visible on the lower cloud deck.