The poles of the Sun are the next frontier to cross

The Sun is scrutinized from every angle, whether from the ground or by probes set out to meet it. But there are still very poorly known areas: the poles. However, it is these parts of our star that are crucial for understanding the phenomena that take place there. For researchers, this is the next frontier to overcome.

It seems logical that the advance of scientific knowledge answers the questions of Humanity. But, concerning the Sun, the more this knowledge has taken on volume, the more the mysteries around our star are numerous. Today, between ground-based telescopes and space probes, there is no shortage of viewpoints on the Sun. However, some questions remain desperately unanswered, notably about the Sun’s magnetic field.

It must be said that, despite appearances, we finally see a fairly small part of the Sun: the Earth being in the plane of the equator, everything close to the center is familiar to us, but the poles remain almost invisible. This problem is at the heart of a paper pre-published on ArXiv (it has not yet been published in a scientific journal) on December 30, 2022, by a team of international researchers. “It is above all an incentive intended for space agencies. Developing a mission to see the poles could be very interesting to learn more about the Sun“. 

The researcher from the Institute for Research in Astrophysics and Planetology, in Toulouse, is particularly interested in the cycles of the Sun. Movements of the magnetic field are well observed and documented, but theoretical models are based on some uncertainties.

Small field course (solar magnetic)

To summarize the problem, it should be known that the solar magnetic field reverses in an uninterrupted cycle every eleven years. Initially, it is poloidal, with a field that goes from South to North, but the rather complex rotation of the Sun causes it to change places. “If you are sitting on the equator, explains Laurène Jouve, you will do three revolutions of the Sun while a person sitting at the pole will only do two. And, this asymmetry is the source of many disturbances“. 

A phenomenon on the Sun observed by the Parker probe. // Source: NASA’s Goddard Space Flight Center/Conceptual Image Lab/Adriana Manrique Gutierrez

Little by little, the magnetic field will be attracted towards the equator by the rotation which is faster there. It will stretch, twist, and eventually be expelled from the surface, creating the well-known phenomenon of sunspots. Then this energy will gradually converge again towards the poles until it returns to the original configuration, but this time from North to South.

This cycle is known as the Babcock-Leighton mechanism, and it is quite studied regarding the Sun. That said, seeing directly what happens in the poles when the magnetic field reverses would be a huge step forward. A thirst for knowledge that is not purely theoretical, since the challenge is also to better predict the intensity of the next cycles. Laurène Jouve explains: “We know that the strength of a magnetic field at the beginning of its cycle influences the number of sunspots present during the maximum. And, this knowledge is invaluable“. 

Solar flares have a significant effect on telecommunications satellites, but also on space probes exposed to radiation, not to mention manned missions. The harmful effects of this activity are numerous: satellites can stop working, and astronauts, who are not protected by the Earth’s atmosphere, can receive a surplus of radiation, and therefore present an increased risk of cancer. That doesn’t include space station passengers who are still fairly close to Earth, but as manned missions to the Moon come into focus, the question will arise.

“There is no technological obstacle”

Going to see the poles of the Sun is therefore particularly important and interesting, whether for pure science or for more practical aspects related to space exploration. But how to do it? It turns out that if no probe has tried to see before, it’s because it’s rather complex. The Earth being at the level of the equator, it is necessary to launch a probe that would have a significant change of trajectory to go up or down towards the poles. “The one that comes closest to it is Solar Orbiter,” adds Laurène Jouve. The probe moves away from its plane, but not more than 30 degrees, so it’s still quite a distance from the poles“. 

The European Space Agency’s probe uses gravitational assistance mechanisms to change planes in this way, which is possible in particular thanks to the planet Venus, used here as a slingshot. This, therefore, implies a fairly complex trajectory, because several rotations are necessary to end up arriving at the right destination, not to mention the energy reserves that are still significant to modify the trajectory of the machine.

Artist’s impression of Solar Orbiter. // Source: ESA/ATG medialab (cropped photo)

“It’s more complex, recognizes Laurène Jouve, but there is no real technological obstacle. What is missing would be the willingness of agencies to embark on such a project, and we think it can be worth it“. 

In the meantime, resources are not lacking to begin a mission to the solar poles. The probes directed towards our star, in particular, Solar Orbiter which must approach a little closer to the poles, bring a quantity of crucial data to develop the mission which will exceed the current borders. It could be that the first passage of Solar Orbiter closer to the poles, scheduled for 2025, encourages projects of this type.

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