Supergranulation and Poleward Migration of the Magnetic Field at High Latitudes of the Sun

TL;DR

This study presents the first out-of-ecliptic observations of the Sun's south pole with Solar Orbiter, focusing on polar supergranulation and the latitudinal migration of the magnetic network at high latitudes. By integrating SO/PHI-HRT spectropolarimetry with EUI/FSI304 EUV imaging, the authors quantify polar supergranular scales of $20$--$40$ Mm and detect a net poleward magnetic transport with speeds around $10$--$20$ m s$^{-1}$, including localized faster motions up to $-105$ m s$^{-1}$ for individual features. The findings suggest a faster, potentially radially varying transport mechanism at high latitudes compared to ecliptic measurements, with significant implications for polar magnetic-field buildup and the solar cycle. The work demonstrates the critical role of high-latitude campaigns in constraining polar solar dynamics and informs the design of future polar missions.

Abstract

Magnetoconvection at the solar surface governs the dynamics in the upper solar atmosphere and sustains the heliosphere. Properties of this fundamental process are poorly described near the solar poles. Here we report the first out-of-ecliptic remote-sensing observations of the south pole of the Sun from a high-latitude campaign of the Solar Orbiter spacecraft which reveal spatial and temporal evolution of supergranular convective cells. The supergranular cells have spatial scales of 20--40 Mm. From eight days of observations starting on 2025 March 16, our analysis shows that the magnetic network migrates poleward, on average, at high latitudes (above 60\textdegree), with speeds in the range of 10--20 m s$^{-1}$, depending on the structures being tracked. These results shed light on the buildup of the polar magnetic field that is central to our understanding of the solar cycle and the heliospheric magnetic field.

Figures (8)

• Figure 1: South pole of the Sun. 15-hour temporally averaged maps of the line-of-sight components of the photospheric velocity (a), and magnetic field (b) observed by the SO/PHI-HRT instrument are shown. The maps are displayed in the zenithal equidistant projection, with the zenith placed at the south pole at $-90$° latitude (red dot). The velocity map is saturated at $\pm1$ km s$^{-1}$, with darker (lighter) shaded pixels indicating plasma flows toward (away from) the observer (gray is zero). The magnetic field map is saturated at $\pm5$ G. Lighter (darker) patches have magnetic field component toward (away from) the observer. In panel (a) the blue colored lines passing through the pole are longitudes, going from 55° to 125° in a counter-clockwise direction, with 10° spacing. The yellow shaded region is a $\pm$1° longitudinal band. The concentric circles in both panels represent lines of latitude with a separation of 5°. An animation of panel (b) at 2-hour cadence is available online. The animation has a play back time of 2 s and contains 7 frames with time stamps from 2025 March 21 UT 10:29 to 2025 March 21 UT 22:29, with $\sim$2 hour increments. See Appendix \ref{['app:phi']} for more details.
• Figure 2: Spatial scales of supergranules near the poles. In each panel we plot the spatial autocorrelation function of the mean line-of-sight velocity (blue colored curves) from the corresponding meridian band in Fig. \ref{['fig:phi_map']}, as labeled. This autocorrelation function is plotted as a function of spatial lag in degrees (lower abscissa) and megameters (upper abscissa). The yellow shaded curves are the 1$\sigma$ standard deviation in the autocorrelation signal. The dotted horizontal lines mark the zero of the autocorrelation function. See Appendix \ref{['app:auto']} for more details.
• Figure 3: Supergranular structure and evolution near the pole. An image sequence showing the 2-hour time-averaged maps of the line-of-sight components of the photospheric velocity, in the same projection as in Figure \ref{['fig:phi_map']}a. The maps are saturated at $\pm1.5$ km s$^{-1}$. The dashed and solid circles are $-85$° and $-80$° latitudes, respectively. The yellow and blue contours outline the line-of-sight component of the magnetic field (above 7 G) directed toward and away from the observer. The white squares outline a sample of four distinct supergranular cells near the pole. An animation of this figure is available online. The animation has a play back time of 2 s and contains 7 frames with time stamps from 2025 March 21 UT 10:29 to 2025 March 21 UT 22:29, with $\sim$2 hour increments.
• Figure 4: The latitudinal migration of supergranular structures. Latitudinal signal of the longitudinally-averaged line-of-sight components of the velocity (a), and magnetic field (b) are displayed as functions of time. In panel (a) we highlight a feature that is drifting toward the pole with a velocity of $-105$ m s$^{-1}$. The negative sign indicates that the flow is away from the north pole, i.e., toward the south pole. See Appendix \ref{['app:auto']} for more details.
• Figure 5: Latitudinal migration of the magnetic network at high latitudes. (a) A 15-hour temporally averaged EUI/FSI 304 Å intensity map in the zenithal equidistant projection is shown. The south pole is identified by the white dot, with dashed concentric circles denoting latitudes with 5° spacing. The yellow dotted wedge subtends an angle of 120° at the pole. EUV bright points within this wedge are tracked for their motion on the Sun. (b) Latitudinal tracks of EUV bright points with lifetimes of at least 10 hours (90 in total) are plotted with gray symbols. The various colored solid lines are linear fits to the observed tracks. The colors themselves indicate the latitudinal velocity of the moving bright points (we assign negative values for the bright points moving toward the south pole). (c) Longitudinally and temporally averaged velocity signal, derived from a Fourier local correlation tracking technique, as a function of solar latitude is plotted as gray symbols. The red horizontal line marks the $0$ m s$^{-1}$ level. In this representation too, negative velocities correspond to motions toward the south pole. See Appendices \ref{['app:eui']} and \ref{['app:bright']} for more details.