The outermost layer of the solar atmosphere, known as the corona, is more than 150 times hotter than its surface. This is scientifically impossible, because the only known source of energy on the Sun is thermonuclear reactions at its centre, yet there are irrefutable measurements to prove this indeed happens. Despite extensive efforts, solar physicists are still baffled by this 7-decade old discovery and cannot provide a concrete explanation.
Let’s illustrate this conundrum by the analogy of a burning candle. Close to the flame you can feel the heat. Further away, the amount of heat you feel diminishes. We have learnt that the source of heat is the hottest and the further away we are the colder it should get. This is a fundamental law of Thermodynamics and it should apply to all physical systems, including the Sun. The core of the Sun is at temperatures of 15 – 7 million °C, while its surface, what we see as a yellow ball and known as the photosphere, has a temperature of 5430°C. The drastic drop in temperatures is what we expect since the photosphere is about 700 thousand km from the core. The layers above the photosphere are collectively referred to as the solar atmosphere. Moving further away from the heat source (i.e. core), past the photosphere to the corona, the temperatures surprisingly begins to rise again to about an ambient million °C. There was quite a stir when this anomaly was first discovered in 1939 by Walter Grotrian and has since been known as the “Coronal Heating Problem”.
Spectral analysis is a useful tool to get information on temperature, density, composition, electric and magnetic fields etc. of distant objects such as the Sun and other stars. While studying the spectrum of the corona, several spectral lines were discovered which did not match any known elements on Earth. At first, it was thought that a new element called “Coronium” was discovered. This was surprising because no such element was predicted by the periodic table. However, laboratory experiments established that the observed lines were formed by known elements at very high stages of ionization. Level of ionization is an indicator of temperature because thermal energy is needed to strip away the electrons held by each atomic nucleus and ionize the gas. At higher temperatures, elements display increasing stages of ionization. This was the first indication that the coronal gas is at temperatures greater than a million °C.
The solar atmosphere is not an exception to the laws of physics. Which means there has to be either a yet unseen source of energy within the corona or new means by which energy is transferred to the corona. Various heating mechanisms have been proposed and by far the best-regarded candidates are:
1) Wave heating: Waves can cause heating e.g., microwaves heating food in your oven. Scientists speculate that the corona is heated by waves generated in the photosphere. However, most such waves are either reflected or refracted by the strong changes in density between the surface and the many layers of the solar atmosphere. Only “Alfvén” waves can efficiently carry energy from the photosphere to the corona without being deflected. The problem is, these Alfvén waves are very hard to detect. Recent observations from high-resolution telescopes onboard the “Solar Dynamics Observatory” satellite have shown the first evidence of Alfvén waves in the corona. Nevertheless, we still do not know if these waves can transfer enough energy to explain a million degrees temperature discrepancy.
2) Magnetic Reconnection: The Sun is a turbulent and violent environment with many sources of magnetic fields that are in constant motion. When oppositely charged magnetic field lines come close to each other, they attract and explosively “reconnect” releasing a lot of energy. This process is very similar to a lightning strike. It is suspected that small-scale reconnection events are widespread and occur frequently on the Sun. They are called nanoflares and are a million times stronger than a lightning strike. Such weak events are difficult to observe and we only have indirect evidence of their presence obtained from the recently launched high-resolution spectrometer called the “Interface Region Imaging Spectrograph”. Due to the limited field-of-view of this instrument, exhaustive studies on these events have not yet been performed.
Whether one candidate hypothesis can solve the problem or a combination of several mechanisms is the solution is yet to be seen. Newer and better, instruments, an image analyzing techniques, and computers are being used to reveal the secrets of the Sun.