Well, solar max is here. The Sun has reached the height of its 11-year activity cycle. The peak is important for us because it means solar flares and coronal mass ejections are more frequent (and they create stormy space weather), but it also means that the Sun becomes much more visually interesting. The activity cycle is driven by the Sun’s evolving magnetic field and this most obviously manifests itself at the surface of the Sun as dark and relatively cool features known as sunspots. But in the atmosphere things get much more interesting.
Sunspots are sources of strong magnetic field but they are two a 2D slice of what is a 3D structure. The magnetic field extends from the sunspots up into the atmosphere. The magnetic field extends into the Sun too, but its harder to detect there. In the atmosphere, the magnetic field reveals itself because it traps the million-degree electrically charged gas that comprises the outer layer of the Sun. And this hot gas glows in ultra-violet and X-ray light, illuminating the magnetic structures. The consequence is that we see glowing giant arches rising above pairs of sunspots – analogous to the shapes that iron filings take when scattered around a bar magnetic.
My colleagues at the Naval Research Laboratory (NRL) have made some beautiful images of the Sun showing just how different the magnetised atmosphere looks at the minimum and the maximum phases of the solar cycle. The images are made from data taken by the EIS telescope on the Japanese Hinode satellite. The solar max sections show how the Sun looks in ultraviolet light now, the solar min sections show how the Sun looked a few years ago. You can easily see that at solar max the Sun’s atmosphere is full of magnetic structures in stark contrast to solar minimum. NRL has a long history in solar physics having made their first observations using rockets in the late 1940s. They are still very active in solar physics today. Thanks to Harry Warren, Ignacio Ugarte-Urra and Guillermo Stenborg at NRL for making these stunning images.