What type of solar radiation is most responsible for ionization in the outer atmosphere?

Ultraviolet (UV) radiation from the sun carries enough electromagnetic energy to knock electrons loose from gas atoms in the upper atmosphere. This continuous bombardment of UV photons is the primary driver of the ionization that creates the ionosphere. While the sun does emit infrared and microwave radiation, these are lower-energy wavelengths that simply heat the atmosphere rather than ionizing it. Solar particles also affect the ionosphere during solar storms, but extreme ultraviolet light is the most responsible, consistent day-to-day source of ionization.