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A Solar Radiation Storm

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see captionJuly 14, 2000 -- This morning NOAA satellites and the orbiting Solar and Heliospheric Observatory (SOHO) recorded one of the most powerful solar flares of the current solar cycle. Space weather forecasters had been predicting for days that an intense flare might erupt from the large sunspot group 9077, and today one did.

"Energetic protons from the flare arrived at Earth about 15 minutes after the eruption," says Gary Heckman, a space weather forecaster at the NOAA Space Environment Center. "This triggered a category S3 radiation storm."

Right: This SOHO animation of the July 14th X-class solar flare was recorded by the spacecraft's Extreme-ultraviolet Imaging Telescope at 195 angstroms. A 350 kb GIF movie nicely shows the flare, followed by a torrent of energetic particles that arrived about 15 minutes later, creating snow on the images as the particles bombarded the camera's electronic detectors.

According to NOAA space weather prediction scales, an S3 storm can cause the following effects on satellites: single-event upsets, noise in imaging systems, permanent damage to exposed components/detectors, and decrease of solar panel currents. It can also expose air travelers at high latitudes to low levels of radiation, the equivalent of a brief chest x-ray.

The wave of solar particles - known as a solar proton event - is already four times more intense than any other event detected since the launches of SOHO in 1995 and ACE in 1997. At mid-afternoon (UT) on July 14th, the storm of particles from the Sun was still intensifying.

"The energetic particles from this flare arrived very quickly," continued Heckman. "Protons have been observed in the past to arrive between 15 minutes and several hours after a flare. This event was definitely one of the fastest, but we don't yet have all the numbers to say exactly how it ranks in this category."

"Given the information I have now, if I was advising my own daughter I would tell her it's safe to fly. In fact, if she were going to come visit me today (she lives in Florida and I'm in Oklahoma), I would tell her to take a plane rather than drive because even during this solar radiation event flying would be safer." -- Dr. Wallace Friedberg of the Federal Aviation Administration's Civil Aeromedical Institute in Oklahoma City, July 14, 2000.

see captionSoon after the solar flare, which occurred at 1024 UT (6:24 a.m EDT), coronagraphs on board the ESA/NASA Solar and Heliospheric Observatory recorded a "full-halo" coronal mass ejection (CME). CMEs are gigantic bubbles of electrified gas carrying away as much as 10 billion tons of solar material. This one appears to be heading toward our planet at 1300 to 1800 km/s.

Right: A full halo coronal mass ejection recorded on July 14, 2000, by SOHO's C2 coronagraph. "Halo events" are CMEs aimed toward the Earth. As they loom larger and larger they appear to envelop the Sun, forming a halo around our star. The many speckles in the latter half of this animation are energetic particles from a related solar flare bombarding SOHO's electronic detectors. "The SOHO instruments look like someone aimed a Gatling gun at them," commented NOAA's Gary Heckman. Click on the image for a larger animation.

There's no cause for alarm. When a CME hits the magnetosphere -- the region around Earth controlled by our planet's magnetic field -- most of the incoming material is deflected away. If the shock wave is very strong, as this one might be, it can compress the magnetosphere and unleash a geomagnetic storm. In extreme cases, such storms can induce electric currents in the Earth that interfere with electric power transmission equipment. Satellite failures are possible, too. Geomagnetic storms can also trigger beautiful aurorae. These "Northern Lights" are usually seen at high latitudes, but they have been spotted farther south than Florida during intense disturbances. The last time this happened was April 6, 2000.

see caption"At this time, I'm making plans to look for the aurora Saturday night," says Heckman, who lives in Colorado.

Whether or not an auroral display is triggered by the blast depends on the orientation of the magnetic field within the CME's approaching shock wave. Magnetic fields with a southward directed component can create a weak point in Earth's magnetic defenses and make auroras more likely.

Above: This rare red-colored aurora over North Carolina was photographed by Chuck Adams on April 6, 2000. The bright object near the horizon is the Moon. Also visible in the background are the Pleiades, Taurus, and Orion. The photographer used a Nikon FM2 camera equipped with a 28mm f/2 lens. The exposure time was one minute on Kodak Elite 100 slide film. (Copyright 2000, Chuck Adams, all rights reserved.)

The Moon will be full this weekend when the solar disturbance is expected to hit. Bright moonlight will outshine faint Northern and Southern Lights, but vivid aurora could still be visible in spite of the lunar interference. Observers across the Pacific might be treated to the very rare sight of shimmering colorful aurora during the total lunar eclipse of July 16th!

see captionSunspot group 9077, which triggered today's solar flare and coronal mass ejection, exhibits a complex magnetic field that harbors energy for powerful eruptions.

Left: This July 14th image of the Sun was captured by the Michelson Doppler Imager on board the Solar and Heliospheric Observatory. The large sunspot group, numbered 9077, is near the center of the Sun's visible disk.

"Solar flares and CMEs occur whenever there's a rapid, large-scale change in the Sun's magnetic field," explains David Hathaway, a solar physicist at the NASA Marshall Space Flight Center. "The solar active region that produced the eruptions [on July 14] had a complicated magnetic configuration. Oppositely-directed magnetic fields were seen right next to each other."

Active region 9077 is very near the center of the Sun's visible disk. Solar rotation will carry the sunspot group toward the Sun's western limb by late next week. For the next few days, however, any additional eruptions are likely to be Earth-directed. The fireworks may not be over yet!

Stay tuned to Science@NASA for news and updates about the coming geomagnetic disturbance.

Below: Solar Flares are classified by their x-ray flux in the 1.0 - 8.0 Angstrom band as measured by the NOAA GOES-8 satellite. On July 14, 2000, a solar flare from active region 9077 registered as a powerful X5-class eruption. Another X-class flare from 9077 was recorded on July 12, 2000.

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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.

Web Links

NOAA Space Environment Center -official forecaster of space weather events -daily updates and news about solar flares, coronal mass ejections and geomagnetic activity

More about the "magnetotail" and what causes aurora - from the NASA/Goddard Space Flight center

All about aurora - from the University of Alaska Geophysical Institute

Aurora FAQ - from the University of Alaska Geophysical Institute

Thursday's Classroom -- lesson plans and educational activities about space weather. - from NASA/MSFC