Coronal Mystery

NASA has just sent out the Parker Solar Probe in order to study the atmosphere of the Sun, called the corona. Considered one of the great unanswered questions of solar physics, the mystery of coronal temperatures has puzzled scientists for decades. Temperatures should drop the farther away from a heat source, yet the corona, is much hotter than the sun's surface. Known as the photosphere, the surface is around 5,500° Celsius, while coronal temperatures can reach several million degrees - quite a dramatic jump. The corona is a diffuse cloud of plasma, which is molded by electromagnetic fields into huge loops, noticeable during a solar eclipse.

The German scientist Walter Grotrian first noticed spectral lines from the corona during a total eclipse in 1869. At first, these lines were thought to be a new element which was called coronium, but Grotrian figured out that they were actually caused by ions of iron; which have been stripped of electrons due to the intense heat. The higher the heat, the more electrons are removed, which allows scientists to use spectral lines to determine coronal temperatures.

Magnetic fields are generally thought to be responsible for the high temperatures, but just how they achieve this is the subject of much research. Intense magnetic fields at the photosphere impel solar weather phenomena, including sunspots, mass ejections, and coronal loops. One hypothesis has to do with solar tornados; swirling gas vortices common in the chromosphere. The strong magnetic impulses that form these vortices exert intense pressure on the gas in the atmosphere, propelling them in spiral trajectories up into the corona. The accelerations achieved could be high enough to explain the anomalous temperatures.

Another potential explanation is sound. The photosphere is thought to be incredibly loud, due to seething columns of hot fluids. Some of this sound rises into the corona, where dissipative processes could convert this energy into heat. Due to the intensity of the audio energy, large amounts of heat can be created. An additional explanation is presented by interactions between convective motions in the corona with the magnetic structure of the photosphere. These are loops of magnetic flux of all sizes that are twisted and pushed into writhing tangles by the boiling gases, creating strong electric currents along field lines. Large amounts of heat can be released when the magnetic field lines "untangle" themselves into simpler arrangements. Similar to electrical wires short-circuiting, this process is known in solar astronomy as reconnection.