The sun comes up every day in the east and goes down in the west, always looking the same; it’s so constant that it has made its way into many idioms as a symbol of immutability. But for those who study the sun, the picture is much more complex. The boiling surface can change in mere minutes, like a pot of water on high heat. Twisted magnetic fields worm their way out through the solar surface, crackling with energy and releasing vast amounts of matter as they erupt. Just above this roiling surface is the sun’s atmosphere, the outermost portion of which is called the corona.
There are two major mysteries that account for much of the research in solar physics. The first is how the corona can reach temperatures in the millions of kelvins (that is, degrees above absolute zero) when the surface below it is only a few thousand kelvins. This has been an open question for over 50 years, and although instruments and observations have made huge strides in that time, a conclusive answer has eluded researchers.
The second involves the solar wind, the constant stream of plasma that bathes every planet, moon, asteroid and comet in the solar system. The corona, like the rest of the sun, is populated by plasma, a high-energy gas whose atoms have had some or all of their electrons ripped away. The volume plasma occupies is thus filled with a soup of positively charged ions and electrons; because of this charge, the sun’s incredibly powerful magnetic fields capture the particles, forcing them to spiral along field lines many times per second (think of the way iron filings outline the field around a bar magnet).
Solar wind is typically broken up into two types, known as fast and slow wind. While this is a vast oversimplification that ignores wind composition and acceleration methods, it is traditional in the field. We know fast wind comes from regions on the sun that have open magnetic fields, but we are less certain of the origins of slow wind. A leading theory posits that slow solar wind originates from magnetic field lines alternating between open (one end connected to the sun, the other to some point in space) and closed (both ends connected to the sun) states. Because plasma follows magnetic field lines, it can escape the sun and become part of the solar wind only when a field line is open; the mechanism that describes this alternating open/closed process is known as interchange reconnection.
My research addresses aspects of both mysteries. I observed small structures in the corona called null-point topologies, which have small closed coronal loops surrounded by long open field lines that extend into the solar wind. They look very much like the famous onion domes atop Russian cathedrals. These null-point topologies are host to downpours of coronal rain—packets of plasma that cool dramatically in a matter of minutes and then fall down to the solar surface.
The process is very similar to water rain on Earth, if the water were strongly magnetized. These blobs of plasma form and fall down the magnetic field lines of the null-point dome. The rain forms in two ways, and both of these methods tell us something important about the sun.
The first place we observe rain is on the closed loops under the dome: the rain blob appears high on one side of the loop and then slides down, repeatedly. This can be explained by a process known as thermal nonequilibrium, or TNE. TNE is a well-established phenomenon that depends on strong heating in a restricted area near the bottom of a loop. At the top of the loop, there is no heating, so the plasma cools and falls.
The loops I observed showed evidence of this process, and the fact that they were so short means that coronal heating must be occurring lower in the atmosphere than previously thought. This narrows the range of places we should be searching for the elusive cause of the corona’s intense temperature.
The second place where I observed rain—the most rain, in fact—was along the pointed tips of the domes and the long open field lines above them. TNE could not explain this, but what could? The answer is interchange reconnection: the rain I saw is the result of newly opened field lines readjusting to their new state. This makes null-point topologies important laboratories for studying how the slow solar wind forms. The close proximity of closed and open magnetic fields creates a perfect location for interchange reconnection, so plenty of slow solar wind should be observable coming from these structures.
The sun may be variable after all, but the search for an understanding of it is not—and with missions such as the Parker Solar Probe and the upcoming Solar Orbiter, we can use data from null-point topologies to help uncover the rest of the sun’s mysteries.