Astronomy and Astrophysics
Aims: We aim to explore the relationship between the chromospheric oscillations inferred by NLTE inversion codes and the intrinsic fluctuations in velocity and temperature produced by the waves.
Methods: We computed numerical simulations of wave propagation in a sunspot umbra with the code MANCHA. We used the NLTE synthesis and inversion code NICOLE to compute spectropolarimetric Ca II 8542 Å line profiles for the atmospheric models obtained as the output from the simulations. We then inverted the synthetic profiles and compared the inferences from the inversions with the known atmospheres from the simulations.
Results: 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 by the inversion results. Velocity oscillations at those higher frequencies exhibit clear signs of opacity fluctuations; namely the power of the signal at constant optical depth greatly departs from the power of the oscillations at constant geometrical height. 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 height where the line is sensitive to temperature is revealed as a strong oscillatory power in the inferred fluctuations, which is much stronger than the actual power from the intrinsic temperature oscillations.
Conclusions: Our results validate the use of NLTE inversions to study chromospheric oscillations with frequencies below ∼9 mHz. However, the interpretation of higher-frequency oscillations and the power of temperature oscillations must be addressed with care, as these exhibit signatures of opacity oscillations.
Los campos magnéticos están presentes en todos los plasmas astrofísicos y controlan la mayor parte de la variabilidad que se observa en el Universo a escalas temporales intermedias. Se encuentran en estrellas, a lo largo de todo el diagrama de Hertzsprung-Russell, en galaxias, e incluso quizás en el medio intergaláctico. La polarización de la luz
Los campos magnéticos son uno de los ingredientes fundamentales en la formación de estrellas y su evolución. En el nacimiento de una estrella, los campos magnéticos llegan a frenar su rotación durante el colapso de la nube molecular, y en el fin de la vida de una estrella, el magnetismo puede ser clave en la forma en la que se pierden las capas