Coronal Mass Ejections

tte most spectacular solar eruptions are gigantic magnetic bubbles, called coronal mass ejections, which expand outward from the Sun (Fig.7.16). A typical coronal mass ejection carries about 10 billion tons, or 10 million million kilograms, of coronal material as it lifts off into space, removing about a tenth of the total coronal mass, tte outward-moving coronal mass ejections stretch the magnetic field until it snaps, leaving behind only bright rays rooted in the Sun. ttey can expand to become larger than

FIG. 7.16 Coronal mass ejection A contorted coronal mass ejection is seen in this coronagraph image, taken on 4 January 2002. The white circle denotes the edge of the photosphere, so the length of the ejected material is about twice the size of the visible disk of the Sun. The dark area corresponds to the occulting disk that blocked the intense sunlight from the photosphere, revealing the surrounding faint corona. This image was taken with the Large Angle Spectrometric COronagraph, abbreviated LASCO, on the SOlar and Heliospheric Observatory, or SOHO for short. (Courtesy of the SOHO LASCO consortium and NASA. SOHO is a project of international collaboration between ESA and NASA.)

the visible solar disk, streaming outward past the planets and dwarfing everything in their path.

ttousands of mass ejections have been observed during the last few decades with white-light coronagraphs aboard the seventh Orbiting Solar Observatory, OSO-7 (1972), Skylab (ATM, 1973-1974), P 78-1 (Solwind, 1979-1985), the SolarMaximum Mission (C/P, 1980 and 1984-1989), and the Solar and Heliospheric Observatory, abbreviated SOHO (LASCO, 1996-2005).

Such events work only in one direction, always moving away from the Sun into interplanetary space and never falling back in the reverse direction, ttey often exhibit a three-part structure - a bright outer loop, followed by a depleted region, or cavity, that rises above an erupted prominence, tte leading bright loop, or coronal mass ejection, is a rapidly expanding, bubble-like shell 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.

tte material moves outward, an immense cosmic storm that moves with speeds of several hundred kilometers per second, tte energy of this mass motion is comparable to the net radiated energy of a large solar flare, with the destructive potential of a million hurricanes on Earth.

Mass ejections erupt from the Sun as self-contained structures of hot material and magnetic fields, apparently resulting from a rapid, large-scale restructuring of magnetic fields in the low corona, ttey are most likely triggered by large-scale magnetic reconnection events in the corona, similar to those that ignite solar flares.

Although subatomic particles are accelerated to high energies during solar flares, it is now thought that coronal mass ejections may be the main source of the most energetic solar particles arriving at the Earth, ttey are responsible for most, if not all, of the largest energetic proton events that bombard the Earth. Strong mass ejections serve as pistons to drive huge shock waves ahead of them, plowing into the slower-moving solar wind, like a car out of control, and producing shock waves millions ofkilometers across. Electrified particles energized by the shocks travel outward with it, somewhat like surfers riding the ocean waves.

Coronal mass ejections can energize subatomic particles across large regions of interplanetary space, ttose that head toward Earth usually take three or four days to get there. In contrast, the particles accelerated during flares follow well-defined paths described by the interplanetary magnetic spiral, and they can take only minutes to arrive at Earth.

When a coronagraph in a satellite near Earth detects a mass ejection ballooning out from the Sun's apparent edge, the ejection is not headed for Earth, ttose directed toward or away from our planet can be observed as a halo around the Sun, detected by a differencing technique. Another method of detecting an Earth-directed coronal mass ejection is the dimming of soft X-ray or extreme-ultraviolet radiation, caused by the removal oflow corona material near the center of the visible solar disk.

When coronal mass ejections were discovered in the early 1970s, it was thought that they were an explosive consequence of the bright flares; at the time flares were the most energetic eruptive phenomena known on the Sun. tten a key piece of evidence was provided by the heretofore largely ignored prominences. When astronomers began to make association studies of coronal mass ejections and other forms of solar activ ity, they found to their surprise that the erupting prominences were best associated with the mass ejections. Since intense flares do not usually accompany erupting prominences, it was concluded that flares were not required to drive mass ejections.

In addition, the physical size of the mass ejections dwarfs that of flares and even the active regions in which flares occur, and flares are much more common than mass ejections. Most flares therefore probably originate in a somewhat different environment than the mass ejections, but perhaps by a similar magnetic process, tte main differences between the two may just be a matter of physical size, with the compact flares occurring more often than the larger mass ejections.

tte mass ejection, with its accompanying erupting prominence, represents a large-scale rearrangement of the Sun's magnetic structure. It blows open the previously closed magnetic structure, like a hot-air balloon that breaks its tether (Fig. 7.17). tte magnetic cage is torn asunder, releasing pent-up magnetic energy, perhaps triggered by magnetic reconnection, as solar flares seem to be. Sometimes an associated flare might be formed as the result of the energy released by magnetic coupling of the open field lines as they pinch below the rising prominence. Post-flare loops can be subsequently observed, shining from the newly closed magnetic loops in the flare's thermal afterglow (Fig.7.18).

FIG. 7.17 Ejections from the corona A double set of coronal mass ejections seems to be heading in opposite directions from the Sun on 8 November 2000. The dark area corresponds to the coronagraph's occulting disk that blocks the intense sunlight from the photosphere, revealing the surrounding faint corona. An extreme ultraviolet image has been superposed at a location corresponding to the visible solar disk; it was taken on the same day at a wavelength of 30.38 nanometers, emitted by singly ionized helium, denoted He II, at a temperature of about 60,000 kelvin. The coronagraph image was taken with the Large Angle Spectro-metric COronagraph, abbreviated LASCO, on the SOlar and Helio-spheric Observatory, or SOHO for short, and the superposed image was taken with the Extreme-ultraviolet Imaging

Telescope, abbreviated EIT, on SOHO. (Courtesy of the SOHO LASCO and EIT consortia and NASA. SOHO is a project of international collaboration between ESA and NASA.)

FIG. 7.18 Stitching up the wound An arcade of post-flare loops shines in the extreme ultraviolet radiation of eight and nine time ionized iron, Fe IX and Fe X, at a temperature of about 1.0 million kelvin. This image was taken on 8 November 2000,just after a solar flare occurred in the same active region; at least one coronal mass ejection also accompanied the event (see Fig. 7.17). High-energy particles from the flare entered the Earth's radiation belts, and an associated coronal mass ejection produced a strong geomagnetic storm. This image was taken from the Transition Region And Coronal Explorer, abbreviated TRACE. (Courtesy of the TRACE consortium and NASA; TRACE is a mission of the Stanford-Lockheed Institute for Space Research, a joint program of the Lockheed-Martin Solar and Astrophysics Laboratory, or LMSAL for short, and Stanford's Solar Observatories Group.)

FIG. 7.18 Stitching up the wound An arcade of post-flare loops shines in the extreme ultraviolet radiation of eight and nine time ionized iron, Fe IX and Fe X, at a temperature of about 1.0 million kelvin. This image was taken on 8 November 2000,just after a solar flare occurred in the same active region; at least one coronal mass ejection also accompanied the event (see Fig. 7.17). High-energy particles from the flare entered the Earth's radiation belts, and an associated coronal mass ejection produced a strong geomagnetic storm. This image was taken from the Transition Region And Coronal Explorer, abbreviated TRACE. (Courtesy of the TRACE consortium and NASA; TRACE is a mission of the Stanford-Lockheed Institute for Space Research, a joint program of the Lockheed-Martin Solar and Astrophysics Laboratory, or LMSAL for short, and Stanford's Solar Observatories Group.)

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