Alfvén shocks: Polarimetric detections in the umbral chromosphere

The formation of shocks within the solar atmosphere remains one of the few observable signatures of energy dissipation arising from the plethora of MHD waves generated close to the solar surface. Active region observations offer exceptional views of wave behaviour and its impact on the surrounding atmosphere. The stratified plasma gradients through the lower solar atmosphere allow for the potential formation of many theorised shock phenomena. In this study, we observe a large sunspot using spectro-polarimetry of the chromospheric Ca II 8542 Å line and space-based observations of the photosphere, to examine fluctuations in plasma parameters manifesting in the aftermath of powerful shock events. Modern inversion techniques are employed to uncover perturbations in temperature, velocities and magnetic fields occurring at a range of chromospheric optical depths synonymous with shock formation. Classification of these non-linear signatures is carried out by applying slow, Alfvén and fast MHD shock solutions to the Rankine-Hugoniot boundary conditions. Here, we show systematically that the Alfvén shock solution provides the closest match between theory and observations at optical depths consistent with the shock boundary location. This work reveals first-time evidence of chromospheric Alfvén shocks and provide scope for future studies of elusive shocks with next generation facilities such as DKIST, EST, NLST and Solar-C.