Predicting Explosions On The

Solar flares and coronal mass ejections emit energetic particles, intense radiation, powerful magnetic fields and strong shocks that can have enormous practical implications when directed toward the Earth, ttey can disrupt radio navigation and communication systems, pose significant hazards to astronauts, satellites and space stations near Earth, and interfere with power transmission lines on the ground. National space environment centers and defense agencies therefore continuously monitor the Sun from ground and space to forecast explosions on the Sun.

tte ultimate goal is to learn enough about solar activity to predict when the Sun is about to unleash its pent-up energy. Such space-weather forecasts will probably involve magnetic changes that precede solar flares and coronal mass ejections, especially in the low corona where the energy is released or at lower levels in the photosphere where we can watch internal motions pulling and twisting the magnetism around.

It has been supposed that the energy required for eruptions is stored in stressed magnetic structures, and that the magnetic fields rearrange themselves into a simpler configuration after the event. Solar flares do, in fact, occur in regions of strong magnetic shear in the photosphere, and the low solar corona is in a constant state of agitation and metamorphosis. Coronal loops are magnetically reconfigured as they twist and writhe in response to internal differential rotation and convection motions, and it is likely that coronal loops join and reconnect to trigger solar flares.

Scientists may have discovered one method for predicting the sudden and unexpected outbursts. When the bright, X-ray emitting coronal loops are distorted into a large, twisted sigmoid (S or inverted S) configuration, a coronal mass ejection from that region becomes more likely (Fig. 7.19). In some instances, a coronal mass ejection occurs just a few hours after the magnetic fields have snaked into a sinuous S-shaped feature; the mass ejection arrives at the Earth three or four days later. In the meantime, just after the mass had been expelled from the Sun, the X-ray emitting region dramatically changes shape, exhibiting the telltale, cusp-like signature of magnetic reconnection and an X-ray fading or dimming due to the mass removal. In other words, the magnetism gets stirred up into a complex, stressed and twisted situation before it explodes, and then relaxes to a simpler, less-stressful situation.

However, some regions that exhibit magnetic shear and twist never erupt, so contorted magnetism may be a necessary but not sufficient condition for solar flares or coronal mass ejections. And the Sun's sudden and unexpected outbursts often remain as unpredictable as most human passions, ttey just keep on happening, and even seem to be necessary to purge the Sun of pent-up frustration and to relieve it of twisted, contorted magnetism.

Since this erratic, unpredictable, impulsive behavior of the Sun is of enormous practical interest to us humans on Earth, it is also critically important to know if the material sent out from the solar outbursts is headed toward our planet. Coronal mass ejections that are expelled from near the visible edge of the Sun will not impact Earth, but threaten other parts of space. Mass ejections are most likely to hit us if they originate near the center of the solar disk, as viewed from the Earth, and are sent directly toward the planet, tte outward rush of such a mass ejection appears in coronagraph images as a ring or halo around the occulting disk, but the coronagraph data are unable to determine if the halo-like ejection is traveling toward or away from the observer.

tte Earth-directed coronal mass ejections may nevertheless be preceded by coronal activity at extreme ultraviolet and X-ray wavelengths near the center of the solar disk as viewed from the Earth, tte mass ejection is itself sometimes associated with

FIG. 7.19 Sigmoid and Cusp The full-disk X-ray image (top) shows the Sun with a twisted, sigmoid present on 7 April 1997. It produced a halo Coronal Mass Ejection, abbreviated CME, on the following day. The inset (bottom left) shows the soft X-ray sigmoid before eruption of the CME. The other inset (bottom right) shows the soft X-ray cusp and arcade formedjust after the CME took place. These images were taken with the Soft X-ray Telescope (SXT) on Yohkoh. (Courtesy of Richard C. Canfield, Alphonse C. Sterling, NASA, ISAS, LMSAL, the National Astronomical Observatory of Japan and the University of Tokyo.)

FIG. 7.19 Sigmoid and Cusp The full-disk X-ray image (top) shows the Sun with a twisted, sigmoid present on 7 April 1997. It produced a halo Coronal Mass Ejection, abbreviated CME, on the following day. The inset (bottom left) shows the soft X-ray sigmoid before eruption of the CME. The other inset (bottom right) shows the soft X-ray cusp and arcade formedjust after the CME took place. These images were taken with the Soft X-ray Telescope (SXT) on Yohkoh. (Courtesy of Richard C. Canfield, Alphonse C. Sterling, NASA, ISAS, LMSAL, the National Astronomical Observatory of Japan and the University of Tokyo.)

dimming X-rays or extreme ultraviolet radiation with associated waves running across the visible disk, like tidal waves or tsunami going across the ocean. Moreover, NASA's Solar-TErrestrial RElations Observatory, abbreviated STEREO (Fig. 7.20), uses two spacecraft to track coronal mass ejections from the Sun to Earth (Focus 7.2).

tte high-energy electrons that accompany solar flares follow the spiral pattern of the interplanetary magnetic field, so they must be emitted from active regions near the west limb and the solar equator to be magnetically connected with the Earth. Solar flares emitted from other places on the Sun are not likely to hit Earth, but they could be headed toward interplanetary spacecraft, the Moon, Mars or other planets.

FIG. 7.20 STEREO An artist's conception of the Solar-TErrestrial RElations Observatory, abbreviated STEREO, in which two spacecraft will provide the images for a stereo reconstruction of Coronal Mass Ejections, or CMEs for short. One spacecraft will lead Earth in its orbit and one will be lagging. When simultaneous telescopic images from the two spacecraft are combined with data taken from observations on the ground or in low Earth orbit, the buildup of magnetic energy, the lift off, and the trajectory of CMEs can be traced in three dimensions. (Courtesy of Johns Hopkins, Applied Physics Laboratory.)

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