You may know that Pink Floyd had an album called The Dark Side Of The Moon, but did you know that it’s possible to get a look at the far side of the sun?
It is! In fact, that’s what’s in the center of the picture to the left. There is a technique astronomers have developed called helioseismic holography that allows us to map what is happening on the far side of the sun. Here’s the story on the image you’re looking at:
The false colors represent condensations of magnetic flux — that is, sunspots. This holographic map captured April 12, 2001, shows the giant sunspot AR9393 on the back side of the Sun a full week before it emerged into direct view over the Sun’s eastern limb. MDI holographic images reveal the Earth-facing side of the Sun 70 degrees from the disk center, and the far side of the Sun 50 degrees from disk center.
You may be wondering how all this works, well . . .
The Sun is a hummimg ball of sound waves launched by turbulent convective motions in our star’s outer layers. “The waves we monitor [using MDI] have a period of about 5 minutes,” says Phil Scherrer of Stanford University, principal investigator for the MDI instrument. “That’s roughly the turn-over time of the California-sized bubbles that appear as granulation of the photosphere.” Solar granulation is what excites the Sun’s internal sound waves.
Solar sound waves are mostly trapped inside our star — they refract away from the Sun’s hot core and reflect back and forth between different parts of the photosphere. . . . By monitoring the Sun’s vibrating surface, helioseismologists can probe the stellar interior in much the same way that geologists use seismic waves from earthquakes to probe the inside of our planet.
Intense magnetic fields around sunspots affect the transit times of sound waves bouncing from one side of the Sun to the other, variations that the MDI can detect and transform to reveal magnetic condensations (i.e, sunspots) on the hidden side of the Sun. Called “helioseismic holography,” this technique can produce actual images of the far side of our star.
For more info on how all this works, including QuickTime movies of the process, see here and here.
Possibly a dumb question here, but since the Earth goes all the way around the Sun each year, how can there be a “far side”? Does it turn like the Moon, always keeping the same face toward us? I didn’t think stars did that.
Not a dumb question at all, Anne. Actually, the sun does “turn like the moon” in a much more exact sense than you may have meant — it rotates on its axis slightly less than once a month (every 27 days or so).
However, since this rate of rotation is MUCH faster than the cycle time of the earth around the sun (i.e., once a year), obviously the sun does not, like the moon, keep one side perpetually facing the earth.
So the “far side of the sun” is not a side of the sun we wouldn’t ordinarily ever see. It’s just the side of the sun that happens to be facing away from us at the moment. What’s changed is that scientists can now “see through” the sun to the side that’s turned away from us at the moment.
One benefit of this is that scientists can now see explosive areas of solar activity as much as a week before they come into view. Solar gas ejections from these storms can disrupt communications systems and power grids, so being able to predict them could be important.
“…scientists can now `see through’ the sun to the side that’s turned away from us at the moment.”
Science aside, what a glorious picture that phrase brings to mind; all light and fire.
Many thanks for your explanation.