Sun: First glimpse of polar magnetic field in motion

The Sun is governed by a strict rhythm. The magnetic activity of the Sun displays a cyclic variation, reaching a maximum approximately every eleven years. Two enormous plasma circulations, each in one solar hemisphere, set the pace for this rhythm thus defining the Sun’s eleven-year cycle: near the surface the plasma flows carry the magnetic field lines from the equator to the poles; in the solar interior, the plasma flows back to the equator in a huge cycle spanning the entire hemisphere.

Important details of this solar “magnetic field conveyor belt” are still poorly understood. The exact processes at the Sun's poles are likely to be crucial. From Earth, scientists have only a grazing view of this region making it impossible to determine the properties of the magnetic field. Most space probes have a similarly limited perspective.

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“To understand the Sun's magnetic cycle, we still lack knowledge of what happens at the Sun's poles. Solar Orbiter can now provide this missing piece of the puzzle.”
Sami Solanki, MPS Director and co-author of the new study

Since February 2020, ESA's Solar Orbiter spacecraft has been travelling in elongated ellipses around the Sun. In March of this year, it left for the first time the plane in which the planets – and almost all other space probes – orbit the Sun. From a trajectory tilted by 17 degrees, Solar Orbiter now for the first time has a better view of the Sun's poles.

In the new publication, which appears today in the journal Astrophysical Journal Letters, researchers led by MPS analyze data from Solar Orbiter's Polarimetric and Helioseismic Imager (PHI) and Extreme-Ultraviolet Imager (EUI). The PHI data are from March 21 of this year; the EUI data cover the period from March 16 to 24. The measurements provide information about the direction of plasma flows and the magnetic field on the solar surface. 

The data reveal a refined picture of the supergranulation and magnetic network of the Sun at the south pole for the first time. Supergranules are cells of hot plasma, about two to three times the size of Earth, which densely cover the surface of the Sun. Their horizontal surface flows wash magnetic field lines to their edges, creating the Sun's magnetic network: a web of strong magnetic fields.

To the surprise of the researchers, the magnetic field is seen to drift toward the poles at approximately 10 to 20 meters per second, on average, almost as fast as their counterparts at lower latitudes. Previous studies based on the ecliptic-plane observations have seen much slower drifts of the magnetic field near the high polar latitudes. Their motion offers important clues about the Sun’s global plasma and magnetic field circulation. 

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“The supergranules at the poles act as a kind of tracer. They make the polar component of the Sun's global, eleven-year circulation visible for the first time.”
Lakshmi Pradeep Chitta, research group leader at MPS and first author

It is still unclear whether the Sun's global “magnetic conveyor belt” does truly not slow down near the poles. The data now published only show a brief snapshot of the entire solar cycle. Further observational data, ideally covering longer time periods, are needed.