Surges and Si IV Bursts in the Solar Atmosphere: Understanding IRIS and SST Observations through RMHD Experiments

Surges often appear as a result of the emergence of magnetized plasma from the solar interior. Traditionally, they are observed in chromospheric lines such as H alpha 6563 angstrom and Ca II 8542 angstrom. However, whether there is a response to the surge appearance and evolution in the Si IV lines or, in fact, in many other transition region lines has not been studied. In this paper, we analyze a simultaneous episode of an Ha surge and a Si IV burst that occurred on 2016 September 03 in active region AR 12585. To that end, we use coordinated observations from the Interface Region Imaging Spectrograph and the Swedish 1-m Solar Telescope. For the first time, we report emission of Si IV within the surge, finding profiles that are brighter and broader than the average. Furthermore, the brightest Si IV patches within the domain of the surge are located mainly near its footpoints. To understand the relation between the surges and the emission in transition region lines like Si IV, we have carried out 2.5D radiative MHD (RMHD) experiments of magnetic flux emergence episodes using the Bifrost code and including the nonequilibrium ionization of silicon. Through spectral synthesis, we explain several features of the observations. We show that the presence of Si IV emission patches within the surge, their location near the surge footpoints and various observed spectral features are a natural consequence of the emergence of magnetized plasma from the interior to the atmosphere and the ensuing reconnection processes.