NASA – The most intense burst of solar radiation in five decadesaccompanied a large solar flare on January 20. It shookspace weather theory and highlighted the need for newforecasting techniques, according to several presentationsat the American Geophysical Union (AGU) meeting this week inNew Orleans.
The solar flare, which occurred at 2 a.m. EST, trippedradiation monitors all over the planet and scrambleddetectors on spacecraft. The shower of energetic protonscame minutes after the first sign of the flare. This flarewas an extreme example of the type of radiation storm thatarrives too quickly to warn interplanetary astronauts.
“This flare produced the largest solar radiation signal onthe ground in nearly 50 years,” says Dr. Richard Mewaldt ofthe California Institute of Technology, Pasadena, Calif, who is a co-investigator on NASA’s Advanced Composition Explorer(ACE) spacecraft. “But we were really surprised when we sawhow fast the particles reached their peak intensity andarrived at Earth.”
Normally it takes two or more hours for a dangerous protonshower to reach maximum intensity at Earth after a solarflare. The particles from the January 20 flare peaked about15 minutes after the first sign.
“That’s important because it’s too fast to respond with muchwarning to astronauts or spacecraft that might be outsideEarth’s protective magnetosphere,? Mewaldt says. “Inaddition to monitoring the sun, we need to develop theability to predict flares in advance if we are going to sendhumans to explore our solar system.”
The event shakes the theory about the origin of protonstorms at Earth. “Since about 1990, we’ve believed protonstorms at Earth are caused by shock waves in the inner solarsystem as coronal mass ejections plow through interplanetaryspace,? says Professor Robert Lin of the University ofCalifornia at Berkeley, who is principal investigator for theReuven Ramaty High Energy Solar Spectroscopic Imager(RHESSI). “But the protons from this event may have comefrom the sun itself, which is very confusing.”
The origin of the protons is imprinted in their energyspectrum, as measured by ACE and other spacecraft, whichmatches the energy spectrum of gamma-rays thrown off by theflare, as measured by RHESSI. “This is surprising because inthe past we believed the protons making gamma-rays at theflare were produced locally and the ones at the Earth wereproduced instead by shock acceleration in interplanetaryspace,” Lin says. “The similarity of the spectra suggeststhey are the same.”
Solar flares and coronal mass ejections (CMEs), associatedgiant clouds of plasma in space, are the largest explosionsin the solar system. They are caused by the buildup andsudden release of magnetic stress in the solar atmosphereabove the giant magnetic poles we see as sunspots. TheTransitional Region and Coronal Explorer (TRACE) and theSolar and Heliospheric Observatory (SOHO) spacecraft aredevoted to observing the sun and identifying the root causesof flares and CMEs, with an eye toward forecasting them.
“We do not know how to predict the flow of energy into andthrough these large flares,” says Dr. Richard Nightingale ofthe Lockheed Martin Solar and Astrophysics Laboratory inPalo Alta, Calif. “Instruments like TRACE give us new clueswith each event we observe.”
TRACE has identified a possible source of the magneticstress that causes solar flares. The sunspots that give offthe very largest (X-class) flares appear to rotate in thedays around the flare. “This rotation stretches and twiststhe magnetic field lines over the sunspots,” Nightingalesays. “We have seen it before virtually every X-flare thatTRACE has observed since it was launched and more than halfof all flares in that time.”
However, rotating sunspots are not the whole story. Theunique flare came at the end of a string of five other verylarge flares from the same sunspot group, and no one knowswhy this one produced more sudden high energy particles thanthe first four.
According to Lin, “It means we really don’t understand howthe sun works. We need to continue to operate and exploitour fleetof solar-observing spacecraft to identify how it works.”
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