On Tuesday 23 July 2002 space scientists recorded the largest of four powerful solar flares, all occurring in the space of 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 thousand million megatonnes of high explosive. Solar astronomers watch flares closely because they can disrupt high-technology systems. The Solar and Heliospheric Observatory (SOHO) spacecraft gave a preview of the stormy solar weather even 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 15 July 2002, and an X 1.8 flare on 18 July 2002. On 20 July 2002, AR 10036 blasted an X 3.3 flare, and an X 4.8 flare, the most potent of the series, exploded on 23 July 2002 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.
Nowadays, space weather experts watch the Sun more closely than ever, because modern systems are much more vulnerable to solar disturbances than old technology. Even such experts are taken by surprise when 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 United States project scientist for SOHO at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, United States.
“It adds some fun to see things coming,” said Dr Philip Scherrer, principal investigator for MDI at Stanford University. Scherrer sent an e-mail to Gurman warning of the imminent appearance of AR 10039.
SOHO orbits a special point in space 1.5 million kilometres from Earth in line with the Sun, so it cannot see the far side of the Sun directly. However, the MDI instrument can form an image of far-side active regions by analysing ripples on the Sun’s surface. Sound waves reverberating through the Sun generate the ripples, which are analysed by computer to form an image of the far side and the solar interior. Analysis of solar sound waves is the science of helioseismology. This science has opened the Sun’s gaseous interior to investigation in much the same way as seismologists learned to explore the Earth’s rocky interior using earthquake waves.
‘Halo’ CMEs are so called 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 watch halo CMEs closely because they can be on a collision course for Earth. These halo CMEs were from active regions on the far side of the Sun and heading in the opposite direction, so they posed no threat. However, they were useful as signals 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.
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