Comparison of Direct Magnetic Field Measurements in a Sunspot by Ten Spectral Lines of Fe I, Fe II, Ti I, and Ti II

Abstract

Direct magnetic field measurements in sunspots by many spectral lines are important for elucidating the true magnitude and structure of the magnetic field at different levels of the solar atmosphere. Currently, magnetographic measurements are the most widespread, but such measurements mainly represent the longitudinal component of the magnetic field. In the sunspot umbra, such measurements give unreliable information and do not allow for determining the actual value of the module (absolute value) of the magnetic field. Such data can be obtained from spectral-polarization observations, thanks to which the magnetic field can be determined directly from Zeeman splitting, rather than as calibrated polarization in line profiles. The presented work presents the results of the study into the magnetic field in the sunspot on July 17, 2023, which was observed on the Echelle spectrograph of the horizontal solar telescope of the Astronomical Observatory of Taras Shevchenko National University of Kyiv. The I ± V profiles of ten photospheric lines of Fe I, Fe II, Ti I, and Ti II were analyzed in detail. The strongest magnetic field measured by the Fe I lines reaches 2600 G, and the difference in the measured intensities by these lines is sometimes at the level of 50–80%. The umbral lines of Ti I show, in general, the same magnetic fields as Fe I lines, while the lines of Fe II and Ti II show significantly weaker fields. Although the lateral field profile in the spot by most of the Fe I lines is smooth, quasi-Gaussian, one of the lines, namely Fe I λ 629.10 nm, shows a “dip” at 400–600 G in the sunspot umbra, which, most likely, is real. The obtained data probably indicate a combination of at least two effects: the dependence of measurements on the height of line formation in the solar atmosphere and the manifestation of Zeeman “saturation” in lines with different Lande factors. It also turned out that the umbral line of Ti I λ 630.38 nm shows somewhat stronger magnetic fields compared to non-umbral lines. The obtained data are planned to be used to clarify the general picture of the magnetic field in the spot by means of simulation.