Impact of opacity effects on chromospheric oscillations inferred from NLTE inversions

Chromospheric inferences rely on the interpretation of spectral lines that are formed under Non-Local Thermodynamic Equilibrium (NLTE) conditions. In the presence of oscillations, changes in the opacity impact the response height of the spectral lines and hinder the determination of the real properties of the fluctuations. We aim to explore the relationship between the chromospheric oscillations inferred by NLTE inversion codes and the waves' intrinsic fluctuations in velocity and temperature. Numerical simulations of wave propagation in an umbra have been computed with the code MANCHA. The NLTE synthesis and inversion code NICOLE has been used to compute spectropolarimetric Ca II 8542 Å line profiles for the models obtained from the simulations. The synthetic profiles have been inverted and the inferences from the inversions have been compared with the known simulated atmospheres. NLTE inversions of the Ca II 8542 Å line capture low-frequency oscillations, including those in the main band of chromospheric oscillations around 6 mHz. In contrast, waves with frequencies above 9 mHz are poorly characterized. Velocity oscillations at those higher frequencies exhibit clear indications of opacity fluctuations. The main response of the line to velocity fluctuations comes from low chromospheric heights, whereas the response to temperature shows sudden jumps between the high photosphere and the low chromosphere. This strong variation in the heights where the line is sensitive to temperature is revealed as a strong oscillatory power in the inferred fluctuations. Our results validate the use of NLTE inversions to study chromospheric oscillations with frequencies below 9 mHz. The interpretation of higher frequency oscillations and the power of temperature oscillations must be addressed with care since they exhibit signatures of opacity oscillations.