Unlocking the secrets of the Sun’s poles

The Sun’s poles are one of the last uncharted frontiers in solar physics. While space-based satellites and ground-based telescopes have provided extraordinary views of the solar surface, atmosphere, and magnetic field, our vantage point has usually been constrained to the ecliptic plane — the thin slice of space in which the Earth and most other planets orbit around the Sun. This perspective leaves the high-latitude polar regions of the Sun poorly observed and understood. Yet, the magnetic fields and dynamic processes in these regions play a crucial role in the solar magnetic cycle and in supplying mass and energy to the fast solar wind, ultimately being vital in controlling solar activities and driving space weather.

 

Why the Poles Matter

At first glance, the Sun’s poles may appear quiet compared to the active regions located approximately between latitudes of ±35°, where sunspots, solar flares, and coronal mass ejections (CMEs) dominate. However, the magnetic fields in the solar polar regions participate in the solar global dynamo process and may serve as a seed field for the subsequent solar cycle, characterizing the solar dipole magnetic field. Additionally, the in-situ measurements obtained by the Ulysses spacecraft reveal that  the fast solar wind largely originates from big coronal holes in the polar regions. Thus, understanding the Sun’s poles is essential for answering three of the most pressing questions in solar physics:

 

1. How does the solar dynamo work and drive the solar magnetic cycle?

The solar magnetic cycle refers to the periodic variation in sunspot number on the solar surface, typically on a time scale of approximately 11 years. During each cycle, the Sun’s magnetic poles undergo a reversal, with the magnetic polarities of the north and south poles switching. The Sun’s global magnetic fields are generated through a dynamo process. Key to this process are the differential rotation of the Sun that generates the active regions, and the meridional circulation that transport magnetic flux toward the poles. Yet, decades of helioseismic investigations have revealed conflicting results about the flow patterns deep within the convection zone. Some studies even suggest poleward flows at the base of the convection zone, challenging the classical dynamo models. High-latitude observations of the magnetic fields and plasma motions could provide the missing evidence to refine or rethink these models.

 

1. What drives the fast solar wind?

The fast solar wind - a supersonic stream of charged particles - originates primarily from the polar coronal holes, and permeates the majority of the heliospheric volume, dominating the physical environment of interplanetary space. However, critical details regarding the origin of this wind remain unresolved. Does the wind originate from dense plumes within coronal holes or from the less dense regions between them? Are wave-driven processes, magnetic reconnection, or some combination of both responsible for accelerating the plasma in the wind? Direct polar imaging and in-situ measurements are required to settle the debate.

 

1. How do space weather events propagate through the solar system?

Heliospheric space weather refers to the disturbances in the heliospheric environment caused by the solar wind and solar eruptive activities. Extreme space weather events, such as large solar flares and CMEs, can significantly trigger space environmental disturbances such as severe geomagnetic and ionospheric storms, as well as spectacular aurora phenomena, posing a serious threat to the safety of high-tech activities of human beings. To accurately predict these events, scientists must track how magnetic structures and plasma flows evolve globally, not just from the limited ecliptic view. Observations from vantage point out of the ecliptic would provide an overlook of the CME propagation in the ecliptic plane.

 

Past Efforts

Scientists have long recognized the importance of solar polar observations. The Ulysses mission, launched in 1990, was the first spacecraft to leave the ecliptic plane and sample the solar wind over the poles. Its in-situ instruments confirmed key properties of the fast solar wind but lacked imaging capability. More recently, the European Space Agency’s Solar Orbiter has been gradually moving out of the ecliptic plane and is expected to reach latitudes of around 34° in a few years. While this represents a remarkable progress, it still falls far short of the vantage needed for a true polar view.

A number of ambitious mission concepts have been proposed over the past decades, including the Solar Polar Imager (SPI), the POLAR Investigation of the Sun (POLARIS), the Solar Polar ORbit Telescope (SPORT), the Solaris mission, and the High Inclination Solar Mission (HISM). Some envisioned using advanced propulsion such as solar sails to reach high inclinations. Others relied on gravity assists to incrementally tilt their orbits. Each of these missions would carry both remote-sensing and in-situ instruments to image the Sun’s poles and measure key physical parameters above the poles.

 

The SPO Mission

The Solar Polar-orbit Observatory (SPO) is designed specifically to overcome the limitations of past and current missions. Scheduled for launch in January 2029, SPO will use a Jupiter gravity assist (JGA) to bend its trajectory out of the ecliptic plane. After several Earth flybys and a carefully planned encounter with Jupiter, the spacecraft will settle into a 1.5-year orbit with a perihelion of about 1 AU and an inclination of up to 75°. In its extended mission, SPO could climb to 80°, offering the most direct view of the poles ever achieved.

The 15-year lifetime of the mission (including an 8-year extended mission period) will allow it to cover both solar minimum and maximum, including the crucial period around 2035 when the next solar maximum and expected polar magnetic field reversal will occur. During the whole lifetime, SPO will repeatedly pass over both poles, with extended high-latitude observation windows lasting more than 1000 days.

The SPO mission aims at breakthroughs on the three scientific questions mentioned above. To meet its ambitious objectives, SPO will carry a suite of several remote-sensing and in-situ instruments. Together, they will provide a comprehensive view of the Sun’s poles. The remote-sensing instruments include the Magnetic and Helioseismic Imager (MHI) to measure magnetic fields and plasma flows at the surface, the Extreme Ultraviolet Telescope (EUT) and the X-ray Imaging Telescope (XIT) to capture dynamic events in the solar upper atmosphere, the VISible-light CORonagraph (VISCOR) and the Very Large Angle CORonagraph (VLACOR) to track the solar corona and solar wind streams out to 45 solar radii (at 1 AU). The in-situ package includes a magnetometer and particle detectors to sample the solar wind and interplanetary magnetic field directly. By combining these observations, SPO will not only capture images of the poles for the first time but also connect them to the flows of plasma and magnetic energy that shape the heliosphere.

SPO will not operate in isolation. It is expected to work in concert with a growing fleet of solar missions. These include the STEREO Mission, the Hinode satellite, the Solar Dynamics Observatory (SDO), the Interface Region Imaging Spectrograph (IRIS), the Advanced Space-based Solar Observatory (ASO-S), the Solar Orbiter, the Aditya-L1 mission, the PUNCH mission, as well as the upcoming L5 missions (e.g., ESA’s Vigil mission and China’s LAVSO mission). Together, these assets will form an unprecedented observational network. SPO’s polar vantage will provide the missing piece, enabling nearly global 4π coverage of the Sun for the first time in human history.

 

Looking Ahead

The Sun remains our closest star, yet in many ways it is still a mystery. With SPO, scientists are poised to unlock some of its deepest secrets. The solar polar regions, once hidden from view, will finally come into focus, reshaping our understanding of the star that sustains life on Earth.

The implications of SPO extend far beyond academic curiosity. A deeper understanding of the solar dynamo could improve predictions of the solar cycle, which in turn affect space weather forecasts. Insights into the fast solar wind will enhance our ability to model the heliospheric environment, critical for spacecraft design and astronaut safety. Most importantly, better monitoring of space weather events could help protect modern technological infrastructure — from navigation and communications satellites to aviation and terrestrial power systems.

 

See the article：

Probing Solar Polar Regions https://www.cjss.ac.cn/cn/article/doi/10.11728/cjss2025.04.2025-0054

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