Solar and Stellar Atmospheres
The solar corona is the outer atmosphere of the Sun and is hotter than the suface of the Sun (the photosphere). To be more specific, the corona is over 1,000,000 Kelvin (over 1.8 million degrees fahrenheit), while the photosphere is around 6,000 K (10 thousand degrees fahrenheit). The solar corona is less dense than the solar photosphere and thus much dimmer (the photosphere is 1 million times brighter than the corona). So the unaided human eye (the human eye without the aid of external optics like telescopes or binoculars), just observes the bright solar disc in visible light (never stare at the Sun without eclipse glases or other special Sun viewing equipment!!). Only when the light from the solar disc is blocked out, can the corona be easily viewed by the naked eye in visible light. This happens during solar eclipses (see video from the August 21, 2017 solar eclipse to the left).