Thank you for the slack. Yes, the 'corona', not photosphere.
My 'bad'.
No problem. You are very polite. The glib answer (according to a google on "why is the corona hot". I am not a specialist on this either) is that we don't yet know. There are hypotheses to do with magnetic energy being emitted from the sun into the corona where it converts to heat but these have not as yet been tested.