How does an ion tail form on a comet?

An ion tail, also known as a plasma tail or Type I tail, forms when volatile gases in a comet's coma are ionized by ultraviolet photons from the Sun or through charge exchange with the solar wind.12

Once these molecules and free radicals become ionized, they are captured by the Sun's magnetic field and accelerated away from the Sun at high velocities by the solar wind. Because the solar wind velocity is significantly higher than the comet's orbital velocity, these tails point almost directly away from the Sun. These structures appear as long, narrow, and straight trails. While they often contain CO+ molecules that cause them to glow blue, the primary ion present is H2O+, which does not emit light at visible wavelengths.12