Coronal hole

Coronal holes are regions of the Sun's corona that emit low levels of ultraviolet and X-ray radiation compared to their surroundings. They are composed of relatively cool and tenuous plasma permeated by magnetic fields that are open to interplanetary space.^[1] Compared to the corona's usual closed magnetic field that arches between regions of opposite magnetic polarity, the open magnetic field of a coronal hole allows solar wind to escape into space at a much quicker rate. This results in decreased temperature and density of the plasma at the site of a coronal hole, as well as an increased speed in the average solar wind measured in interplanetary space.^[2]

Streams of fast solar wind originating from coronal holes can interact with slow solar wind streams to produce co-rotating interaction regions. These regions can interact with Earth's magnetosphere to produce geomagnetic storms of minor to moderate intensity. During solar minima, CIRs are the main cause of geomagnetic storms.