'Treasure trove' of data reveals the anatomy and evolution of solar flares.
Astronomers seeking to predict solar storms are receiving the first trickles of a wealth of new data, scientists reported this week at a meeting of the Committee on Space Research in Bremen, Germany.
The data come from a new satellite, the Solar Dynamics Observatory (SDO), which became operational in late April. Launched on 11 February and costing US$856 million (including the first five years of operation), it is the inaugural mission in NASA's Living With a Star Program, intended to better understand the sources of solar magnetic storms.
One instrument on board uses the Doppler effect to measure the movement of ionized gases in the Sun's atmosphere in response to changes in the Sun's magnetic field — the magnetic flux. These changes are important, says Yang Liu, a research scientist at Stanford University in California and member fo the SDO team, because very often they trigger solar flares and coronal mass ejections. These, in turn, can send radiation sleeting toward Earth, knocking out satellites, disrupting power grids and endangering astronauts.
Previous instruments had not been consistently able to detect early changes in magnetic flux preceding these events, Liu says, because they either had narrow fields of view — which meant that they would only be pointing by chance at the right part of the Sun when an event began — or they didn't update their flux maps quickly enough to catch the rapid changes that occur in the initial stages of an event.
The SDO, by contrast, photographs the entire Sun at 750-kilometre resolution every few seconds and produces vector maps of the disk's entire magnetic field every 15 minutes. Virtually every second, one of the observatory's six 16-megapixel cameras is beaming an image back to Earth, says Alan Title, a scientist at Lockheed Martin's Space Systems Advanced Technology Center in Palo Alto, California, who is principal investigator for another of the SDO's instruments.
Currently, the Sun is unusually quiescent, but scientists hope to use the SDO to watch how it changes as it moves into the next sunspot cycle.
The new observatory has already produced results. "We are still in the deepest solar minimum for more than a century," Title says. "Nevertheless, the Sun has presented us with a number of coronal mass ejections, filament eruptions, small flares and even a few moderately large flares."
"The beauty of the SDO's magnetic flux instrument is to measure the whole disk of the Sun 24/7 very rapidly," Liu added.
The SDO is also equipped with an array of 15-centimetre telescopes that measure the temperature of the Sun's corona, using spectrometers that look for telltale emission lines from iron and helium ions, corresponding to temperatures from 20,000 kelvin to 20,000,000 kelvin.
To make these images sharp enough for high-resolution mapping (with a pixel size as small as 730 km), Title says the observatory must be so finely stabilized that the cameras shift target by no more than 7 kilometres. "This is equivalent to keeping a laser pointed at a target circle a half millimetre in diameter from a distance of 10 kilometres," he says. The camera also has a fast shutter speed, allowing it to track 'puffs' of hot gases, which can be moving at speeds of 1,000 to 2,000 km per second.
One new detail, not well-studied by previous cameras, are waves that propagate quickly along the magnetic field lines as the flare evolves. "We've seen these waves before," says Tom Woods, of the University of Colorado's Laboratory for Atmospheric and Space Physics in Boulder, "but we're seeing them in a lot better detail."
Also, Woods says, "These fast waves hit another active region and seem to set off sympathetic flares. It's kind of like one solar storm forcing another solar storm somewhere else on the Sun. You can see it propagate over most of the solar disk. It's pretty amazing."
The lesson, Woods adds, is that solar scientists have taken an overly small-scale view of the Sun. "It's really revolutionized how we think about the Sun in terms of global-scale action," he says.
Woods own research involves the SDO's third instrument, which measures the Sun's emissions of 'extreme' ultraviolet light (wavelengths 0.1-105 nanometres). Startlingly, Woods says, the observatory has already found that the X-ray emissions of flares are followed, an hour or two later, by a pulse of extreme ultraviolet containing three times more energy than the initial X-ray burst. "We've been studying the tip of the iceberg," he says of previous research. "We don't really understand quite yet what it means in terms of the physics of what's going on."
Meanwhile, data are pouring in. "Unfortunately," says Title, "the awful truth is that the spectroscopic telescopes alone generate about 3.5 terabytes of uncompressed images per day."
But the real challenge — and "the really exciting" thing about the SDO, says Frank Eparvier, also of the University of Colorado, is the coordination between the satellite's three instruments.
"Because of the sheer size of the treasure trove of data we are getting from SDO," he says, "it will require the entire scientific community to sift though and make the connections that advance the science."