Line profile asymmetries in the transition region: models and observations

Asymmetries in spectral line profiles provide a wealth of information on the properties of the emitting plasma along the line-of-sight. Asymmetries can be produced by the superposition of profiles with different line-of-sight velocities and/or widths resulting from the variation of the velocity and/or temperature from emission sources along the line of sight. Spectral line asymmetries from synthetic transition region and coronal lines constructed from realistic 3D models appear similar to those observed with Hinode/EIS. The simulations span the upper layer of the convection zone to the lower corona and include horizontal magnetic flux emergence. We use the state of the art Bifrost code to solve the full MHD equations with non-grey and non-LTE radiative transfer and thermal conduction along the magnetic field line. Here, we perform a detailed study of the various physical, dynamical and observational processes that can lead to spectral line asymmetries at the transition region footpoints of loops in 3D radiative MHD simulations of the solar atmosphere and compare these with observations. Our models show that the spectral asymmetries are a sensitive measure of the velocity gradient with height in the transition region of coronal loops. In our models the TR shows a large gradient of velocity that increases with height: this occurs as a natural consequence of ubiquitous, episodic heating at low heights in the model atmosphere.