7. The Violent Sun
Coronal mass ejections and eruptive prominences
Another dramatic, magnetically energized type of solar explosion is called a Coronal Mass Ejection, or CME for short. These are giant magnetic bubbles that rapidly expand to rival the Sun in size, and hurl billions of tons of million-degree gas into interplanetary space at speeds of about 400 thousand meters per second, on average, reaching the Earth in about four days. Their associated shocks accelerate and propel vast quantities of high-speed particles ahead of them.
Coronal mass ejections are detected during routine visible-light observations of the corona from spacecraft such the Solar and Heliospheric Observatory, or SOHO. With a disk in the center to block out the Sunís glare, the coronagraph sees huge pieces of the corona that are blasted out from the edge of the occulted photosphere (Fig. 7.13). The mass ejections are seen as moving loop-like features in light scattered by coronal electrons.
Coronal mass ejections are huge. Their average angular span of 45 degrees along the disk edge implies a size near the Sun that is comparable to that of the visible solar disk, and they can expand to even larger sizes further out.
Coronal mass ejections usually have expanding curvilinear shapes that resemble the cross sections of loops, shells or filled bubbles, suggesting magnetically closed regions that are sporadically blown out by the eruption. The upper portions of the magnetic loops are sometimes carried out by the highly-ionized material, while remaining attached and rooted to the Sun at both ends. In other situations, the expelled material stretches the magnetic field until it snaps, taking the coiled magnetism with it and lifting off into space like a hot-air balloon that breaks its tether. Whenever, a big, closed loop of magnetism is unable to hold itself down, a coronal mass ejection takes off.
Coronal mass ejections often exhibit a three part structure - a smooth, bright outer loop or bubble of enhanced density, followed by a dark cavity of low density, within which sits an erupted prominence. The leading bright loop or shell is the coronal mass ejection that opens up and lifts off like a huge umbrella in the solar wind, piling the corona up and shoving it out like a snowplow. About 70 percent of the coronal mass ejections are associated with, and followed by, eruptive prominences (Fig. 7.14, Fig. 7.15).
We can see the magnetic backbone of an erupting prominence regroup and close up again in soft X-ray images, retaining a memory of its former stability. When the prominence erupts, it is replaced by a row of bright X-ray emitting loops (Fig. 7.16), aligned like the bones in your rib cage or the arched trestle in a rose garden. First observed from Skylab, the X-ray loops bridge the magnetic neutral line between opposite polarity regions in the photosphere, stitching together and healing the wound inflicted by emptying that part of the corona.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University