Curving prominence cavity and nearby coronal hole both appear as dark shadows in a blue-coded picture made in ultraviolet light from the hot corona. The sprawling coronal hole at lower right represents a nearly empty void in the corona, where open magnetic field lines allow the gaseous coronal material to escape the Sun.  The curved cavity at left was excavated by refrigeration by a quiescent prominence that still lies hidden within it.
John A. Eddy
1979-07-02
SP-402 A New Sun: The Solar Results From Skylab page 112