Coronal mass ejections (CMEs) are huge expulsions of magnetic field and plasma from the Sun's corona. When CMEs hit Earth, they are responsible for geomagnetic storms and aurora. CMEs are characterized as regions of high densities of plasma with imbedded magnetic structures. CMEs come from two sources on the Sun, filament eruptions and active regions. When CMEs erupt from active regions, they tend to be associated with solar flares. CMEs from erupting filaments tend to be slower while active region CMEs tend to be faster. The fastest CMEs are always from active regions.
CMEs travel outward from the Sun typically at speeds of about 300 kilometers per second but can be as slow as 100 kilometers per second or faster than 3000 kilometers per second. The fastest CMEs reach Earth in as little as 17 hours. Slower CMEs take several days to traverse the distance from the sun to Earth. Because CMEs have an embedded magnetic field that is stronger than the background field of the solar wind, they will expand in size as they propagate outward from the Sun. By the time they reach the Earth, they can be so large as to fill half the volume of space between the Sun and the Earth. Because of their immense size, CMEs can take as long as 24 to 36 hours to pass over the Earth, once the leading edge has arrived. CMEs that are traveling faster than the fast mode wave speed (the space equivalent of the Earth’s sound speed) will generate a shock wave (just like an airplane traveling faster than the speed of sound generates a sonic boom). Often, the first sign of a CME hitting the Earth environment is the shock wave.
CME size, speed, direction, and density are important when trying to predict if or when a CME will impact Earth. We can estimate these properties of a CME with an instrument known as a coronagraph, which blocks the direct sunlight, just as in a total solar eclipse, allowing the solar atmosphere (corona) to be observed. CMEs show up as bright clouds of material moving outward through the solar atmosphere. In order to predict the strength of the resulting geomagnetic storm, the magnetic field strength and direction are important. At the present time, the magnetic field cannot be determined until it is measured as the CME passes over a satellite. The magnetic field direction is never going to be the same throughout an entire CME and it is typical to see it change direction as it passes over Earth, even completely switching to the opposite direction by the time the CME passes over Earth. Thus, most CMEs end up having favorable magnetic field directions for generating geomagnetic storms at some time during their impact.
Images courtesy of? NASA/ESA SOHO LASCO