Apr. 5, 2000 WASHINGTON - Scientists have discovered a new source for some of the large scale eruptions on the Sun known as coronal mass ejections, or CMEs. These eruptions are a key ingredient of strong geomagnetic storms that can cause bright auroras, damage sensitive satellite instruments in space, and even disrupt power generation and transmission by inducing strong electric currents below the surface of the Earth.
Writing in the April 15 issue of Geophysical Research Letters, Dr. Josef I. Khan of University College, London (UK), and Dr. Hugh S. Hudson of the Solar Physics Research Corporation in Tucson, Arizona (USA), report that shock waves launched from solar flares can cause CMEs elsewhere in the solar corona. They found that such mass ejections do not emanate from structures directly above the eruptive flare, a more common pattern, but rather from off to one side.
Khan and Hudson are both currently based at Japan's Institute of Space and Astronautical Science (ISAS) in Kanagawa, where they conducted their investigations using the Yohkoh ("sunbeam") satellite. Studying X-ray images of the Sun, they examined very large hot loops of solar material that sometimes connect sunspot regions in the Sun's northern and southern hemispheres. They found that such loops, known as interconnecting X-ray loops, can simply disappear suddenly, ejecting huge amounts of material.
In particular, Khan and Hudson found three similar disappearances on May 6, 8, and 9, 1998. All of the observed interconnecting X-ray loops disappeared and were followed by looplike CMEs. Each CME event could be tracked far out into the corona via images obtained from another instrument, the Large Angle Spectroscopic Coronagraph (LASCO) on the Solar and Heliospheric Observatory (SOHO) satellite.
The researchers say the association between these disappearing loops and solar flares is novel and interesting. Scientists debate the relationship between CMEs and other phenomena, including solar flares and prominence eruptions. Solar flares release large amounts of energy across the electromagnetic spectrum, while prominence eruptions are the ejection of large suspensions of cool material in the Sun's hot outer atmosphere, or corona. Some, but not all, prominence eruptions and CMEs are associated with flares.
There is, however, a clear relationship between prominence eruptions and CMEs on the one hand and X-ray brightenings seen in the corona below these ejections. The question before scientists is what causes what, and which therefore is the more important phenomenon physically. Khan and Hudson found that for the events they studied, and contrary to the normal pattern, the flare is not located directly below the CME. It is, rather, located off to one side, in a sunspot region outside the structures that erupt to become part of the CME. Put another way, the mass ejected in the CME does not come from the structures directly above the flare.
Khan and Hudson, studying Yohkoh X-ray images, were able to determine fairly accurately the timing of the disappearance of the interconnecting loops. They found that the solar flare occurs before the loops disappear and, therefore, before the start of the coronal mass ejection. Further, by studying these X-ray data and simultaneous data from radio telescopes, they found evidence for shock waves. In every instance they examined, they found that the interconnecting loops disappear when the shocks cross their vicinity.
These observations led to a new scenario Khan and Hudson have put forward to explain some CMEs. A shock wave generated by the flare crosses a large interconnecting loop, causing it to become unstable and erupt. This ejects hot X-ray material, which becomes a significant fraction of the coronal mass ejection. The researchers acknowledge that this hypothesis requires further exploration, and they recognize that it does not apply to all CMEs, only the type they reported.
Yokoh is a mission of the Institute of Space and Astronautical Science (ISAS) of Japan. SOHO is a joint mission of ESA and NASA, stationed at 1.5 million kilometers (930,000 miles) from the Earth, near the so-called Lagrange point L1, where the gravity of Earth and Sun balance each other.
The research for this paper was supported by grants from the Particle Physics and Astronomy Research Council (PPARC) of the United Kingdom and NASA.
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