Credit: NASA; Solar Dynamics Observatory
The Dark Cool Heart of the Sun
The crown of the Sun's atmosphere, its outermost region, is called the corona. The corona is about a thousand times hotter than the surface of the sun, with coronal temperatures topping millions of degrees, and streams from just above the visible surface of the Sun extend outward into space. The corona is very dynamic and changes with the solar sunspot cycle, and on much shorter timescales too. Understanding the solar corona is important, since it's the one part of the Sun that, in a very real sense, materially touches the earth. Eruptions and explosions powered by the tangling of the Sun's magnetic field are responsible for the corona's extreme temperature, and this magnetic energy can expel portions of the corona into space during coronal mass ejection events. Cooler, lower density regions called coronal holes sometimes appear in the corona. An impressively large, heart-shaped one seen in the image of the Sun above was recently photographed by the NASA's Solar Dynamics Observatory on September 13, 2025 in extreme ultraviolet light. The extreme ultraviolet highlights the hot, million-degree gas of the corona, while the optically bright surface of the sun appears dark in this image, since it's not hot enough to emit extreme ultraviolet radiation. At the current time, the Sun is just past the peak of its 11-year solar activity cycle, so, over the next 5 years the number of sunspots and intensity of solar storms battering the earth will be declining.
Published: September 15, 2025
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Page Author: Dr. Michael F. Corcoran
Last modified Monday, 22-Sep-2025 13:35:02 EDT