Abstract
Abstract The formation of shocks within the solar atmosphere remains one of the few observable signatures of energy dissipation arising from the plethora of magnetohydrodynamic waves generated close to the solar surface. Active region observations offer exceptional views of wave behavior and its impact on the surrounding atmosphere. The stratified plasma gradients present in the lower solar atmosphere allow for the potential formation of many theorized shock phenomena. In this study, using chromospheric Ca ii λ8542 line spectropolarimetric data of a large sunspot, we examine fluctuations in the plasma parameters in the aftermath of powerful shock events that demonstrate polarimetric reversals during their evolution. Modern inversion techniques are employed to uncover perturbations in the temperatures, line-of-sight velocities, and vector magnetic fields occurring across a range of optical depths synonymous with the shock formation. Classification of these nonlinear signatures is carried out by comparing the observationally derived slow, fast, and Alfvén shock solutions with the theoretical Rankine–Hugoniot relations. Employing over 200,000 independent measurements, we reveal that the Alfvén (intermediate) shock solution provides the closest match between theory and observations at optical depths of log 10 τ = − 4 , consistent with a geometric height at the boundary between the upper photosphere and lower chromosphere. This work uncovers first-time evidence of the manifestation of chromospheric intermediate shocks in sunspot umbrae, providing a new method for the potential thermalization of wave energy in a range of magnetic structures, including pores, magnetic flux ropes, and magnetic bright points.
Highlights
The desire to understand wave energy transportation, and subsequent dissipation in the solar atmosphere, is a major driver behind much of solar physics research
We have presented high temporal resolution spectropolarimetric Ca II 8542 Aobservations, captured by the Interferometric BIdimensional Spectrometer (IBIS) instrument at the Dunn Solar Telescope
The largest fluctuations occur at optical depths of log10 τ = −4, which is consistent with a geometric height of approximately 625 km, close to the boundary between the upper photosphere and the lower chromosphere
Summary
The desire to understand wave energy transportation, and subsequent dissipation in the solar atmosphere, is a major driver behind much of solar physics research. Ubiquitous detection inside highly magnetic sunspot umbrae (Beckers & Tallant 1969) This phenomenon is commonly referred to as umbral flashes (UFs), and are a consequence of the steepening of slow magnetoacoustic waves as they traverse the rapid density stratification of the umbral atmosphere (de la Cruz Rodrıguez et al 2013; Henriques et al 2017). Arber et al (2016) utilized 1.5D numerical models to show that Pedersen resistivity is able to directly dissipate highfrequency Alfven waves, while Snow et al (2018) revealed theoretical evidence for how vortex motion applied to magnetic flux tubes is able to drive intermediate shocks that propagate upward with speeds of approximately 50 km s−1, transporting energy and momentum into the upper layers of the solar atmosphere. Solar Telescope (DST), in conjunction with modern inversion techniques and analytical theory, to provide unique insights into the dynamic plasma fluctuations associated with the manifestation of intermediate shock fronts in the Sun’s magnetic atmosphere
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