Synthetic observations of wave propagation in a sunspot umbra

TL;DR

This paper uses synthetic spectropolarimetric data from simulations to analyze wave propagation in a sunspot umbra, revealing how opacity fluctuations and shock waves influence observed signals and magnetic field oscillations.

Contribution

It introduces a detailed analysis of wave-induced spectropolarimetric signals in sunspot simulations, highlighting the roles of opacity fluctuations and shock waves in the chromosphere.

Findings

Opacity fluctuations cause intensity variations except at 5-6.5 mHz.

Magnetic field oscillations are linked to wave propagation and low magnetic field gradients.

Shock waves produce emission reversals and abnormal Stokes V profiles.

Abstract

Spectropolarimetric temporal series from Fe I $\lambda$ 6301.5 \AA\ and Ca II infrared triplet lines are obtained by applying the Stokes synthesis code NICOLE to a numerical simulation of wave propagation in a sunspot umbra from MANCHA code. The analysis of the phase difference between Doppler velocity and intensity core oscillations of the Fe I $\lambda$ 6301.5 \AA\ line reveals that variations in the intensity are produced by opacity fluctuations rather than intrinsic temperature oscillations, except for frequencies between 5 and 6.5 mHz. On the other hand, the photospheric magnetic field retrieved from the weak field approximation provides the intrinsic magnetic field oscillations associated to wave propagation. Our results suggest that this is due to the low magnetic field gradient of our sunspot model. The Stokes parameters of the chromospheric Ca II infrared triplet lines show striking variations as shock waves travel through the formation height of the lines, including emission self-reversals in the line core and highly abnormal Stokes V profiles. Magnetic field oscillations inferred from the Ca II infrared lines using the weak field approximation appear to be related with the magnetic field strength variation between the photosphere and the chromosphere.