The Sun’s Magnetic Field and the Solar Cycle
by Sam Atkins
It was recently determined that the Sun has reached its solar maximum period in its solar cycle. What does that mean? What is the solar cycle? What causes it? How does it affect us here on Earth?
NOTE: Tap or hover over images for captions and credits.
Image credit: Kreuzschnabel
While to us it may appear to be a steady unyielding ball of light, the Sun is brimming with chaos and change. Deep within its core is a hellish crucible that generates the light and heat that defines how we experience the Sun. Thermonuclear fusion in the Sun’s core is its most important element, but there is much more going on within and around it. The surface is a raging sea of scorching plasma where turbulent movements give rise to fascinating phenomena. These include things like sunspots, solar flares and coronal mass ejections. We’ll come back to these in a bit. First, let’s focus on what drives this activity - the Sun’s magnetic field.
This diagram shows how the complex motions of plasma and the Sun’s rotation converge to create the star’s complex magnetic structure. The top row is the omega effect and the bottom row is the alpha effect. Image credit: E. F. Dajka
Where does the Sun’s magnetic field come from? It has to do with something called a dynamo.
Dynamos are based around the idea that the complex motion of conductive material generates electric currents, which in turn generates a magnetic field (remember that it’s called electromagnetism for a reason). We see this on a very small scale, like when we were in science class and we connected a battery to a copper wire, which generated a small magnetic field that you can watch shift around iron filings. We can also see this same effect on a very large scale in planets like Earth. The very dense outer core of our planet churns with liquid hot iron and nickel which generates a magnetic field that reaches tens of thousands of kilometers into space.
The Sun’s dynamo is similar. However, instead of liquid metal in the core, the conductive material is the plasma (superheated gas) churning around in the Sun’s interior. This matter is so hot from the immense pressure and thermonuclear fusion in the core, that the electrons are ripped from their atoms which electrically charges them, hence the generation of magnetic energy.
That being said, it’s not enough to simply generate the magnetic energy. It requires complex motion.
The Sun rotates just like planets do but because it is not a solid body, it rotates faster at the equator than it does at the poles. This is called differential rotation. Because plasma is flowing at different speeds, the magnetic field generated by it becomes stretched like a rubber band. The more the Sun rotates, the more exaggerated this becomes until the magnetic lines are wrapping around the Sun’s equator within its interior. This is often called the omega effect. The stretching of the magnetic field in this way also serves to strengthen and amplify it.
As you may have guessed, the omega effect is paired with another, called the alpha effect. The plasma within the Sun’s interior is extremely hot and regularly undergoes convection with hotter plasma rising to the surface, then cools and sinks back down. Combined with the Sun’s rotation, this causes the magnetic field to twirl into coiling loops, resembling a helix. The flow of these loops can even realign the magnetic field into a poloidal direction (pole-to-pole movement). This is similar to the Coriolis effect we see in Earth’s atmosphere.
The interplay between the omega and alpha effects are fundamental to sustain the Sun’s dynamo as well as its cyclical complexity, which we will get into shortly.
The Sun’s differential rotation and plasma convection both take place within the convection zone, the outermost layer of the Sun’s interior. For this reason, the magnetic field is often said to begin at a narrow region at the bottom of the convection zone called the tachocline. Below this region, towards the Sun’s center, rotation seems to become significantly more uniform in speed.
A model of the Sun’s magnetic field at a speed of a few months per second. The green and purple lines represent north and south polar magnetic lines while the white lines represent more localized loops of magnetic variation that arc from the surface. Notice how the magnetic lines bunch up around sunspots.
Video credit: NASA Goddard
Imagine a magnetic field as a series of lines that protrude from within the Sun, arc out in every direction around it, then converging back in through the other side. Keep in mind that magnetic fields are, just like gravity, an invisible force but nonetheless can be detected by looking at their effects in and around the object that spawns them. (NOTE: We actually can see the magnetic lines in the Sun’s corona region during solar eclipses.) Also, the Sun’s magnetic field is not static but always stretching and contorting and shifting which reflects the constant flow of the material that generates it. In fact, magnetic fields that are generated through a dynamo process create a feedback loop in which that magnetic field interacts with and manipulates the Sun’s own internal material and currents.
Sunspot image credit: The Royal Swedish Academy of Sciences/The Institute for Solar Physics
Solar flare image credit: NASA/SDO
Coronal mass ejection image credit: NASA/GSFC/SOHO/ESA
These movements of the Sun’s magnetic field are responsible for much of the activity we see on the surface. Sunspots, for example, are temporary dark regions where the magnetic field inhibits convection. This makes the area cooler, and thus darker. As the magnetic field twists in and out of shape, it builds up magnetic energy like a spring until eventually the energy is released, violently. An explosive burst of energy called a solar flare ejects plasma out into space. A larger scale version of this is the coronal mass ejection (CME) in which a gargantuan bubble of gas erupts from the surface. This phenomenon can move much slower but carry significantly more solar material.
