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Alan Stahler: The eclipse: What happened?

Close-up of magnetic coronal loops near the base of the corona. Earth would fit thirty times across the base of one loop.
Courtesy of NASA |

Our homes are filled with technologies that would have made 19th century scientists drool — microwave ovens, GPS receivers in phones, ballpoint pens … and AM radios.

Electricity creates radio waves. Turn on an AM radio to hear the crackle of lightning from an approaching storm before you hear the thunder.

Even AM broadcasts are informative.



Tune in a weak AM station at night, and listen to it go in and out. It’s “twinkling,” not unlike the stars.

Leave the dial set there. Chances are, you won’t be able to hear the station at all the following day until night falls again. AM signals propagate farther at night. Most AM radio stations must, by law, reduce their transmitter power when the sun goes down, lest they step on each other.




The “twinkling” of AM radio is due to Earth’s upper atmosphere.

The least understood part of the sun is visible only during total eclipse: the corona, or “crown” of the sun. The corona is hot — not the mere ten-thousand-or-so degrees of the visible “surface” of the sun that lights the Earth, but millions of degrees Fahrenheit.

Put a pot on the stove and it won’t get hotter than the burner. How could the corona be way hotter than the sun below, from which it gets its heat? Something must be focusing energy into the corona. But no one knows how.

Astronomers have invented telescopes — coronagraphs — to create an artificial eclipse to study the corona. But according to Jay Pasachoff of Williams College, even the best coronagraph blocks out the lowest part of the corona — right where energy is likely focused. We can only see this part of the sun during total eclipse.

As a toaster turns on, the filaments first glow red, then orange, as they get hotter. If they could get hotter yet, they’d glow yellow, then white.

Beyond white-hot, the filaments would emit ultraviolet; and hotter than that, they’d emit x-rays.

The sun’s corona is x-ray hot. Spewing extreme ultraviolet and x-rays through the solar system, the sun sterilizes the airless surfaces of Mercury, Mars and the moon. It would sterilize the surface of Earth, too, but for our atmosphere.

Atoms are composed of a positively-charged core — the nucleus — surrounded by a cloud of negatively-charged electrons. When electrons absorb x-rays and EUV, they get hot, bounce around, and escape from the atom. The dangerous radiation is thus absorbed, but it’s busted up atoms of air.

Missing an electron, the atom becomes a positively-charged ion. The region of the atmosphere hosting busted-up atoms is the ionosphere.

The mix of electrons and ions in the ionosphere is a plasma. Josh Semeter of Boston University describes a plasma as a sort of “gaseous metal.”

Metals can both reflect and absorb radio waves. In the ionosphere, how much a radio wave is reflected or absorbed depends on how many electrons and ions are in the plasma, and on the wavelength — the “color” — of the radio wave.

Semeter observes radio waves, transmitted by GPS satellites, as they travel through the ionosphere. On Monday, he would be watching, with radio, day become night, and night once more become day, all in a matter of minutes.

Focusing on the lowest region of the ionosphere — the region that prevents AM radio waves from getting through during the day, but allows them to pass through and reflect back to Earth at night — Morris Cohen, of Georgia Tech, would be watching how the crackle of lightning bolts, around the world, bounced around in the on-again, off-again ionosphere.

Monday was for collecting data. The coming months and years will be for figuring out what to make of this data. Stay tuned.

Al Stahler enjoys sharing nature with students of all ages. His science stories can be heard on radio station KVMR (89.5 FM), and he may be reached at stahler@kvmr.org


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