Bridging the gap


Dr. Gherardo Valori

We’re well into the new academic year now. Our latest PhD student – Jamie Ryan – has joined us in the solar group to work on sunquakes, which are bursts of seismic activity that are triggered by currently little understood mechanisms. And we have a new senior researcher in the group – Dr. Gherardo Valori. Gherardo will be working with me and Lidia van Driel-Gesztelyi on a Leverhulme Trust funded project called “Solar magnetic activity: bridging the gap between observation and theory”. The project aims to bring together the work Lidia and I do using solar observations to probe the evolution of magnetic fields in the Sun’s atmosphere with Gherardo’s approach of using a computational model of these magnetic fields. By closing the gap between these two approaches we will be able to learn more about how energy is built up and then released to produce events like coronal mass ejections and solar flares.

Solar flares are flashes of radiation across the electromagnetic spectrum and coronal mass ejections blast vast amounts of electrically charged gas and magnetic field into the Solar System. Both phenomena drive space weather here at Earth and so we’re interested in understanding them from a fundamental physical point of view but also to understand more about how living near a dynamic star affects us.

So, over the next four years, I’ll be using observations of the Sun’s atmosphere made in X-ray and ultraviolet light to approach this problem. Gherardo on the other hand will be using the magnetic field measured at the Sun’s visible surface to create what is known as a nonlinear force-free field extrapolation. This allows him to reconstruct the full three-dimensional magnetic field in the region of the atmosphere we are studying. From this reconstruction we can probe how the magnetic field is storing energy and how (and perhaps when) it is likely to be released. Having a three-dimensional reconstruction of the magnetic field also means we can identify the locations where the magnetic field is stressed and see where the energy is stored. These sites are where solar flares and coronal mass ejections form, so if our work goes well, we should see our two approaches converging on what we find. Gherardo’s technique allows the amount of energy that can be released from the magnetic field at these sites to be calculated and by following the magnetic field over time allows us to probe why the energy is released when it is. It’s going to be a busy time.



Solar Max

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.


Sun at min and max     Sun at min and max