I wanted to write a short note on the use of the terms “sunspots” and “active regions” when discussing structure on the Sun’s surface. To do that, however, requires an aside as to the nature of the Sun’s surface itself. The Sun has a core where hydrogen is built into helium at a temperature of millions of degrees. As we move further away from the core, the temperature drops (as one would expect). At some point, we reach a point where the density has dropped to a point where photons can escape and stream outwards. This radius, at 695,500 km (or 1 solar radius), is deemed the location of the Sun’s surface, which we call the photosphere.
The photosphere is 5800 K in temperature (nearly 10,000 degrees Fahrenheit!), and above it lies the lowest part of the solar atmosphere, called the chromosphere. The chromosphere is at similar temperatures to the photosphere. However, between the chromosphere and the outer solar atmosphere, called the corona, there is a region of intense temperature increase, and the corona reaches temperatures of over one million Kelvin.
It’s important to take a moment to realize the absurdity of this: the region further from the Sun is hotter than the surface of the Sun. Everything we know about hot objects relies on the simple fact that they feel cooler the further we are from them. If you put your hand near an open flame, it is heated. As you pull your hand away, it cools. The light grey curve in the plot below shows what we expect for the temperature of the Sun as we move away from its surface at r = 1 solar radius. However, the maroon line shows on a log-log scale what the previous graph was showing us: there is a thin transition region where temperatures skyrocket to millions of degrees. Although there is a whole field of research looking into the physical processes to explain this, scientists have not yet definitively determined the causes. Crazy stuff!Back to the matter at hand, however. I’ve introduced the terms for the Sun’s surface (the photosphere) and the Sun’s atmosphere (the chromosphere and corona). This distinction is key to understanding the relation between sunspots and active regions on the Sun.
Sunspots are cooler patches of plasma at the photosphere. They are at a lower temperature because magnetic field lines are bursting out of the Sun’s interior and restricting the flow of hot plasma from reaching those parts of the surface. Sunspots come in pairs, since these emerging field lines form loops with the two opposite-polarity footpoints defining the locations of the sunspot pairs. To take observations of the photosphere of the Sun, we need to use visible wavelengths, such as the 4500 Angstrom observation in the middle of the three-image figure below. If we instead look at shorter wavelengths, corresponding to ultraviolet light, we are seeing plasma that has been trapped along these magnetic field loops and has been transported into the corona. Big, bright active regions in ultraviolet observations are usually found directly above the sunspots seen in visible-light observations. Scientists find the shorter wavelength observations much more useful because they reveal so much detail. See for yourself! Note that some active regions lie above sunspots that are small enough (or warm enough) not to appear in the visible-light images.
All in all, the short answer of what the difference is between sunspots and active regions is that there isn’t one in a sense. Both are specific indicators of strong magnetic fields above the surface of the Sun, the kinds of magnetic fields that can release flares and coronal mass ejections. It kind of makes you think twice when you look up at the rather uniform bright yellow spot in the sky.
“Some painters transform the sun into a yellow spot,
others transform a yellow spot into the sun.”