# Magnetic fields inferred by Solar Orbiter: A comparison between SO/PHI-HRT and SDO/HMI

**Authors:** J. Sinjan, D. Calchetti, J. Hirzberger, F. Kahil, G. Valori, S. K. Solanki, K. Albert, N. Albelo Jorge, A. Alvarez-Herrero, T. Appourchaux, L. R. Bellot Rubio, J. Blanco Rodríguez, A. Feller, A. Gandorfer, D. Germerott, L. Gizon, J. M. Gómez Cama, L. Guerrero, P. Gutierrez-Marques, M. Kolleck  
**arXiv:** [2303.16771](https://arxiv.org/abs/2303.16771)  
**Date:** 2023-03-29  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/2303.16771v1/

## Key points

- Solar Orbiter spacecraft (SO) and the Solar Dynamics Observatory (SDO) both use polarised light images to infer the photospheric magnetic field.
- The High Resolution Telescope (HRT) of the Polarimetric and Helioseismic Imager on board SO and the Helioseismic and Magnetic Imager (HMI) on board SDO are two instruments that provide magnetic field maps.
- SO/PHI is the first magnetograph to move out of the Sun-Earth line, providing unprecedented access to the Sun's poles.
- Researchers aimed to compare magnetic field maps from these two instruments using data obtained during Solar Orbiter's inferior conjunction on 7 March 2022.
- Comparison showed that SO/PHI-HRT and HMI produce remarkably similar line-of-sight magnetograms with a Pearson correlation coefficient of 0.97.
- In weak signal regimes (< 600 G), SO/PHI-HRT measures stronger and more horizontal fields than HMI due to greater noise in its data.
- In strong field regimes (> 600 G), HRT infers lower field strengths but with similar inclinations (a slope of 0.92) and azimuths (a slope of 1.02).
- Overall, this study provides valuable insights into similarities and differences between magnetic field maps obtained by SO/PHI-HRT and SDO/HMI instruments which highlights their importance in gaining a more comprehensive understanding of the Sun's magnetic field when used simultaneously.

## Summary

The Solar Orbiter spacecraft (SO) and the Solar Dynamics Observatory (SDO) both use polarised light images to infer the photospheric magnetic field. The High Resolution Telescope (HRT) of the Polarimetric and Helioseismic Imager on board SO and the Helioseismic and Magnetic Imager (HMI) on board SDO are two instruments that provide magnetic field maps. SO/PHI is the first magnetograph to move out of the Sun-Earth line, providing unprecedented access to the Sun's poles. This presents an excellent opportunity for new research where both instruments' magnetic field maps can be used simultaneously. In a recent study, researchers aimed to compare the magnetic field maps from these two instruments and discuss any possible differences between them. They used data obtained during Solar Orbiter's inferior conjunction on 7 March 2022. The HRT data were additionally treated for geometric distortion and degraded to the same resolution as HMI, while HMI data were re-projected to correct for the $3^{\circ}$ separation between the two observatories. The comparison showed that SO/PHI-HRT and HMI produce remarkably similar line-of-sight magnetograms, with a slope coefficient of $0.97$, an offset below $1$ G, and a Pearson correlation coefficient of $0.97$. However, SO/PHI-HRT infers weaker line-of-sight fields for the strongest fields. As for vector magnetic fields, SO/PHI-HRT was compared to both the $720$-second and $90$-second HMI vector magnetic field: SO/PHI-HRT has a closer alignment with the $90$-second HMI vector. In weak signal regimes ($< 600$ G), SO/PHI-HRT measures stronger and more horizontal fields than HMI due to greater noise in its data. In strong field regimes ($\gtrsim 600$ G), HRT infers lower field strengths but with similar inclinations (a slope of $0.92$) and azimuths (a slope of $1.02$). The slope values are from the comparison with the HMI $90$-second vector. Overall, this study provides valuable insights into similarities and differences between magnetic field maps obtained by SO/PHI-HRT and SDO/HMI instruments which highlights their importance in gaining a more comprehensive understanding of the Sun's magnetic field when used simultaneously.