Imagine forecasting a hurricane in Miami weeks before the storm was even a swirl of clouds off the coast of Africa -- or predicting a tornado in Kansas from the flutter of a butterfly’s wing in Texas. These are the kind of forecasts meteorologists can only dream about. Could the dream come true? A new study by Stanford University researchers suggests that such forecasts may one day be possible -- not on Earth, but on the Sun. In the August 19 issue of Science, Stathis Ilonidis, together with co-workers Junwei Zhao and Alexander Kosovichev, announced that they can see some sunspots while they are still submerged, before they are visible to the naked eye. “This could lead to significant advances in space weather forecasting,” Ilondis said.

Most Amateur Radio operators are aware of sunspots, those dark blemishes on the surface of the Sun. This is because sunspots influence propagation. They can be a boon to hams interested in HF propagation, as sunspots are the sites of ultraviolet or extreme ultraviolet radiation that creates our ionosphere. But sunspots can also be destructive. They are the source for disturbances, such as flares, and the roots of coronal mass ejections (CMEs) that can hinder propagation. When a CME hits the Earth’s atmosphere, the low bands will be depressed and signals will be weaker the lower the frequency. The absorption rate will be most severe on 160 meters, less on 80 and somewhat better on 40 meters. The maximum usable frequency (MUF) -- the highest frequency by which a radio wave can propagate between given terminals by ionospheric propagation alone, independent of power -- will be lower and auroral propagation on the VHF bands is quite possible.

“Astronomers have been studying sunspots for more than 400 years, and they have pieced together their basic characteristics: Sunspots are planet-sized islands of magnetism that float in solar plasma,” explained NASA’s Tony Phillips. “Although the details are still debated, researchers generally agree that sunspots are born deep inside the Sun via the action of the Sun’s inner magnetic dynamo. From there, they bob to the top, carried upward by magnetic buoyancy; a sunspot emerging at the stellar surface is a bit like a submarine emerging from the ocean depths.”

Phillips explained that the trio’s analysis technique, called “time-distance helioseismology,” is similar to an approach widely used in earthquake studies: “Just as seismic waves traveling through the body of Earth reveal what is inside the planet, acoustic waves traveling through the body of the Sun can reveal what is inside the star. Fortunately for helioseismologists, the Sun has acoustic waves in abundance. The body of the Sun is literally roaring with turbulent boiling motions.” This sets the stage for early detection of sunspots.

Instruments onboard two spacecraft -- the Solar and Heliospheric Observatory (SOHO) and the newer Solar Dynamics Observatory (SDO) -- constantly monitor the Sun for acoustic activity. Submerged sunspots have a detectable effect on the Sun’s inner acoustics -- namely, sound waves travel faster through a sunspot than through the surrounding plasma. A big sunspot can leapfrog an acoustic wave by 12 to 16 seconds. “We can’t actually hear these sounds across the gulf of space, but we can see the vibrations they make on the Sun’s surface,” Ilonidis told Phillips. “By measuring these time differences, we can find the hidden sunspot.” Ilonidis cautioned that there are limits to the technique: “We can say that a big sunspot is coming, but we cannot yet predict if a particular sunspot will produce an Earth-directed flare.”

According to Phillips, the technique seems to be most sensitive to sunspots located about 60,000 km beneath the Sun’s surface. The team isn't sure why that is the “magic distance,” but Phillips noted that it’s a good distance because it gives the team as much as two days’ advance notice that a spot is about to reach the surface.

“This is the first time anyone has been able to point to a blank patch of Sun and say ‘a sunspot is about to appear right there,’” said Ilonidis’ thesis advisor Dr Phil Scherrer of the Stanford Physics Department. “It’s a big advance.”

So far, Ilonidis, Zhao and Kosovichev have detected five emerging sunspots: four with SOHO and one with SDO. Of those five, two went on to produce X-class flares, the most powerful kind of solar explosion. “This encourages the team to believe their technique can make a positive contribution to space weather forecasting,” Philips said. “Because helioseismology is computationally intensive, regular monitoring of the whole Sun is not yet possible, but Ilonidis believes that it is just a matter of time before refinements in their algorithm allow routine detection of hidden sunspots.”  -- Thanks to Dr Tony Phillips for the information