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Right: An artist's concept depicts a flare evolving into a Coronal Mass Ejection. Color-color diagrams may help scientists predict which flares will evolve into potentially dangerous Coronal Mass Ejections (CMEs), and which ones will fade back into the solar atmosphere. Links to 930x730-pixel, 90KB JPG. Credit: NASA/Marshall Space Flight Center. This finding, presented today at the Centennial Meeting of the American Astronomical Society in Chicago, could yield new insight into how particles may be accelerated to high energies in solar flares. It could become especially important over the next few years as the Sun's activity peaks during solar maximum. "Right now, we can basically account for the gross properties of solar flares in our numerical simulations," explained Dr. Elizabeth Newton, a solar physicist at NASA's Marshall Space Flight Center. These "gross properties" are like having a basic understanding that people have two arms, two legs, and walk upright. It's when you look more closely that people begin to take on different characteristics, like fingerprints. The same is apparently true with flares, but only to a point. |
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"We call this variation with time 'spectral evolution,'" Newton continued. "Is it the same in every flare? Are they all different? Are there 'classes' of flares? This is what we're after." Solar flares are tremendous explosions on the surface and in the atmosphere of the Sun. In a matter of just a few seconds they heat material to many millions of degrees and release as much energy as a billion megatons of TNT.
They occur near sunspots, usually along the dividing line (neutral line) between areas of oppositely directed magnetic fields, where the fields have become stressed (sheared). In some cases, these flares are associated with eruptions of the Sun's matter into space called "coronal mass ejections." These events release a million tons of particles traveling at a 1.6 million km/h (1 million mph) - occasionally directed at the Earth. In a solar flare, the released magnetic energy accelerates particles - electrons and protons - to extremely high energies. When these particles crash into the solar atmosphere, their kinetic energy is converted into X-rays and gamma-rays that are detected by orbiting satellites. The spectral evolution of a flare can therefore be thought of as a direct fingerprint of the mechanism that accelerates particles to high energies in the Sun and the type of target they interact with. "It's a mark of the throttle for getting these particles up to high energy in a flare and what material these particles are interacting with," remarked Newton. "And what we've found is that there is striking similarity from flare to flare in how the X-rays these particles produce vary in time and energy." |
As a first step to tell whether a flare's evolving X-ray energy distribution carries clues about its cause, Newton borrowed a technique called "color-color diagrams" that was developed for studying X-ray binary stars. Tim Giblin, a graduate student working at NASA/Marshall, earlier applied it to gamma-ray bursts and found that bursts fell into a half-dozen or more patterns. |
In a color-color diagram, a scientist plots an event's count ratio, or "color", in one energy band against its count ratio in another energy band. It's not unlike using your stereo system's graphic equalizer and taking the ratio of the sound's loudness in two "tweeter" channels and comparing it to the ratio of the sound's loudness in two "woofer" channels. Instead of the amplitude of sound, however, scientists use the diagram to look at the brightness of X-rays and gamma-rays. "These diagrams are very useful," commented Newton, "as they don't rely on any assumptions about what the spectrum looks like. They're completely model-independent, empirical tools for examining what's going on in the flare." Previous investigations into spectral evolution have had to assume a spectral model for the flare before characterizing the evolution. Both Newton and Giblin use data from the Burst and Transient Source Experiment (BATSE) aboard the Compton Gamma-Ray Observatory. While BATSE was designed to seek the locations of gamma-ray bursts in deep space, it will detect any gamma-rays or X-rays from any object in space, including the Sun. BATSE detects radiation from flares between 20,000 and 1.87 million electron volts (slightly higher than the energy in the photons in a typical dental X-ray), and divides it into sixteen energy channels, providing multiple 16-channel snapshots of a flare each second. Newton took a ratio of the brightness in lower energy channels and the ratio of brightness in the higher energy channels, and plotted the two ratios against each other. To help the human eye track the flare in time from start to finish, a different visual color is assigned to each point in time for which one takes the brightness ratios. Two views of the same event |
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Left: The two graphs at left show the brightness profiles of two solar flares as detected by BATSE. The first, on Nov. 10, 1991, was a strong M7.9 flare; the second was a slightly weaker M4.1 flare on Dec. 4, 1991. At right are their color-color diagrams showing the "Lazy V" and "Inverted Crescent" patterns that dominate some 36 flares analyzed with this tool. Links to 650x551-pixel, 95K JPG. Credit: NASA/Marshall The color diagram results don't point directly to an answer to the mystery of what accelerates material in flares, but it will help scientists focus their investigations. "We need to fully characterize the behavior first before we can explain it," commented Newton. "We hope to find some relationship between a flare's color-color diagram and the flare's type. We'd like to predict which ones will cause coronal mass ejections and which ones will just be a flash in the pan. |
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"In other words, in order to accurately model what accelerates particles in solar flares, we have to know observationally what baseline behavior the model is supposed to meet. We're not seeing anything that would support a model of random acceleration, as that would likely result in all the flares' color diagrams being as unique as fingerprints. There's some sort of local control involved in how the X-rays are produced, and this local control seems to be the same in a significant majority of flares." Scientists are looking forward to the launch of the next instrument to be applied in this area, the High Energy Solar Spectroscopic Imager (HESSI) scheduled for July 4, 2000. HESSI's primary mission is to explore the basic physics of particle acceleration and explosive energy release in solar flares and will provide better spectral and temporal resolution, in addition to images of flares. |
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