Astrophysicists have been looking for Dark Matter, something invisible but gives extra mass to galaxies since Fritz Zwicky measured the rotation of galaxies in 1930s. And looking for Dark Energy, something invisible in the space between galaxies that leads to gravity pushing those galaxies apart, since Adams Reiss and Saul Perimutter, accurate measured galaxies motions with supernova, in the late 1990s. But what astrophysicist rarely mention is that they have a problem much more visible and much closer to home. It is the Sun itself, and thus blindingly bright, but no less of a problem. Its to do with the transition between the surface of the sun, the photosphere, which a temperature of 5800K, the chromosphere lives above the photosphere, has similar temperature for 1500 Kilometer upwards then suddenly  within  100Km comes the Solar Corona at over million Kelvin. The Temperature of the hydrogen gas was been boosted incredibly quickly within a very short distance.
   
If you think about any object that held together by gravity and heated from within, you'd expect it
to be a fuzzy ball with no surface, which a density and temperature profile that steady reduces with height. The Sun doesn't look like this at all, but then the Sun is made of Plasma and not gas. In a Plasma a varying proportion of the gas is ionised, split into free electrons and positive ions lacking electrons. The equation of motion of Plasma are know, but are not easy to calculate. In a plasma
electric and ion currents my flow, magnetic flux may be trapped, it will emit and absorb UV radiation in both continuous and fixed absorption frequencies. It a complex situation that hasn't been perfectly modelled yet. But one thing is certain something invisible is pumping a huge amount of energy from either the surface or deep inside the sun, into the Solar Corona. The heat pressure of the Corona then push back on the chromosphere and the photosphere resulting in a Surface for the Sun, the photosphere.

In gas and space, only light propagates, but in a plasma many more types of wave travel, including Alfven waves, wave lettered, X,O,L,R, magnetic sonic waves, Upper and Lower Hybrid Oscillations, ion acoustic waves and Langmuir waves. There might also be effects from unknown particles physics. Indeed a any particle with a mass less than a million kelvin over kc^2 or about 100eV, might be constantly created in the hot regions and annihilated or decay in the cold ones.

Naturally between Solar Plasma Physicists and Particle Physics there's quite a different attitude as to the cause of Corona heated. Most Solar Physics expect the heating to be from plasma effects, often Alfven waves, or release or energy from trapped magnetic flux, perhaps even absorption of ultraviolet light from nanoflares. For particle physics this can be a place to test new particles. Many particle physics think they need a low mass particle called an axion to solve a problem with QCD and stop the
neutron from having an electron dipole moment (which isn't observed). Any new particle would lead to extra energy transport in the Sun, and there is quite a lot of work to see if these could heat the solar corona. I myself calculated to see if a sterlie neutrino made deep in the Sun might be the culprit. It would need to decay within a hundred of a millisecond and have a mass in the tens of electron volts, then it would do the job of pumping energy into the corona.

Despite having many solar telescopes and quite a few researchers I don't think we yet have proven solution, good enough models or observations of the Sun to correctly understand Solar Corona heating. So if your very good at understand plasma and not expected a job in fusion, perphaps your bright mind, might want to turn to the mystery of the heating of the Suns Corona.