by Mauricio Peredo, Nicola Fox, and Barbara Thompson, NASA Goddard Space Flight Center, Greenbelt, Md.
An international group of scientists studying the Sun-Earth relationship with a fleet of spacecraft and ground-based networks had the unique opportunity to witness and track a forceful explosion of matter from the Earth to the Sun. By studying the event's evolution, they learned more than ever before about coronal mass ejections and how particle emissions from the Sun affect Earth.
Last year satellites tracked a solar eruption all the way from the Sun to the Earth for the first time ever. The resulting coronal mass ejection traveled four days through space before arriving at Earth, where it violently disturbed the magnetic environment and produced spectacular auroral displays. The initial expulsion occurred on the Sun on January 6, 1997, and the resulting magnetic cloud hit the Earth on January 10.
The Sun often erupts. It flings out white-hot gas (actually hotter than white-hot, to where it glows in X-rays) with explosive violence. Only occasionally is this gas aimed at Earth, however, and it is even more unusual for scientists to be watching the coronal mass ejection just as it leaves the Sun. This made it possible for scientists from the International Solar-Terrestrial Physics (ISTP) "Observatory," which observed the event, to alert other scientific teams of possible activity they might observe two to three days later. It normally takes that long for such ejecta to travel the 150-million-kilometer void from Sun to Earth. Thus while this is not the first, or the largest, event to be detected, the ISTP Observatory comprises a complement of spacecraft and ground-based missions that allows study of this "space storm" on a scale never accomplished before.
Initial evidence of the event was reported January 7 during an ISTP Science Workshop at NASA's Goddard Space Flight Center, Greenbelt, Md. Scientists involved in the joint NASA-European Space Agency satellite Solar and Heliospheric Observatory (SOHO) showed evidence that a CME had been emitted from the Sun and was moving toward Earth. Based on previous observations of such events, SOHO scientists predicted the CME would arrive at Earth on January 10.
Through its radio-burst tracking capability, the WAVES instrument on NASA's WIND spacecraft recorded the evolution of the coronal mass ejection throughout its journey (see figure 1).
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| Figure 1: Illustration of how the event evolved as tracked by the WAVES instrument onboard NASA's WIND spacecraft. |
The event was first detected in SOHO observations (see figure on top of p. 12) from January 6, 1997, by the Large Angle and Spectrometric Coronagraph (LASCO). This coronal mass ejection produced a storm of solar energetic particles and contorted magnetic fields, spewed out from the Sun, and flung into interplanetary space. That it was directed along the LASCO line of sight suggested that its effect on Earth systems could be significant.
As predicted by SOHO scientists, the CME arrived near the Earth's environment on January 10. In orbit approximately 1.4 million kilometers to the sunward side of the Earth, SOHO was the first to detect the interplanetary signature of the event with the particle monitors of the Charge, Element, and Isotope Analysis System (CELIAS) instrument. Closer to home—at 640,000 kilometers—WIND, with its full complement of plasma and magnetic field instruments, was able to identify the event as a magnetic cloud approximately 48-million-kilometers thick, and traveling toward Earth at a speed of 450 kilometers per second.
About 30 minutes later, the magnetic cloud reached Earth, and it took an entire day to pass. During its first 12 hours the cloud exposed the magnetosphere to a southward-slanted magnetic field, a condition that "opens the valve" and allows direct transfer of energy between the matter flowing to Earth from the Sun and the Earth's magnetic field. As a result, a series of magnetic substorms were observed during this period. The cloud's magnetic field then rotated northward orientation, shutting down the pathway.
Nature, however, provided an unexpected twist. At the trailing edge of the cloud, a huge density increase lasting about one hour was observed. The Solar Wind Experiment on board NASA's WIND spacecraft, which studies the Earth-Sun relationship, detected high-density values approximately 30 times greater than normal.
This created a large pressure pulse that struck an already "loaded" magnetosphere like a hammer hitting a bell. This second blow, completing the one-two punch, was sufficient to push the outer boundary of the magnetosphere far closer to Earth, inside geosynchronous orbit (at a radius of about 42,000 kilometers, where many communications satellites orbit).
The near-Earth radiation environment went haywire. At the equator, convection of plasma in the magnetosphere increased, and multiple magnetic substorms explosively released stored energy, generating auroras—beautiful displays of light. A magnetic field depression of 150 nano Teslas measured in China revealed a magnetic storm. Subsequent measurements suggested that the enhanced state of the radiation belts persisted for several days. On the ground, the effects of the cloud were equally spectacular. The electric field surged to levels twice as high as those usually observed during disturbed periods.
The intense auroral displays resulting from the violent substorm activity were captured in unprecedented detail by the visible, ultraviolet, and X-ray imagers on NASA's POLAR spacecraft (see figure below). Movies illustrating the dynamic response of the auroras near the poles are available from the event home page (see next section).
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| Observations of the solar corona with the SOHO spacecraft's LASCO instrument, showing the evolution of the Coronal Mass Ejection as it flows away from the Sun. |
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| Auroral activity recorded by imagers on the PLOAR spacecraft. The upper panels are near the peak of activity just before 1100 UT on January 10, while the bottom panels correspond to approximately 30 minutes later when the ativity had decreased significantly. |
The ISTP science teams are now trying to chronicle in detail the history of this event as it evolved from start to finish, including its three-dimensional structure and influence on the magnetosphere. An effort is also underway to help fill in gaps in the data. Fortunately, the observations cover the event fairly thoroughly and can readily be used to gauge the ability of theoretical or numerical models to "predict" the effect of events like the one in question. These data are available to agencies interested in conducting technical investigations of the possible effects on communications systems and Earth-orbiting spacecraft.
For more information, see http://www-istp.gsfc.nasa.gov/istp/cloud_jan97/event.html on the World Wide Web. The event has generated a great deal of interest in the space physics community, the media, and the general public. News of the event resulted in 12,000 hits on the event home page on January 23, and news of another incoming coronal mass ejection in the spring prompted nearly 23,000 hits on April 10.
Currently, the Sun is near the minimum of its solar cycle; over the next five years the activity on the Sun will increase until it reaches solar maximum. The many spacecraft and ground-based networks of the ISTP Observatory are uniquely suited to study this transition from solar minimum to solar maximum, beginning with what we used to call "quiet sun." Based on SOHO observations to date, and after witnessing the chain of events associated with this CME, it is obvious that the term "quiet sun" is quite outdated. The ISTP program offers the first real possibility of understanding Sun-Earth events to the point where "space weather" prediction becomes feasible.
Source: Eos, October 28, 1997, p. 477.