Simulation reveals sun’s polar vortex

Simulation reveals sun’s polar vortex

Simulation of the polar vortex on the Sun in its initial state. Later, the vortices migrate further poleward and only two remain. © University Corporation for Atmospheric Research

The Earth has them, Jupiter and Venus too: Almost all planets in the solar system have polar vortices in their atmosphere. However, it is unclear whether our home star, the Sun, also has these typical vortex formations. Their plasma is strongly magnetic and this could change or even prevent the otherwise typical flows. A magnetohydrodynamic simulation now provides more information. Accordingly, characteristic vortices also form in the solar plasma. These arise as an orderly ring of counter-rotating vortices at around 55 degrees solar latitude and then migrate poleward over the course of the solar cycle. As a result, their number decreases. However, how many polar vortices remain and how ordered they are depends on the magnetic strength of the respective solar cycle, as the team reports. During the solar maximum, these polar vortices also disappear.

Polar vortices are a typical phenomenon for planetary atmospheres: the rotation of the planets creates a Coriolis force that shapes the prevailing air currents even in polar regions. This creates a ring of strong winds on Earth around both the North and South Poles. These polar vortices trap the cold air at the poles and, depending on their stability and permeability, influence the regional weather up to temperate latitudes, but also the ozone depletion over the poles. Such polar vortices also exist on other planets in the solar system. They are particularly spectacular on the large gas planets Jupiter and Saturn, which are characterized by raging storms. This means that several densely packed hurricanes form over the poles on Jupiter – eight over the North Pole and five at the South Pole. On Saturn, the air currents create a hexagonal-shaped flow band at the north pole, while the south pole vortex is circular. These differences give planetary scientists clues about the structure and dynamics of these planets’ atmospheres.

What do the Sun’s polar vortices look like?

However, the sun is a special case: it also rotates and is therefore exposed to similar forces. But unlike the atmospheres of planets or moons, their solar plasma is magnetic. “Therefore, these magnetic fields are expected to influence the existence or non-existence, the appearance and also the evolution of the polar vortices on the Sun,” explain Mausumi Dikpati from the National Center for Atmospheric Research in Colorado and her colleagues. In addition, the solar magnetic fields change over the course of the eleven-year solar cycle: At each maximum of the solar cycle, the magnetic field reverses its polarity, so that there are 22 years between two maxima of the same polarity. In parallel, currents of highly magnetized plasma gradually move poleward over the course of the cycle. However, the biggest problem in solar research is a lack of observation data: the Earth and all other planets orbit the sun at its equator. That’s why we only ever see our star from the side. And the solar observation observatories in space also remain in the planetary plane. No solar probe has yet been able to investigate in more detail what the poles of the sun look like.

That’s why Dikpati and her team have now examined what’s happening at the solar poles using magnetohydrodynamic simulations. Based on physical laws, they reconstructed how the plasma flows should behave with and without a magnetic field over the course of a solar cycle. The simulations showed that the sun also forms polar vortices. The complex pattern of these magnetic plasma vortices is more similar to that on the gas planet Jupiter than to the simple ring flow on Earth. “Our main finding is that a ring of vortices forms over the course of the sunspot cycle,” the team reports. This vortex ring of counter-rotating currents arises at approximately 55 degrees solar latitude and then moves further and further poleward. “The ring shrinks and loses vortices as it moves, until finally only a pair of vortices remains very close to the pole,” write Dikpati and her team. They identified Rossby waves in the solar plasma, large-scale wave-like currents that arise from the interaction of the sun’s rotation with the plasma, as the driving force for this polar concentration.

Number of vortices depends on magnetic field strength

How many vortices form in the polar solar plasma and how they develop depends on how strong the magnetic field is in the solar cycle in question. For an average cycle with a rather weak magnetic field, the simulations showed a ring of eight regularly arranged plasma vortices, which then reduced to two at the pole. If, on the other hand, the magnetic field is stronger, fewer vortices initially form, which are also less clearly ordered. After their polar migration, two to four vertebrae, which are also less regularly arranged, remain. “The shape and formation of the polar vortices depends crucially on the strength of the drifting background magnetic field,” state Dikpati and her colleagues. Weaker magnetic fields seem to promote the formation of vortices.

The simulations showed another important result at the solar maximum: When solar activity reaches its peak in the eleven-year solar cycle, the polar vortices disappear – presumably in the context of the polarity reversal of the solar magnetic field. This also means that future missions to investigate the solar poles would have to be adjusted accordingly. “Otherwise you would start a solar mission that observes the solar poles at completely the wrong time,” explains Dipati’s colleague Scott McIntosh. This also affects the European solar probe Solar Orbiter, which was launched in 2020. It currently orbits the sun in highly elliptical orbits, getting closer and closer to the sun and moving further and further away from the equatorial plane. At its closest point to the Sun, the Solar Orbiter will orbit in an orbit tilted 24 degrees from the equator. This should give solar researchers a first, albeit oblique, look at the sun’s polar regions. This phase of the mission begins in December 2026 – it could be that the initial images do not yet spot polar vortices because the solar maximum that began in 2024 is not yet far enough behind. “In any case, our results highlight the need for future missions that can observe the solar poles at times other than solar maximum,” the team writes.

Source: Mausumi Dikpati (National Center for Atmospheric Research, Boulder) et al., Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2415157121

Recent Articles

Related Stories