Why is Sun's atmosphere hotter than its surface? Can magnetic reconnection be the answer to why temperature of corona is so much higher than the surface of sun. Magnetic reconnection is a physical process occurring in highly conducting plasmas in which the magnetic topology is rearranged and magnetic energy is converted to kinetic energy, thermal energy, and particle acceleration. Can this be exactly what is happening in corona due to which a part of magnetic energy is converted to thermal energy which might be the reason why corona is so much hotter than surface of sun.
 A: The most popular theories are Alfven wave heating and magnetic reconnection and nanoflares, but I do not believe that this question is fully resolved yet (as of 2022). The Parker Solar Probe will aim to collect more data by flying closer and closer to the Sun!
A: The problem is actually more likely the other way around: the temperature of the corona is pretty much the natural temperature of the Sun, reflecting its gravitational potential energy (corresponding to about  $10^7 K$). So the question should rather be: why is the temperature in the photosphere so low? There are no complicated theories needed to answer this, as cooling of a gas is always due to inelastic collisions, in this case the inelastic collisions  of high energy ions and electrons with neutral atoms, which turn the high kinetic energy of the plasma into radiation. Below the photosphere the density is too high for atoms to exist (the interior of the sun is just a plasma of bare nuclei and electrons). The photosphere is the region where the density (which decreases outwards) has become low enough for atoms to exist, which then in turn cool the plasma due to inelastic collision. This defines the 'surface' of the Sun with its low temperature (which corresponds actually almost exactly to a cooling factor (electron mass/proton mass). The hot coronal plasma is just likely due to the few plasma particles that make it through the photosphere without being slowed down by inelastic collisions.
So, as this is very much a non-LTE problem (given the huge energy range of about a factor 1000 involved), the attempt to address this with methods of thermodynamics (as is usually done), are destined to yield wrong answers and/or result in just more questions. On the other hand, with a suitably complex non-LTE computational model, it should only be a matter of properly implementing the relevant physical processes, as indicated above, with the the required fine detail such as to reproduce the main features of the solar corona, that is without need for any exotic physical processes.
For more details see my web page regarding coronal heating
