Coronal Mass Ejections

    Coronal mass ejections (CMEs) are huge explosions of magnetic field and plasma from the Sun's corona. They carry up to ten billion tons (1016 grams) of plasma away from the Sun. CMEs travel outward from the Sun typically at speeds of about 300 km/s, but can be as slow as 100 km/s or faster than 3000 km/s. CMEs often display spatial structures, commonly referred to as 'three-part-structure', i.e., a bright frontal loop, a dark cavity, and a bright core which is shown to be the erupting prominence (Figure 1).

Figure 1 Images of CMEs taken by the LASCO/C2 and C3 coronagraph onboard the SOHO mission. To view the emission from CMEs, the solar disk is occulted and marked as the white circle.

1. Why study CMEs?
    Coronal mass ejections are more likely to have a significant effect on human's activities than flares because they carry more material into a larger volume of interplanetary space, increasing the likelihood that they will interact with the Earth's atmosphere. When a CME reaches the Earth, it interacts with the Earth's atmosphere and disturbs the Earth's magnetosphere, setting off a geomagnetic storm and leading to space weather effect. A CME typically takes 3 to 5 days to reach the Earth after it leaves the Sun. Observing the initiation of CMEs from the Sun provides an early warning of geomagnetic storms.     The most severe effects of CMEs on human activities occur during major geomagnetic storms are:
    ● One serious problem that can occur during a geomagnetic storm is damage to Earth-orbiting satellites, especially those in high, geosynchronous orbits;
    ● The energetic particles from a CME would be dangerous to an astronaut on a mission to the Moon or Mars;
    ● Large geomagnetic storms may, among other things, cause electrical power outages in a large region.

    Therefore, to understand and predict 'space weather' and the effect of solar activity on the Earth, an understanding of both CMEs and flares is required.


Figure 2 Composite image of the Sun (left in the figure) showing a solar eruption in progress. Bearing right is an artist's sketch of the terrestrial magnetosphere being impacted by solar ejecta. Distances are not to scale. (Courtesy of SOHO consortium)

Figure 3 Cartoon that illustrates the basic picture of solar eruptions.

2. Current understanding of solar eruptions
    Solar flares, filament eruptions, and CMEs are related phenomena, but one does not cause the other. They are believed to be different manifestations of a single physical process (i.e., magnetic reconnection) that involves a disruption of the coronal magnetic field. The most commonly accepted solar eruption models is shown in Figure 3, which embodies the three-part-structure of a CME with the erupting prominence as the bright core of the CME. It is believed that the energy released during eruptions is stored in the coronal magnetic field prior to the eruption.




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