What Is the Solar Cycle?

On Earth, we go through many cycles, such as the cycle of the day and night and the cycle of the four seasons. There is a cycle about the Sun too, and it’s the (approximately) 11-year cycle of its magnetic field. Let’s find out what a solar cycle is in this article.

During the solar cycle, the Sun’s magnetic field changes. Sometimes, the magnetic field is very strong at the poles but doesn’t have much activity at the center. Sometimes it’s all messed up with many internal structures, as the strength at the poles drops to zero and then reverses polarity. Therefore, every cycle, the Sun’s magnetic field flips polarity. That means north becomes south, and south becomes north.

This causes changes in the Sun’s giant magnetic field that can affect how the Sun behaves and affects the Solar System. Each cycle, there is a solar minimum — a period where the Sun’s activity is minimal. And then, about half a cycle later, it is followed by a solar maximum — a period where the Sun is at its most active. This is also when the polarity flip happens. Note that the period of the cycle is irregular — sometimes it lasts longer than 11 years (as long as 14 years), and sometimes it is shorter (as short as 8 years).

Currently, we are in Solar Cycle 25, the 25th solar cycle properly documented, which started in December 2019. The last solar cycle was average in length, being almost exactly 11 years long. But this time, solar maximum is coming sooner than the 11-year periodicity would suggest, and is already happening this year (2024).

Here is a comparison between the Sun in solar minimum and in solar maximum (video from NASA Goddard):

Sunspots result from fluctuations in the Sun’s magnetic field. When fluctuations in the Sun’s magnetic field lowers the surface temperature of a region on the Sun’s surface, that region appears darker than the background and becomes a sunspot. Therefore, the occurrence of sunspots is intertwined with the magnetic activity of the Sun, and thus the 11-year solar cycle.

During solar maximum, there are a lot of fluctuations in the Sun’s magnetic field. Therefore, the number of sunspots increases, particularly in the mid-latitude region. But during solar minimum, the magnetic field is calm and doesn’t exhibit much activity. Therefore, the number of sunspots decreases significantly, dropping down to close to zero at times.

Another phenomenon related to the solar cycle is the coronal mass ejections (CMEs). Sometimes, the Sun’s magnetic field lines can become twisted, forming flux ropes. These flux ropes form loops that can be visible from outside the Sun. When they release the magnetic energy suddenly, they can cause a large bundle of charged particles to be ejected from the Sun, in an event known as a coronal mass ejection. During solar maximum, when the Sun’s magnetic activity is at its maximum, there tends to be more CMEs and flux ropes on the surface of the Sun.

When the coronal mass ejection reaches Earth, it could disrupt Earth’s magnetic field and cause a geomagnetic storm. This may cause the spectacular event of an aurora, but it could also disrupt power systems and lead to severe outages.

In addition to releasing charged particles, the Sun’s magnetic field can also release its energy via large amounts of high-energy electromagnetic radiation, sometimes reaching the energy level of X-rays and gamma rays. When they hit the Earth, the high-energy light rays can disrupt the ionosphere and hinder radio communications. Like CMEs, solar flares occur most often in solar maximum, as the magnetic activity is richer in that period.

To conclude, the solar cycle is a periodic (with an irregular period) variation of the Sun’s magnetic field activity. It results in a flipping of the Sun’s magnetic poles about every 11 years, and it is the reason for the variation in the number of sunspots, solar flares, coronal mass ejections, and auroras. If you would like more information about the solar cycle and its effects, check out the articles in the references below.

1. Dorbijevic, D. (2022, April 26). Solar cycle: What is it and why does it matter? Retrieved May 31, 2024, from https://www.space.com/solar-cycle-frequency-prediction-facts
2. Bartlett, R. (2023, October 11). What is the 11 Year Cycle of the Sun? Retrieved May 31, 2024, from https://www.highpointscientific.com/astronomy-hub/post/astronomy-101/what-is-the-11-year-cycle-of-the-sun
3. (n.d.). NASA: Understanding the Magnetic Sun. Retrieved May 31, 2024, from https://www.nasa.gov/science-research/heliophysics/nasa-understanding-the-magnetic-sun/
4. Roach, J. (2004, September 28). Why Does Earth’s Magnetic Field Flip? Retrieved May 31, 2024, from https://www.nationalgeographic.com/environment/article/magnetic_field_flip
5. (n.d.). Sunspots/Solar Cycle. Retrieved May 31, 2024, from https://www.swpc.noaa.gov/phenomena/sunspotssolar-cycle
6. Dorbijevic, D. (n.d.). Sunspots: What are they, and why do they occur? Retrieved May 31, 2024, from https://www.space.com/sunspots-formation-discovery-observations
7. (n.d.). Coronal Mass Ejections. Retrieved May 31, 2024, from https://www.swpc.noaa.gov/phenomena/coronal-mass-ejections
8. (n.d.). Solar Flares (Radio Blackouts). Retrieved May 31, 2024, from https://www.swpc.noaa.gov/phenomena/solar-flares-radio-blackouts