Formation of the helium extreme-UV resonance lines

Context. While classical models successfully reproduce intensities of many transition region lines, they predict helium extreme-UV (EUV) line intensities roughly an order of magnitude lower than the observed value. Aims. Our aim is to determine the relevant formation mechanism(s) of the helium EUV resonance lines capable of explaining the high intensities under quiet Sun conditions. Methods. We synthesised and studied the emergent spectra from a 3D radiation-magnetohydrodynamics simulation model. The effects of coronal illumination and non-equilibrium ionisation of hydrogen and helium are included self-consistently in the numerical simulation. Results. Radiative transfer calculations result in helium EUV line intensities that are an order of magnitude larger than the intensities calculated under the classical assumptions. The enhanced intensity of He