The index is defined over a period of one day. An A index can be defined for any location on earth and also for the entire globe. A very useful planetary index is the Ap index. They can trigger long-lasting geomagnetic storms when they strike Earth's magnetosphere. These provide a good visual explanation of the cannibal effect.
Coronal holes are regions where the corona is dark.
These features were discovered when X-ray telescopes were first flown above the earth's atmosphere to reveal the structure of the corona across the solar disc. The high-speed solar wind is known to originate in coronal holes. Magnetic fields buckle releasing previously constrained hot material from the upper atmosphere of the Sun. As a result, hot gas streams out into the Solar System, impacting planets, moons, spacecraft, and making space a dangerous place for astronauts. CMEs are more common but less intense than solar flares.