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A fleet of solar-orbiting spacecraft observed the April 9 solar eruption in unprecedented detail, confirming previously unseen features that are predicted by computer models. Solar eruptions, or coronal mass ejections (CMEs), occur when a giant bubble of hot plasma and magnetic field lines blast outwards from the Sun, ejecting billions of kilograms of matter into the interstellar medium at speeds of 2,500 kilometres per second. These eruptions happen periodically, and, if aimed at the Earth, can be a potential threat to astronauts, satellites or communications and power systems on the Earth.
A prominence leaps from the surface of the Sun and heads into interstellar space. Image: SOHO (ESA & NASA). The April 9 CME occurred on the edge of the Sun as viewed from Earth, which gave astronomers the relatively rare opportunity to observe the X-ray brightening of a solar flare, which is usually hidden from view. The Sun-watching satellites were able to take long exposures which uncovered fainter structures than previously seen. The X-ray telescope (XRT) on board the Hinode satellite showed a spiral magnetic structure unwind as it left the Sun during the CME, a process associated with a release of energy, which thereby helps to power the eruption. Hours later, the same instrument revealed an inflow of material towards a current sheet, an electrified sheet of gas that appears as a bright line when viewed edge-on, where oppositely directed magnetic field lines annihilate one another in a process known as magnetic reconnection. The extended observations from XRT show that magnetic fields flow in toward the current sheet for many hours after the eruption, progressing first toward the sheet and then down to the Sun's surface. Computer models of CMEs predict such movements of magnetic field lines, but observing them has proven difficult. The unique positioning of this CME on the Sun's limb allowed astronomers to measure those motions. They also determined that the temperature of the current sheet is between 2.7 and 10 million degrees Celsius, which matches previous measurements higher up in the corona by the SOHO spacecraft. Together, those observations will provide a more complete picture of the source and evolution of CMEs.
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