A spectacular solar show
Posted: November 3, 2002

Image courtesy of SOHO/NASA/ESA
Not to be outdone by fall's explosion of color, the Sun issued a series of spectacular solar eruptions, which were photographed by the Solar and Heliospheric Observatory (SOHO) spacecraft.

These beautiful but violent solar events are important to solar science and can affect high-technology systems. "Understanding the processes beneath the Sun's surface that drive these eruptions is critical to understanding the Sun's behavior and its influence on our planet," said Dr. Gareth Lawrence of the Catholic University of America, who is stationed at NASA's Goddard Space Flight Center, Greenbelt, Md., as the Operations Scientist for the Large Angle and Spectrometric Coronagraph (LASCO) instrument on SOHO.

The first CME, perplexed even seasoned solar physicists with its unique appearance. Coronal mass ejections tend to take on a shape that resembles a "lightbulb". This one, however, had more the shape of a dark keyhole, or a bulb on top of a column. The most likely explanation appears to be a combination of several coincidences - the chance juxtaposition of unrelated solar structures, enhanced by the image processing that removes the "background image", and a visual illusion that enhances the apparent contrast of the column. Image courtesy of SOHO/NASA/ESA
The solar fireworks began October 24 with an immense eruption of electrically charged gas (plasma) called a coronal mass ejection (CME). It was observed with the LASCO instrument, which has a disk that blocks direct light from the Sun so the much dimmer solar atmosphere can be seen. The CME had a unique appearance and has been named the ³Keyhole CME².

"We've observed thousands of CMEs, but none ever looked like this one," said Dr. Stein Vidar Hagfors Haugan of the European Space Agency, a solar scientist with the SOHO program, also stationed at NASA Goddard. SOHO scientists believe a combination of effects from nearby solar structures, SOHO's viewing position, and an enhancement from image-processing techniques produced the dark area resembling a keyhole in the image.

A second CME, also seen with LASCO, erupted October 25 and vaguely resembles a corkscrew, with twisted lines bursting from the Sun. According to SOHO scientists, its unusual appearance is due to twisted solar magnetic fields, which steer the flow of the CME plasma. Part of the Sun's interior magnetic field becomes twisted from activity deep inside the Sun. It is eventually ejected from the Sun following an explosive energy release process. Details regarding how the fields become twisted and the exact mechanisms that propel CMEs into space are the focus of intense research activity.

The second CME (above, left) showed an unusually high level of structure, with a definite "twist" to the material being ejected. Image courtesy of SOHO/NASA/ESA
CMEs are billion-ton eruptions of plasma from the Sun. CMEs are blasted from the Sun at high speeds, ranging from 125 to 1,550 miles per second (about 200 to 2,500 kilometers per second). Normally the ones directed at Earth are potentially harmful, depending on the orientation of the magnetic field in the CME plasma cloud, the CME's speed, and the plasma density.

If directed at Earth, the CME plasma is harmless to people but slams into the Earth's magnetic field, distorting it like a jellyfish buffeted by a strong current. The alteration in the shape of the Earth's magnetic field accelerates electrically charged particles (electrons and atomic nuclei) trapped within. The rapidly-moving particles sometimes generate beautiful auroral displays (northern and southern lights) when they collide with the upper atmosphere around the polar regions. The most severe CME impacts cause geomagnetic storms capable of disrupting satellites, radio communications, and power systems. Neither the keyhole nor the corkscrew CME were heading toward Earth, so they aren't expected to produce strong geomagnetic storm effects.

Three images from CME. Image courtesy of SOHO/NASA/ESA
The Sun punctuated its performance with an eruptive prominence on October 25. It was seen with the Extreme ultraviolet Imaging Telescope (EIT) instrument on SOHO, which looks directly at the solar disk and can observe cooler gas near the surface. Prominences are huge clouds of relatively cool, dense plasma suspended by magnetic fields in the Sun's hot, thin, outer atmosphere (corona). They occur when dense plasma at the solar surface becomes trapped in magnetic fields that are propelled high into the corona.

They frequently linger for a week or two and sometimes suddenly erupt, escaping into space. Other times, the plasma slowly drains back to the solar surface under the influence of the Sun's powerful gravity, 28 times stronger than Earth's. More research is needed before solar scientists can agree on what pushes prominences into the corona and why they sometimes fly off into space. Part of the eruptive prominence was Earth-directed, although it did not generate significant storm activity.

Illustration with size comparison to Earth. Image courtesy of SOHO/NASA/ESA

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