Terms

C

 * Composition. What an object is made of.
 * Corona. The outer part of the Sun's "atmosphere". In the outer region of the corona, particles travel away from the Sun and stretch far out into space. The corona can only be seen during total solar eclipses, appearing as a halo around the moon.
 * Coronagraph. SOHO/LASCO coronagraph image of the SunAn instrument that makes it possible to observe the corona at times other than during an eclipse. A simple lens focuses the Sun onto a disk that blocks the light from the solar disk, providing an artificial eclipse. This image of the Sun is from the SOHO/LASCO coronagraph instrument.
 * Coronal holes. Large regions in the corona that are less dense and cooler than surrounding areas. There is a constant flow of low density plasma escaping from the holes.
 * Coronal mass ejections (CMEs). Huge bursts of solar wind rising above the Sun's corona. One of the biggest explosions in our solar system.
 * Corotating. Sharing the rotation of a planetary magnetic field, as in Corotating Interaction Regions.
 * Corotating Interaction Regions (CIRs). The area in the heliosphere where there is an interaction between high and low speed streams of solar wind that causes a shock wave of high density, pressure and magnetic field strength.

H

 * Helio-. Prefix so as to referr to the Sun.
 * Heliocentric. The principal of the Sun being in the center.
 * Heliopause. The gradual boundary between the heliosphere and the interstellar gas outside our solar system. See the diagram with the definition of "heliosphere" below.
 * Heliosphere. The area in space that contains our solar system, solar wind, and the entire solar magnetic field. It extends well beyond the orbit of Pluto, out to the heliopause.
 * Helium. The gas made from hydrogen in the core of stars by nucleosynthesis. Each atom of helium contains two protons.
 * Hemisphere. One half of a sphere or globe. The northern hemisphere on Earth is divided from the southern hemisphere by the equator.
 * . An instrument that determines the three-dimensional path of a particle through it. Hodoscopes are used in both the and  on the.
 * Hydrogen. The most common gas in the universe. Each atom of hydrogen contains one proton.