Limitations of the Ca ii 8542 Å Line for the Determination of Magnetic Field Oscillations
- 1 September 2021
- journal article
- research article
- Published by American Astronomical Society in The Astrophysical Journal
- Vol. 918 (2)
- https://doi.org/10.3847/1538-4357/ac111c
Abstract
Chromospheric umbral oscillations produce periodic brightenings in the core of some spectral lines, known as umbral flashes. They are also accompanied by fluctuations in velocity, temperature, and, according to several recent works, magnetic field. In this study, we aim to ascertain the accuracy of the magnetic field determined from inversions of the Ca ii 8542 angstrom line. We have developed numerical simulations of wave propagation in a sunspot umbra. Synthetic Stokes profiles emerging from the simulated atmosphere were computed and then inverted using the NICOLE code. The atmospheres inferred from the inversions have been compared with the original parameters from the simulations. Our results show that the inferred chromospheric fluctuations in velocity and temperature match the known oscillations from the numerical simulation. In contrast, the vertical magnetic field obtained from the inversions exhibits an oscillatory pattern with a similar to 300 G peak-to-peak amplitude, which is absent in the simulation. We have assessed the error in the inferred parameters by performing numerous inversions with slightly different configurations of the same Stokes profiles. We find that when the atmosphere is approximately at rest, the inversion tends to favor solutions that underestimate the vertical magnetic field strength. On the contrary, during umbral flashes, the values inferred from most of the inversions are concentrated at stronger fields than those from the simulation. Our analysis provides a quantification of the errors associated with the inversions of the Ca ii 8542 angstrom line and suggests caution with the interpretation of the inferred magnetic field fluctuations.Funding Information
- Ministerio de Ciencia, Innovación y Universidades (PGC2018-097611-A-I00)
This publication has 56 references indexed in Scilit:
- PROPAGATING WAVE PHENOMENA DETECTED IN OBSERVATIONS AND SIMULATIONS OF THE LOWER SOLAR ATMOSPHEREThe Astrophysical Journal, 2012
- Spectropolarimetric Investigation of the Propagation of Magnetoacoustic Waves and Shock Formation in Sunspot AtmospheresThe Astrophysical Journal, 2006
- Magnetoacoustic Waves in SunspotsThe Astrophysical Journal, 2003
- Full-disk magnetic oscillations in the solar photosphereAstronomy & Astrophysics, 2003
- Systematic errors in measuring solar magnetic fields with a FPI spectrometer and MDIAstronomy & Astrophysics, 2002
- Anomalous Circular Polarization Profiles in Sunspot ChromospheresThe Astrophysical Journal, 2000
- Anomalous Polarization Profiles in Sunspots: Possible Origin of Umbral FlashesScience, 2000
- Characteristics of Magnetohydrodynamic Oscillations Observed with the Michelson Doppler ImagerThe Astrophysical Journal, 1999
- Photoelectric observations of chromospheric sunspot oscillations. IV - The CA II H line and He I 10830 AThe Astrophysical Journal, 1986
- Photoelectric observations of chromospheric sunspot oscillations. II - Propagation characteristicsThe Astrophysical Journal, 1984