If the Sun's outer atmosphere - corona - is so hot, why does it always look so cool?
The Sun's visible surface is 'only' 10,000 degrees Fahrenheit, but as you move outward the temperature shoots up to millions of degrees. It's like a campfire that feels hotter the farther away you stand. That defies common sense, but so do dogs named Checkers and Esther Williams swimming pools so let's talk about coronal loops.
Coronal loops are shaped like an upside-down U and show where magnetic field lines are funneling solar gases or plasma. The best photos of the Sun suggest that these loops are a constant width, like strands of rope. However, a new study presented at a meeting of the American Astronomical Society Solar Physics Division in Bozeman, Montana shows that this is an optical illusion; the loops are actually tapered, wider at the top and narrower at the ends. This finding has important implications for coronal heating.
"You need less energy to heat the corona if the loops have a tapered geometry, which is exactly what we found," says lead author Henry Winter of the Harvard-Smithsonian Center for Astrophysics (CfA).
Winter and his colleagues constructed a computer model of a tapered loop using basic physics. Then they processed their model to show how it would look when photographed by instruments like the High-resolution Coronal Imager (Hi-C) or the Solar Dynamics Observatory's Atmospheric Imaging Assembly (AIA).
They found that even the best available images wouldn't have the resolution to show the loop's true structure. As a result, a tapered loop would appear tubular even though it wasn't.
Photo of the Sun's edge, taken with the Solar Dynamics Observatory Atmospheric Imaging Assembly, shows coronal loops in a variety of sizes. Although the loops appear to have a constant width, like strands of rope, new work suggests that this is an optical illusion. The loops are actually tapered, wider at the top and narrower at the bottom. Credit: NASA/SDO
"In science we always compare theory to reality. But if your view of reality is incorrect, your theory will be wrong too. What we thought we saw could be just an effect of the instrument," explains Winter.
Historically, as we have gotten better and better photos of coronal loops, they have revealed more and more structure. What first appeared to be a single loop turned out to be made of many smaller strands. The team's work shows that better instruments with higher resolution are still needed to reveal the true shape and structure of the loops.
"Coronal loops are like Russian nesting dolls. We keep pulling them apart but we haven't gotten to the smallest one yet," says Winter.
Winter's co-authors are Chester Curme of Boston University, Katharine Reeves (also of CfA) and Petrus Martens of Montana State University.