杏吧原创

A new technique predicts with 90% accuracy which regions of the Sun are not likely to spew out flares of particles that could endanger astronauts or knock out power grids on Earth. But the method cannot yet calculate when the eruptions will occur 鈥� a crucial requirement for accurate space weather forecasting.

Astronomers use satellite observations of the Sun鈥檚 roiling magnetic fields to try to identify which are likely to twist and break free, sending charged particles hurtling to Earth in solar flares and coronal-mass ejections. The particles can arrive at Earth in as little as 15 minutes and their radiation can harm astronauts and disrupt satellites and power grids.

鈥淚n some cases, the magnetic structure looks so weird that any solar physicist looking at it says, 鈥榯his is going to flare鈥�, simply because there is no way for that kind of structure to be maintained,鈥� says Karel Schrijver, a physicist at Lockheed Martin Solar and Astrophysics Laboratory in Palo Alto, California, US.

鈥淏ut at times we get surprised by the magnitude of flares and coronal mass ejections from parts of the Sun that just don鈥檛 look to be that threatening,鈥� says Joseph Kunches, chief of the Forecast and Analysis Branch at the National Oceanic and Atmospheric Administration鈥檚 Space Environment Center in Boulder, Colorado, US.

TRACE evidence

Now, Schrijver and three colleagues have devised two related ways to determine which suspicious regions are likely to erupt. Both methods hinge on detecting which magnetic field lines from within the Sun are carrying up strong electrical currents. The currents appear to drive solar eruptions, but scientists do not yet understand what causes the currents in the first place.

In one method, scientists use magnetic field measurements of the photosphere 鈥� the visible surface of the Sun 鈥� taken by the US-European SOHO spacecraft. Then they put those observations into a computer model that computes how the fields without electrical currents should rise upwards into the Sun鈥檚 outermost layer, or corona.

Next, they compare these modelled fields with those directly observed in the corona using NASA鈥檚 TRACE satellite. If the model matches the TRACE observations, then that suggests the magnetic fields are not packing current and the chance of a flare from that region is low. But if the two data sets disagree, the region is about three times more likely to flare up.

You can view NASA videos (Mpeg format) showing how the model magnetic field lines (in red) in quiescent regions of the Sun but in active regions.

Alternatively, scientists can study just the SOHO data for a day or two to look for the emergence of new magnetic fields from within the Sun that are out of alignment with those already on the photosphere. These new fields seem to induce more current as they interact with the established fields.

All clear

The methods are each 90% accurate in determining whether a particular region about 160,000 kilometres in width carries large electrical currents. These regions then have a 40% chance of erupting in a major flare within four days, according to the new work. 鈥淏ut taking the methods from there to when a flare will happen is a very difficult step,鈥� Schrijver said in a teleconference with reporters on Tuesday.

It is also not possible to predict the size of flares. But a joint Japanese-US-UK spacecraft called Solar-B, due to launch in 2006, should be able to measure the magnitude of the electrical currents to help answer this question.

But while predicting the strength and timing of an expected flare is at least a year away, knowing that no solar flares are expected is also useful, says Richard Fisher, director of NASA鈥檚 Sun-Solar System Connection Division in Washington, DC, US.

鈥淭he advantage of having an 鈥榓ll clear鈥� is you have a factor which you can use in scheduling extra-vehicular activities,鈥� he says, referring to the spacewalks that expose astronauts to even more radiation than they receive while in the space station. The 鈥榓ll clear鈥� prediction would last a few hours for the Sun鈥檚 most active regions.

Journal reference: Astrophysical Journal (vol 628, p 501)