Towards a new view of the photospheric magnetic field with the Polarimetric and Helioseismic Imager on Solar Orbiter
by Jonas Sinjan
Date of Examination:2024-09-05
Date of issue:2024-12-19
Advisor:Prof. Dr. Sami K. Solanki
Referee:Prof. Dr. Sami K. Solanki
Referee:Prof. Dr. Ansgar Reiners
Persistent Address:
http://dx.doi.org/10.53846/goediss-10752
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Abstract
English
Accurately determining the magnetic flux through the solar surface is incredibly important to understanding the dynamics of the Sun and its atmosphere. The magnetic field provides energy for heating the higher layers of the solar atmosphere, as well as controlling the dynamics of the plasma in this region. The number and strength of solar eruptive events, the amount of solar radiation, are all related to its magnetic field. Currently solar magnetographs, instruments that infer the solar magnetic field, can probe the magnetic field in the lowest layer of the solar atmosphere: the photosphere. Maps of the photospheric magnetic field are then used as an input for models to estimate the magnetic flux at higher layers of the solar atmosphere and beyond into the solar system. These solar magnetographs have thus far been restricted to only observe the Sun from one direction, that along the Sun-Earth line. This singular view has its complications: there is a wide consensus that inference of the magnetic field near the edge of the solar disc is unreliable, as there the viewing angle with respect to the surface normal is highly inclined. The Polarimetric and Helioseismic Imager on board the Solar Orbiter spacecraft (SO/PHI), has become the first solar magnetograph to view the Sun from different positions. The new view provided by this instrument is the first opportunity to quantify the inaccuracy of inferring the photospheric magnetic field near the solar limb and in the polar regions. To make this possible first a data reduction pipeline for the High Resolution Telescope of SO/PHI (SO/PHI-HRT) was developed to produce the first maps of the photospheric magnetic field, from raw data taken during the first phases of the mission. This effort determined the instrument's performance in different observational modes and produced magnetograms with noise levels between 6.5-8.5 G. Secondly, a comparison of the photospheric magnetic field inferred by SO/PHI-HRT with that from a space-based magnetograph orbiting Earth, the Helioseismic and Magnetic Imager on board the Solar Dynamic Observatory (SDO/HMI), is made when viewing the Sun from the same direction. The line-of-sight magnetograms aligned very closely, with offsets less than 1 G and a slope of 0.97 when performing linear fitting. Larger differences were found when comparing the vector magnetic field components, which can be mostly attributed due to the different noise levels between the instruments. With this knowledge we can later disentangle differences due to the viewing direction from those originating from differences between the instruments themselves. A first attempt was made to compare the photospheric magnetic field from SO/PHI-HRT and SDO/HMI, when viewing from different positions in the ecliptic plane. Due to limitations in the reprojection algorithm, only the magnetic flux in sunspot umbrae could be compared. There the total unsigned magnetic flux from the two directions, after dividing B_LOS by μ to account for the different viewing direction (where μ=cos(θ)$ and θ is the heliocentric angle), did align closely. Nevertheless large differences in certain areas were found: near the centre of the umbrae, SO/PHI-HRT inferred stronger line-of-sight magnetic fields than SDO/HMI as it viewed the sunspots closer to disc centre, even after the μ-correction is applied. The μ-correction assumes the magnetic field to be radial everywhere, but even in the umbra and especially near its boundaries the field can be strongly inclined. Hence large differences were also found near the boundaries, which was further enhanced in areas where the boundaries did not agree, due to the Wilson depression and different instrument PSFs. To compare the magnetic fluxes over larger areas and from different magnetic features the reprojection algorithm must be developed to accurately resample magnetograms. From investigations of inclined viewing angles on 3D magnetohydrodynamic simulations of the photosphere with a unipolar magnetic field, it was found that the line-of-sight magnetic field is underestimated at all angles and worsened with low spatial resolution at high μ. Only at disc centre and with high spatial resolution was the ground truth magnetic flux within the simulation retrieved. Furthermore at μ ≤ 0.5 the spatial resolution had very little impact. These results were found for two commonly used photospheric spectral lines and reproduced by four different methods of inferring the line-of-sight magnetic field. The striking results from the simulations, together with preliminary observational evidence, imply that significantly different line-of-sight magnetic fields are retrieved when viewing from different directions. This may contribute to the resolution of the open flux problem, where the total solar open magnetic flux when propagated to 1 au is 2-3 times lower than that directly measured at 1 au.
Keywords: solar physics; magnetic field; magnetohydrodynamics; magnetogram; magnetograph; photosphere; Solar Orbiter
