The first out-of-ecliptic observations of the polar magnetic field of the Sun

TL;DR

This work addresses the long-standing difficulty of measuring the solar polar magnetic field by leveraging Solar Orbiter's out-of-ecliptic vantage with SO/PHI-HRT, enabling direct polar-field measurements up to $±17^rac{o}$ in heliographic latitude. The authors analyze two campaigns (south and north poles) using MILOS-based radiative-transfer inversion to obtain radial-field maps, after processing with the SO/PHI-HRT pipeline and applying polarization-based masking and 180° ambiguity resolution. They find a bell-shaped latitudinal flux distribution with opposite net flux signs for the two poles, and distinct flux-density patterns consistent with ongoing high-latitude polarity reversal and viewing-geometry effects, all within the context of limited north-polar data quality. These results demonstrate the critical role of high-latitude observations for advancing solar dynamo understanding and set the stage for continued polar campaigns with Solar Orbiter.

Abstract

Direct remote-sensing observations of the solar poles have been hindered by the restricted view obtained from the ecliptic plane. For the first time ever, Solar Orbiter with its remote-sensing instruments observed the poles of the Sun from out of the ecliptic in the Spring of 2025. Here we report the first measurements of the magnetic field of the solar poles taken when Solar Orbiter was at heliographic latitudes ranging between 14.9$^\circ$ and 16.7$^\circ$. The data-sets were collected by the High Resolution Telescope of the Polarimetric and Helioseismic Imager (SO/PHI-HRT) on board Solar Orbiter. Two sets of observations, approximately one month apart, for the south and north pole are considered in this work. The magnetic flux and flux density measured during these campaigns are reported as a function of the heliographic latitude observed by SO/PHI-HRT. The net fluxes show a different latitudinal distribution for the two polar caps. We also discuss the observed dependence of the measured fluxes on the viewing angle. These first results highlight the importance of high-resolution direct measurements of the polar field, paving the way to the high-latitude observations planned for SO/PHI-HRT in the coming years.

Figures (6)

• Figure 1: Magnetic field at the solar poles. Partial field-of-view of the line-of-sight magnetic field acquired during the south (left panel) and north (right panel) polar campaigns. The dashed lines show the heliographic longitudes and latitudes with 30$^\circ$ and 5$^\circ$ spacing respectively. The acquisition time is specified on top of each panel. The orientation of the field-of-view is caused by the spacecraft roll. An animation of this figure is available online. The animation has a playback time of 16 s and shows the full field-of-view of the data acquired during both campaigns.
• Figure 2: Histogram showing the absolute value of the radial magnetic field measured during the polar campaigns.
• Figure 3: Magnetic fluxes for the north (upper panel) and south pole (bottom panel) campaigns. The green and pink lines show the absolute positive and negative fluxes respectively, and the dark gray line the net flux as a function of the heliographic latitude. The latitudinal bins have a size of 2$^\circ$. The thick lines show the flux averaged over the different observations, the shaded areas show the $1\sigma$ variance between the individual snapshots. The blue line shows the average percentage of area coverage through the campaign at different latitudes. In both panels the respective pole is located on the left side.
• Figure 4: Number density of the magnetic elements as a function of the magnetic fluxes of these elements measured during the south (left panel) and north (right panel) pole campaigns. Only values with absolute heliographic latitude above 70$^\circ$ are considered.
• Figure 5: Magnetic flux densities for the north (upper panel) and south pole (bottom panel). The colors are represented as in Figure \ref{['fig:flux']}.