system
Given the following post, respond as if you're commenting on Reddit:
human
How can the corona of the sun be hotter than the surface?
gpt
The top comment is not correct. Nobody knows how the corona is heated. It is an excellent question to ask but no one can answer you for certain.

The comment suggest flares but they have been ruled out. They simply do not provide the energy that is needed where it is needed; they are too occasional and they are limited in number during solar minima yet the corona does not cool when the Sun has no flares, even for extended 100 year minimums. This would be very surprising if flares were responsible as the corona is radiating strongly so requires a constant supply of heat.

One of the interesting things about the coronal heating problem is that it is not a question of there not being enough energy. Despite the corona being over a million degrees K, due to it's low density, there is far more energy stored down below, in the chromosphere and photosphere. We only need about 1kW per square metre of the Sun's surface to heat the corona to it's several million degree temperature. The problem is instead one of dissipation and transport, we can not deposit the energy in the right places in the right amounts to get this power there.

The two favorite candidates right now are reconnection (but again not flares) and waves. For reconnection we are talking extremely small scale reconnection events, sometimes called nano-flares, that could occur all over the transition region (the area between the chromosphere and the corona) or the chromosphere. They would not be delivering power by energetic particles but by MHD modes or perhaps Ohmic heating. They have never been observed but that maybe is not a surprise due to their small size and our instrument resolution - spatial, spectral and in intensity.

The other strong candidate right now is waves, particularly Alfven Waves (although there is no easier way to start an argument than to discuss categorizing waves at a soolar physics session), these could start off as motion of the plasma in the photosphere causing the embedded magnetic field lines to wiggle. This resulting wave can propagate out to the corona. The problem with these are they are difficult to dissipate in the right place.

Modeling problems are present for both of them, it is not possible to model flares that we do observe let alone ones that we have never seen and it is challenging to get waves to deposit their energy in the right place in the right amount. There are also always new ideas being suggested and studied.

Either way, this is definitely a problem worth solving and people will absolutely keep trying.