Ultraviolet images taken by the Solar Dynamics Observatory showing the Sun at solar minimum (left, December 2019) and solar maximum (right, April 2014). Image credit: NASA’s Goddard Space Flight Center
The Sun gradually ebbs and flows in its level of solar activity in a cyclical way. Sometimes the Sun appears as like a serene lake, relatively speaking, and other times it is a raging maelstrom, constantly bombarding space with plasma energy. This is the result of the Sun’s solar cycle, or more accurately called the sunspot cycle. This pattern was first formally described by German astronomer Samuel Schwabe in 1843 and scientists have learned much about this process in the over 180 years of observations since.
These changes happen in regular intervals of about 11 years (though can vary between 8 and 14 years), starting at a state called solar minimum. This is the point at which the Sun’s magnetic field is relatively stable, triggering little to no sunspots, solar flares or coronal mass ejections. What sunspots do appear throughout the year are small, brief and hugging the equator. Solar flares and coronal mass ejections are weaker and occur maybe a few times a month.
Over time, the internal convection of plasma in the Sun’s interior and the differential rotation stretches and twists up the magnetic field (remember the alpha-omega effects). This gradually builds up magnetic energy and increases the presence of sunspots, solar flares and coronal mass ejections.
The Sun eventually reaches its point of highest solar activity. This is called solar maximum and is driven by the magnetic field erratically contorting and tangling itself up. Sunspots besiege the solar surface, many of them bigger than the Earth, and they tend to curiously drift away from the equator toward higher mid-latitudes. Because these sunspots are where the magnetic energy is most concentrated, this also tends to be where it is released which causes solar flares and coronal mass ejections. At solar maximum, these explosive events can become fifty times more frequent, able to occur multiple times per day. They push out large quantities of plasma which barrage the Earth and its magnetic field, triggering geomagnetic storms.
This can have some cool and not-so-cool effects. For one thing, this can disrupt and damage navigation and communication satellites and even alter their orbits. The induction of electric currents in power lines can overload transformers, ultimately leading to power grid failures and damage. The increased radiation can be dangerous to spacewalking astronauts. On the plus side, geomagnetic storms can interact with our atmosphere and trigger aurorae, beautiful displays of light caused by the excitation of electrons in the atoms by solar winds. When these storms are particularly powerful, aurorae become visible at lower latitudes than usual. Even down here in Maryland as many of us have seen!
Model showing the Sun’s magnetic fields at three different years: Left (1997) Middle (2003) Right (2013). The magnetic poles have reversed.
Image credit: NASA Goddard
Something else very interesting happens around solar maximum. Think of the Sun’s magnetic field like a bar magnet with a north and a south pole. At solar minimum, these two poles are quite distinct but as the Sun approaches solar maximum, the magnetic field becomes more complex and twisted and the separation between the poles becomes less and less clear.
Finally, as it nears solar maximum, the instability of the magnetic field triggers a massive change. The north and south poles flip over, reversing their respective positions. Not instantaneously, mind you, but over the span of a year or two, but sometimes five years (which is the blink of an eye on an astronomical timescale). We humans can barely see it happening so don’t mistake this as some dangerous cataclysm waiting to happen. The last magnetic flip occurred between 2012 and 2013.
The geomagnetic storms are not a direct result of this flip. Rather, the magnetic reversal and the geomagnetic storms are both a product of solar maximum. It remains a mystery to scientists why the magnetic poles reverse under these conditions. We simply don’t have a consistent mathematical model to describe the solar cycle process in full. What we do know is how to predict it, sort of, largely by keeping tabs on the presence of sunspots.
The Sun’s magnetic field emerges from this reversal unwound and weakened. Solar activity slows. Sunspots all but disappear and the frequency of solar flares and coronal mass ejections plummet. Eventually, the sun returns to solar minimum. At this point, the sunspot cycle is complete, and another begins. Given another roughly 11-year cycle and the magnetic poles will once again flip back to their original orientation which marks the end of what’s called a Hale cycle, or the magnetic cycle, which simply consists of two consecutive 11-year sunspot cycles.
A Swiss contemporary of Schwabe, named Rudolph Wolf, was inspired by his theory enough to pour over data on past sunspot observations going back to Galileo and retroactively identified the first recognizable solar cycle in 1755. We are currently at peak solar activity in the middle of the 25th solar cycle. We should expect sunspots, solar flares and coronal mass ejections as well as the geomagnetic storms and aurorae that they spawn to be common over the next few years.
One last interesting fact to wrap this up: As said before, these cycles can vary in longevity, but they can also range in severity, from cycle to cycle. It is here that scientists have found yet another pattern. The intensity of a solar cycle seems to be inversely proportional to its duration. In other words, cycles with overall stronger solar activity reach solar maximum faster than cycles with overall weaker solar activity. This is called the Waldmeier effect after the scientist who first described it. Solar cycle 25 has been found to be slightly stronger than the previous cycle.