Surges are ubiquitous cool ejections in the solar atmosphere that often appear associated with other interesting phenomena such as UV bursts or coronal jets. Recent observations from the Interface Region Imaging Spectrograph show that surges, although traditionally related to chromospheric lines such as H I 6563 Å or Ca II 8542 Å, can exhibit enhanced emission in Si IV and, as a consequence, lead to spectral profiles that are brighter than for the average transition region. However, a theoretical explanation to understand that behaviour was missing. In this talk, we analyse the response of the transition region to surge phenomena. To that end, we carried out two 2.5D radiative-MHD numerical experiments using the Bifrost code and including the non-equilibrium ionisation of silicon and oxygen. In the experiments, a cool and dense surge is obtained as a consequence of magnetic flux emergence. We find that non-equilibrium is key to understand why surges show enhanced emissivity in transition region lines. Studying the properties of emitting surge plasma, we point out the important role of the optically thin radiative cooling and heat conduction for the non-equilibrium ionisation. Furthermore, through the calculation of synthetic spectra of O IV, we provide predictions for future observations.
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