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SOLAR
ERUPTIONS THAT MISS CAN STILL HIT Like
a misdirected punch from a heavyweight boxer, a glancing blow from the Sun can
still pack a wallop. Results from a fleet of observatories in space and on the
ground were used to dissect a solar eruption that caused particle and magnetic
storms at Earth in April, 2001. Although it appeared to hit Earth head-on, astronomers
discovered that the solar eruption, called a Coronal Mass Ejection (CME), was
actually blasted from behind the Sun and expanded sideways to the Earth. Nevertheless,
it was still capable of generating a moderate space storm around the Earth. CME
eruptions can hurl a billion tons of electrified gas (plasma) into space at a
million miles per hour (1.6 million kilometers/hour). Solar astronomers watch
the Sun closely for Earth-directed CMEs, because they can generate severe space
storms when they arrive at Earth. If the orientation of the magnetic fields contained
in the CME plasma cloud is opposite to the direction of the Earth's magnetic field,
the two magnetic fields fuse, and the CME dumps its high-velocity plasma particles
into the space around the Earth, generating an intense space storm. Space storms
occasionally disrupt satellites, and communications and power systems. Fast CMEs
also drive powerful shocks, which generate solar cosmic rays that can be hazardous
to spacecraft electronics. The
new observation shows that astronomers also have to consider CMEs that are not
aimed at the Earth. "This complicates space weather prediction," said
Dr. Natchimuthuk Gopalswamy, a solar astronomer at NASA's Goddard Space Flight
Center in Greenbelt, Md. "The Earth, and the rest of the solar system, is
actually embedded in the atmosphere of the Sun. When a powerful event like a CME
disturbs the solar atmosphere, Earth can feel the effects, even if the CME is
not directed our way." Gopalswamy
combined observations from solar observatories in space and on the ground to analyze
a CME from 18 April 2001. The observation, which involved unprecedented coordination
between solar observatories, will be discussed as part of Gopalswamy's presentation
on coordinated observations of the Sun's atmosphere and CMEs at the American Astronomical
Society's summer meeting in Albuquerque, N.M., June 5. Special
instruments are required to observe CMEs, such as the Large Angle and Spectrometric
Coronagraph (LASCO) instrument on the Solar and Heliospheric Observatory (SOHO)
spacecraft, which uses a disk to create an artificial solar eclipse, blocking
direct light from the Sun so its much fainter outer atmosphere (corona) can be
seen. When Gopalswamy inspected the LASCO images, the April 18 CME resembled other
Earth-directed CMEs, called "halo" CMEs because of their appearance
in the LASCO images as they approach Earth. The SOHO spacecraft is directly between
the Earth and the Sun, and an Earthbound CME plasma cloud appears as a faint,
white ring around the LASCO disk as it gets closer to our planet. However,
recorded observations of the CME in microwaves from the Nobeyama Radio Observatory,
Nagano, Japan, told a different story. Nobeyama doesn't require an occulting disk
like LASCO, so it can look closer, directly at the actual edge, or limb, of the
Sun. This corresponds to the very early phase of the CME, not accessible to LASCO
because of its occulting disk. The microwave observations revealed that the CME
actually erupted slightly behind the Sun, just over the western limb (the right
side of the Sun in SOHO images). Another instrument on SOHO, the Extreme-ultraviolet
Imaging Telescope, which looks directly at the Sun to image very hot plasma in
its atmosphere, confirmed the microwave results. The instrument showed a flash
over the limb, like distant artillery fire behind a mountain range, where the
CME appears in microwaves. The eruption was also seen as a bright enhancement
in an X-ray image obtained by the Soft X-ray Telescope on board the Yohkoh satellite. More
detailed analysis of the LASCO images showed that what appeared as a halo was
in fact a shock wave in the corona caused by the high-speed CME. Although the
corona is very thin, it can cause space weather effects at Earth because it is
permeated by magnetic fields and comprised of electrically charged particles (ions
and electrons). Since these particles feel magnetic forces, they can be accelerated
when the coronal magnetic fields suddenly move, as when a shock wave traverses
the corona. The accelerated particles, called solar cosmic rays, become one type
of particle storm that can damage sensitive electronics on spacecraft. The
Geostationary Orbiting Environmental Satellite, operated by the National Oceanic
and Atmospheric Administration, recorded a particle storm around Earth as a result
of the shock wave driven by the CME. Two other NASA spacecraft, the Advanced Composition
Explorer and the Wind spacecraft, recorded the shock wave close to the Earth approximately
85 hours after the April 18 CME eruption. A
coordinated data analysis workshop is being organized during this summer to analyze
a large number of such storms accompanied by solar cosmic rays:
(http://cdaw.gsfc.nasa.gov/LWS/).
"These
multi-wavelength observations helped us separate the components of a CME eruption,
and we realize that the shock is much more extended than the CME that drives it,"
said Gopalswamy. "This event also helps us realize that all halo CMEs are
not Earth-directed. We need to clearly separate the 'CME object' from the waves
that are associated with the ejection." Back
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