SOLAR
GRAND SLAM While
Tiger Woods was trying to claim the top four golf tournaments, the Sun delivered
a grand slam of its own, blasting four of its most powerful class of solar flares
in just eight days. Solar
flares are tremendous explosions in the atmosphere of the Sun, with the most powerful
class, called the X class, capable of releasing as much energy as a billion megatons
of TNT. Flares are closely watched by solar astronomers because they can disrupt
high-technology systems, but the Solar and Heliospheric Observatory (SOHO) spacecraft
gave a preview of the stormy solar weather while it was still rumbling on the
far side of the Sun, the side opposite the Earth. The
flares came from sites of violent activity on the Sun, called active regions.
Active region (AR) 10030 blazed with an X 3.0 flare on July 15, and an X 1.8 flare
July 18. On July 20, AR 10036 blasted an X 3.3 flare, and an X 4.8 flare, the
most potent of the series, exploded July 23 from AR 10039. These active regions
were all associated with sunspots, planet-sized dark areas on the solar surface
caused by an intense concentration of magnetic fields. Active
regions are much larger than the Earth and consist of strong magnetic fields on
the Sun's surface. Active regions produce flares and eruptions of plasma (hot,
electrically charged gas), called coronal mass ejections (CMEs). The radiation
and plasma from these events sweep past the Earth, sometimes affecting spacecraft
electronics and terrestrial power systems, and disrupting radio communications.
Understanding and forecasting solar eruptions and their consequences is a relatively
new science called space weather. Now
space weather experts watch the Sun more closely than ever, because modern systems
are much more vulnerable to solar disturbances than old technology. The experts
can still be taken by surprise because the Sun rotates, bringing the effects of
hidden active regions to bear on Earth. However,
scientists using SOHO had advance warning that stormy weather was brewing on the
Sun. "Activity from active region 10039 was 'expected,' based on a series
of strong, far-side halo coronal mass ejections during the last week and far-side
observations by the SOHO Michelson Doppler Imager (MDI)," said Dr. Joe Gurman,
the U.S. Project Scientist for SOHO at NASA's Goddard Space Flight Center in Greenbelt,
Md. "It
adds some fun to see things coming," said Dr. Philip Scherrer, Principal
Investigator for MDI at Stanford University. Scherrer sent an email to Gurman
warning of the imminent appearance of AR 10039. SOHO
orbits a special point in space one million miles from Earth in line with the
Sun, so it can't see the far side of the Sun directly. However, the MDI instrument
can form an image of far-side active regions by analyzing ripples on the Sun's
surface. Sound waves reverberating through the Sun generate the ripples, which
are analyzed by computer to form an image of the far side and the solar interior.
Analysis of solar sound waves is the science of helioseismology, and it opened
the Sun's gaseous interior to investigation in much the same way as seismologists
learned to explore the Earth's rocky interior with earthquake waves. "Halo"
CMEs are named because of their appearance in another SOHO instrument, the Large
Angle and Spectrometric Coronagraph (LASCO). A halo CME resembles a faint, white
ring that expands in LASCO's field of view as the ejected plasma cloud moves away
from the Sun. Astronomers pay close attention to halo CMEs because they can be
on a collision course for Earth. Since these were from active regions on the far
side of the Sun, they were heading in the opposite direction and posed no threat.
However, they were useful as harbingers of the angry active regions about to rotate
into our view. SOHO
is a cooperative project between the European Space Agency (ESA) and NASA. The
spacecraft was built in Europe for ESA and equipped with instruments by teams
of scientists in Europe and the USA. Back
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