Coaxing The Most From Sunlight

Spectacular flames of hydrogen gas called prominences hover off the sun's limb in this photo taken yesterday through an H-alpha filter. Growing sunspot group 1393 is at left. This morning the group was just visible to the naked eye through a safe solar filter. Credit: John Chumack

John Chumack’s excellent photo inspired me to think of how many different ways we’re able to study the sun nowadays – visible light, X-rays, ultraviolet, radio, hydrogen alpha, coronagraph – through special filters and all manner of telescopes. It’s like having a full array of designer sunglasses at our disposal. “I think I’ll wear the pink ones today.”

All of our colored glasses essentially do the same thing. They block all forms of light except the desired one. In that narrow sliver we’re able to examine a facet of the sun that would otherwise be lost to glare or simply invisible to the eye. Our instruments extend our eyes into every part of the electromagnetic spectrum, every color of light.

I’ve posted lots of images of the sun taken in UV light, visible light and the rest. Last week we peered into the sun’s chromosphere, a layer of atmosphere above the blinding white surface we see with your eyes, with a hydrogen alpha filter. Astronomers have ways of coaxing more information from the sun. Using a spectrograph, an instrument that finely dissects the light emitted by atoms to tell us their speed and how they’re affected by powerful magnetic fields around sunspots.

A Doppler "photograph" of the sun taken January 9. The darker area is rotating toward us, the lighter half is rotating away. Credit: NASA/SDO/HMI

If you’ve ever heard the pitch of an ambulance siren change as it approaches and then speeds by, you’re familiar with the Doppler Effect. Sound waves bunch up as the vehicle approaches causing the frequency or pitch of the sound to rise higher then drops when the vehicle departs. The very same thing happens with light. As something approaches an observer, the “pitch” of its light rises and becomes bluer. Objects receding in the distance are redder. None of this is noticeable with the eye because at speeds we’re accustomed to here on Earth, the effect is much too small for our eyes to detect.

The ghostly looking sphere above is a Dopplergram of the sun taken with the Helioseismic and Magnetic Imager on the Solar Dynamics Observatory this afternoon. The left half of the sun is darker indicating it’s rotating toward us at a maximum speed of about 5,000 miles per hour. That may sound fast but because the sun’s so huge, it takes nearly a month to make one complete spin. The brighter half on the right is rotating away.

Look closely at the texture. Hot gases well up from the sun’s interior in narrow plumes similar to bubbles of hot air bursting in your overheated microwave oatmeal. The gas reaches the surface and spreads out horizontally from left to right. The texture of stippled dark and light patches tells us gas is moving both up and down. The center of the sun looks smooth because the gas is moving neither toward or away from us but rather horizontally across our line of sight.

The sun's magnetism is revealed in this magnetogram taken this afternoon by SDO. Credit: NASA/SDO

Astronomers can even see the strong magnetism that’s concentrated within the dark groups of sunspots like the one pictured in Chumack’s photo.

Sunspots are regions on the sun where powerful magnetic fields (magnetism) erupt from below onto the sun’s surface and insulate that area from the hot gas currents boiling all around. This insulation cools the region by several thousand degrees which is why sunspots look black compared to the rest of the sun. The blacker the spot, the cooler it is.

Iron filings reveal the invisible magnetic field centered on a bar magnet's north and south poles.

Most sunspot groups have a simple structure like an ordinary magnet with a north and a south pole. In the magnetogram (top), the white patches represent the south pole of each group and the dark patches the north. The grey area is where there is little to no magnetic field.

Studying the sun’s magnetic personality, astronomers have learned that the south pole in a spot group points inward toward the center, while the north points outward. That’s not all our sun is up to. Matter of fact, it hums a little tune every day. No kidding. Stay tuned for more in part 2 tomorrow.

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