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The Sciences

Why is the Sun spinning like a pinwheel?


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First the Sun develops a big hole, and now it's flipping end over end. But once again, not to worry — all is well.


An animation of images taken by NASA's Solar Dynamics Observatory on July 6, 2016 in extreme ultraviolet wavelengths appears to show the Sun spinning like a pinwheel. (Source: NASA) A few days ago, I posted a video showing a gigantic hole in the Sun's atmosphere. Now, NASA has published an animation showing the Sun spinning end over end like a pinwheel. What's going on? For a detailed explanation of the hole in the Sun, go here: What's up with that huge dark hole in the Sun? And now, what's up with the pinwheeling Sun?  In the animation at the top of this post, the Sun does seem to be flipping end over end. But that's not because it has suddenly gone berserk. The pinwheel effect is the result of commands that NASA gave to the Solar Dynamics Observatory spacecraft, which captured the imagery — commands that deliberately caused SDO to do a 360-degree roll on its axis over the course of seven hours. So from our perspective watching the action on our screens, it looks like the Sun is pinwheeling. Why would NASA do such a thing? Here's the agency's explanation:

This maneuver happens twice a year to help SDO’s Helioseismic and Magnetic Imager, or HMI, instrument take precise measurements of the solar limb, the outer edge of the sun as seen by SDO. Were the sun perfectly spherical, this would be a much simpler task. But the solar surface is dynamic, leading to occasional distortions. This makes it hard for HMI to find the sun’s edge when it’s perfectly still. HMI’s biannual roll lets each part of the camera look at the entire perimeter of the sun, helping it map the sun’s shape much more precisely.

Why is it is important to map the Sun's shape with precision? It has to do with violent activity on the Sun, including sudden flares of radiation and gargantuan eruptions of solar material. These explosive events can pose significant risks to us here on Earth by damaging satellites, electrical grids, and communication systems. And the more warning we can get, the better prepared we can be. With that in mind, scientists use the HMI instrument to see how changes in the Sun's limb are related to variations in solar activity over time. Aside from providing insight into basic solar physics, such knowledge may help lead to better forecasts of potentially damaging flares and solar eruptions. So by commanding the SDO spacecraft to pinwheel twice a year — thereby giving us the impression that the Sun is flipping end over end — scientists are doing much more than producing an intriguing animated GIF.

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