Polarity-Resolved Far-Side Magnetograms Based on Helioseismic Measurements

Abstract

Understanding and monitoring solar active regions is essential for operational space-weather forecasting and improved solar dynamo modeling. This requires comprehensive 360-degree observations of the Sun. While space-weather forecasting has long relied successfully on high-quality observations of the Earth-facing hemisphere, a critical gap remains due to the lack of direct, continuous magnetic field measurements of far-side active regions, particularly magnetic field strength, polarity configurations, and related parameters. We present a methodology for inferring magnetic field distributions of active regions in helioseismic maps of the far hemisphere. The analysis focuses on identifying the magnetic polarities of opposing components of a helioseismic signature and applying stable, continuous polarity assignment to large-scale magnetic structures derived from such maps. These helioseismic signatures reliably resolve strong active regions, especially those that later appear as major rotation regions when they rotate into Earth view. Polarity boundaries are identified by analyzing the bimodal longitudinal variance profile of the seismic signal within each region, after which Hales law is applied to establish east-west ordering consistent with the solar cycle. The method produces polarity-resolved far-side magnetograms suitable for integration with near-side observations, enabling construction of full-Sun magnetic boundary conditions for coronal and solar wind modeling and providing a critical step toward improved heliospheric simulations and operational forecasting.

Figures (11)

• Figure 1: Solar Orbiter’s far-side dataset coverage, and viewing geometry. (a) Longitudinal separation between Solar Orbiter and Earth through the years 2022 -- 2024, where values approaching $\pm 180^\circ$ signify the predominant far-side vantages. The horizontal dashed line marks the $0^\circ$ separation angle, corresponding to the alignment between SO and Earth in Carrington longitude. The vertical dashed lines indicate the times when Solar Orbiter crosses this alignment. The blue shaded regions represent the periods selected for this study, during which the longitudinal separation exceeded $\pm 126^\circ$, corresponding to the times when Solar Orbiter observes more than two-thirds of the far hemisphere. The gray background area plot shows the smoothed monthly sunspot number for the current Solar Cycle 25 as a reference. (b) Example of the normalized separation angle, scaled relative to the full $180^\circ$ far-side extent, through one of the Solar Orbiter’s orbits, from mid-2023 to early 2024. (c) Polar view of the Solar Orbiter's position (green square) with respect to Earth (pale blue disc at bottom), showing the Stonyhurst longitudinal separation between Earth and Solar Orbiter on 2023 June 19. The green semicircular disk represents the actual angular separation, $\theta$, on that date. The red radial lines indicate the $\pm 126^\circ$ threshold (i.e., 0.7 normalized far-side coverage), used as a cutoff for defining valid far-side intervals in this study.
• Figure 2: Comparison between GONG and SO/PHI maps and extraction of overlapping far-side regions on 2022 May 21, 12:00 UT. (a) GONG helioseismic phase-shift map with overlaid visibility boundaries for the full far-side (red outline) and SO/PHI (blue). (b) GONG map masked to retain only the region visible with SO/PHI. (c) Horizontally cropped GONG map. (d) Corresponding SO/PHI magnetogram with visibility boundaries for the far-side in red and SO/PHI in blue. (e) SO/PHI magnetogram truncated to include only the region overlapping with the GONG far-side coverage. (f) Horizontally cropped SO/PHI magnetogram.
• Figure 3: (a) Pre-processed GONG phase-shift map on 2022 May 21, 12:00 UT. (b) Far-side ARs masks, identified using the FASTARR model, overlaid with centroid positions (latitude, and longitude) for each region. (c) Statistical summary of the phase-shift signals $(\varphi)$ within each AR: mean $\mu(\varphi)$, minimum $\min(\varphi)$, and standard deviation $\sigma(\varphi)$. (d) Co-temporal SO/PHI magnetogram showing the corresponding far-side magnetic field. (e) Same SO/PHI magnetogram with the FASTARR/AR bounded-boxes, overlaid to extract the magnetic characteristics within each identified region. (f) Magnetic property summary for each region: mean of the unsigned LoS magnetic field ($\mu|B|$), standard deviation ($\sigma|B|$), and maximum field (Max$|B|$).
• Figure 4: Comparison of the helioseismic phase-shift and magnetic field properties for the far-side ARs detected. (a) GONG far-side phase-shift map on 2022 May 21 with the identified AR candidates (labeled 1–4). (b) Zoom-in view of the extracted region-1. (c, d) Latitudinal and longitudinal profiles of the mean (blue) and standard deviation (red) of the absolute phase-shift, $|\varphi|$, across the region-1, respectively. (e) Corresponding SO/PHI magnetic field map, co-aligned with the GONG helioseismic map overlaid with the same identified ARs. (f) Zoom in to region 1. (g, h) Latitudinal and longitudinal profiles of the mean (blue) and standard deviation (red) of the unsigned LoS magnetic field $|B|$, respectively.
• Figure 5: Comparative analysis of ten far-side ARs using helioseismic phase-shift maps, longitudinal variability, and corresponding magnetic field maps. Each row corresponds to one far-side AR, identified by its NOAA number and far-side crossing date. The first column shows the far-side phase-shift maps with overlaid black contours delineating the AR boundaries automatically identified by the FASTARR model. The second column presents the longitudinal standard deviation of the absolute phase shift, $\sigma_{long}(|\varphi|)$, with the fitted double-Gaussian components (blue and red dashed curves) and their combined profile (magenta line) overlaid on the measured distribution (black line). The third column displays the corresponding far-side magnetic maps with the same FASTARR-derived AR contours overlaid. The fourth column shows the longitudinal standard deviation of the absolute LoS magnetic field, $\sigma_{long}(|B|)$, emphasizing the spatial variability of the magnetic field distribution along the Carrington longitude.