June 1, 1999: A tool for taking the fingerprints of gamma-ray bursts from deep space is being used to study the spectra of flares from the Sun. But unlike humans and gamma-ray bursts, whose fingerprints are as unique as...well...fingerprints, new research shows that the detailed spectral behavior of solar flares falls for the most part into just two categories.
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.
Model Behavior
"When we 'build a flare' in a computer simulation, we can reproduce things like the total number of particles being accelerated, the energies they attain, and the timescales over which these things occur," remarked Newton. So the next logical step in the development of their understanding is to probe the details, and see if their computer models can meet more detailed observations, such as how the distribution of emitted energy (called a spectrum) varies with time in a flare.
"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."
Flares usually make little difference to the Sun's brightness in visible light, but stand out well in ultraviolet, X-rays, and gamma-rays. Their output at higher energies can affect the Earth's outer atmosphere and damage electronics on satellites. Scientists have a classification scheme for a flare's intensity that relates to the total X-ray flux or optical brightness. But it does not describe how the energies within the overall X-ray emission are distributed.
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.
Plotting in color 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
Newton analyzed 114 flares that were detected by BATSE and also seen by the Hard X-ray Telescope aboard the Japanese Yohkoh solar physics satellite, thus providing two independent views of the same events on the Sun. To this point, however, the analysis has only focused on the higher sensitivity data provided by BATSE.
Of 36 flares with sufficient data in the highest energy channels, Newton found that about 80 percent of the color diagrams fall into a "Lazy V" pattern, and 18 percent fall into an "Inverse Crescent" pattern. Data on the remaining 2 percent were not sufficient to fall clearly within either category. This behavior of flares is in marked contrast to their "cosmic cousins" the gamma-ray bursts, which show a half-dozen or more distinct patterns in their spectral evolution.
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.
"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.