The corona is the outer atmosphere of the sun.  It's temperature is
very high (millions of Kelvin) and matter in it exists as a thin gas
of ions—mainly electrons and protons.  The chromosphere is also
made of ionized hydrogen gas.  However its temperature is low enough
(and density high enough) that electrons and protons "recombine" to
momentarily form neutral hydrogen.  The subsequent relaxation of
electrons from higher- to lower-energy orbitals causes light emission
characterized by the beautiful crimson color of the chromosphere.