In understanding our own star, solar physicists have one giant question: why is the outside hotter than the inside?
Since nuclear fusion in the core generates the heat, one might expect the atmosphere or corona to be cooler, not hotter, since it is farther away from where the heat comes from in the core. In fact, the Sun's outer atmosphere ( the Corona ) is hotter than 1,000,000ºC (1,800,000ºF) while the visible surface has a temperature of only about 6000ºC (10,000ºF). One theory for why the heat might be greater farther out depends the action of solar flares, the spectacular, magnetically-tied explosions that uplift large masses and push them outward. Among these heating events, the largest are called coronal mass ejections.
|Solar flares issue strong electromagnetic bursts.
According to new data from the University of California, Berkeley's RHESSI satellite, while the sun's big, bright, explosive flares are the attention grabbers, the tiny, more numerous microflares may have nearly as much influence on the solar atmosphere.
Solar flares, the largest explosions in the solar system, propel energetic particles into space and are thought to be the main source of heat pumping the sun's outer atmosphere to a few million degrees Celsius -- hotter than the surface itself.
|Click here for larger image. The Sun, a typical G2V dwarf. G stars are characterized by the presence of metallic lines and weak hydrogen.
Credit: Harvard University
Now, solar observations by the RHESSI (Reuven Ramaty High-Energy Solar Spectroscopic Imager) satellite show that microflares a million times smaller are far more frequent and may together provide a major portion of the heat in the corona.
"The big question for microflares is, are there enough of them? Do they occur frequently enough and dump enough energy into the corona?" said Robert Lin, professor of physics at UC Berkeley and principal investigator for RHESSI. "RHESSI can see these tiny flares to lower energies than before, and our observations are beginning to show that there is more energy released in these tiny flares than people had originally thought."
Since solar flares play a major role in space weather, RHESSI's discoveries about flares and microflares could eventually help predict the big storms that interfere with radio communications on Earth.
Lin presented new data from RHESSI at the meeting of the International Astronomical Union in Sydney, Australia.
RHESSI, launched by NASA in February 2002 to study X-ray and gamma-ray emissions from flares, has observed more than 10,000 microflares in the past year and a half. These microflares are identified by the hard X-rays they emit, which RHESSI is able to detect with 10 to 500 times the sensitivity of any previous instruments flown in space.
These X-ray observations show that microflares are merely smaller versions of their larger cousins, Lin said. Some astronomers have suggested that microflares may be mainly thermal events, heating the sun but not accelerating particles like larger flares. If that were the case, they would produce more low-energy soft X-rays than high-energy hard X-rays. But they do not.
"We've noticed that microflares are very similar to big flares. In big flares, a lot of the energy, perhaps most of it, comes out in accelerated particles -- electrons, protons and heavy nuclei," Lin said. "We are finding the same to be true of microflares."
Interestingly, a subset of microflares appears to be a different animal entirely and responsible for a type of radio burst from the sun studied intensively by pioneering Australian radio astronomer Paul Wild in the 1960s and 1970s. These so-called Type III bursts are characterized by radio signals that decrease in frequency, like the whistle from a departing train.
|Flares reaching the dimension of our star.Credit: SOHO
RHESSI has seen many Type III bursts, and they appear to be associated with microflares that do very little heating of the solar atmosphere. Instead, the stream of high-speed particles they produce seems to jet unchecked out of the sun at speeds up to one-third the speed of light, exciting radio oscillations at lower and lower frequencies as the particles pass through lower and lower density plasma.
"This probably has to do with the magnetic field in the region around the microflare, since particles are pretty much tied to the field lines and have to run along them," Lin said. "We think that for normal microflares, the particle acceleration occurs in a closed magnetic region so the electrons can't get away; they do more heating that way. In Type III bursts, the electrons are accelerated in an open magnetic field, and they have an easy way to escape, so they do less heating in the corona."
Aside from RHESSI's numerous observations of microflares, the satellite's X-ray and gamma-ray instruments have also captured several large flares. These have allowed the RHESSI team to investigate the relationship between flares and coronal mass ejections (CME), which are another type of large stellar explosion that sends shock waves into space. One conclusion, Lin said, is that the fastest coronal mass ejections -- those moving at 1 to 5 million miles per hour (1.6 to 8 million kilometers per hour) -- are linked directly to solar flares.
Flare Starts Everything Off
"With RHESSI, we can image the location of a flare's initial release of energy and accelerated particles," Lin said. "When we look at extremely big and fast coronal mass ejections and extrapolate back to the sun, we find that at the very point where the coronal mass ejection is initiated, that is exactly where the flare energy release happened. The flare starts everything off."
|The dotted magnetic structure of the sun, like an orange rind.Credit: SOHO
These largest of the mass ejections are the ones that have the greatest effect on Earth, exciting geomagnetic storms that can cause power outages and damage communications satellites. The shock wave from coronal mass ejections also produces energetic particles that pose a hazard to satellites and astronauts. R ecent history had shown how serious the sudden changes in space weather can be. In 1989, a solar storm tripped a protective switches in Canadian Hydro-Québec power company. For nine hours, the entire province of Québec was without power. The problem nearly spread to the United States through an interconnected grid, officials said at the time. In a 1997 solar storm, an AT&T Telestar 401 satellite used to broadcast television shows from networks to local affiliates was blacked out. A more serious breakdown of communications occurred in May 1998 when a space storm disabled PanAmSat's Galaxy IV. Then, though SOHO was flying, researchers had not yet developed the sophisticated prediction abilities they have today. Among the Galaxy IV casualties: automated teller machines; gas station credit card handling services; 80 percent of all pagers in the United States; news wire service feeds; CNN's airport network; and some airline weather tracking services.
|Sun as source of energy for life on Earth. Credit: NASA
"If we understood the process, we could begin predicting when coronal mass ejections should happen," Lin said. "We're still a long way from that, but it makes it extremely interesting to discover the relationship between flares and coronal mass ejections."
It is still unclear whether other types of coronal mass ejections are related to solar flares, he said.
Both flares and coronal mass ejections are produced by the roiling magnetic fields in the surface of the star. As the surface churns, magnetic field lines get twisted like rubber bands. When the tension becomes too great, they break, snapping and flinging charged particles outward in a solar flare.
Flares can trigger coronal mass ejections, which are massive rising bubbles of plasma entangled with the magnetic field. But some mass ejections seem unrelated to flares, Lin said. One possible explanation is that these come from magnetic fields that kink as they twist, so the magnetic field intensity doesn't get compressed enough to explode into a flare.
"In this case, the magnetic fields slowly kink and eventually start to rise, dragging plasma with it them," he said. "They're not associated with a flare because they don't break suddenly.
"The very fast, powerful CMEs are probably the breaking kind."
RHESSI will continue its observations of solar flares for at least another two years, and probably longer.
The RHESSI scientific payload is a collaborative effort among UC Berkeley, NASA Goddard Spaceflight Center, the Paul Scherrer Institut in Switzerland and the Lawrence Berkeley National Laboratory. The mission also involves additional scientific participation from France, Japan, The Netherlands, Scotland and Switzerland.
Related Web Pages
International Astronomical Union
The Sun as Art
Solar Full Disk (Astrobiology Magazine)
Real-time Solar Weather (NOAA)
SOHO Latest Images
ESA SOHO Science