Well...it really isn't. The core still takes the prize with aa temperature near 14 million degrees. No other region of the Sun outside the core tops this. You may have heard that the solar corona has a temperature near 1 million degrees, while its surface temperature is only about 5700 degrees K. The surface, however, is definitely cooler than the core so there is no problem there. The problem that bedevils solar physicists is to understand how the corona can sit atop a much cooler surface and still be a million degrees hotter. We think that the answer is that the corona is threaded with magnetic fields that 'reconnect' and release lots of energy locally. So in reality, the magnetic field of the sun, and perhaps shock waves traveling up from the solar surface into the corona, are the missing heating mechanisms.