One of the greatest questions in solar astronomy may have an answer after more than 400 years, thanks to an inquisitive team of German researchers. Every eleven years, the population of sunspots seen on the surface of our local star reaches a maximum, before dying out. Another population of sunspots then begin to appear (this time with their poles reversed from the previous cycle) before they too peak and fade away. This process may be well-known, but the reason for these 11-year peaks has remained a mystery, until now.
The magnetic field of the Sun may be affected by the gravitational forces of Venus, Earth, and Jupiter, resulting in the cyclical sunspot cycle, a new study suggests. Researchers compared solar cycles to the positions of planets, finding the gravitational forces of these three worlds acts like a cosmic clock, regulating the solar cycle.
“There is an astonishingly high level of concordance: what we see is complete parallelism with the planets over the course of 90 cycles. Everything points to a clocked process,” explained Frank Stefani of the German-based research institute Helmholtz-Zentrum Dresden-Rossendorf (HZDR).
You Missed a Spot Right There
Sunspots were first clearly seen between the years 1610 and 1611, in the years following the invention of the telescope. Although Galileo is often given credit for the discovery, several pioneering astronomers of the era reported finding the distinctive dark spots on the Moon around the same time.
The publication of the first paper recognizing these features, by Dutch astronomer Johannes Fabricius, shocked the zeitgeist of early 17th Century society, which always held a belief in a perfect, unchanging, featureless Sun.
“At the time, people believed the sun was an inviolate, unchanging, perfect body. What people like Fabricius and Galileo did was show that these spots traveled around the surface and that the sun rotated,” describes solar physicist Keith Strong of NASA’s Goddard Space Flight Center.
Everybody Line up!
The greatest gravitational force of planets on the Sun occurs once every 11.07 years, when Venus, Earth, and Jupiter come into alignment. Gravitational pull from this arrangement results in tidal forces on the Sun, similar to the way our own Moon draws oceans upward, creating tides.
This effect is not strong enough to affect the interior of our stellar companion, so the timing of this alignment was previously overlooked in earlier studies of sunspot cycles. However, a physical effect known as the Tayler instability is capable of altering the behavior of conductive liquids or a plasma.
The Tayler instability alters the rate of flow of material (the flux) in an object, like the Sun, and can affect magnetic fields. This effect can be triggered by relatively small movements in materials like the plasma found at the surface of the Sun. Due to this effect, these relatively minor tidal forces can alter the relationship of sunspots to their direction of travel. This measurement, known as the helicity of a region of plasma, alters the solar dynamo (the physical process which generates the magnetic field of our parent star).
The Sun, thus seated, by Mechanic Laws, The Earth, and every distant Planet draws;
By which Attraction all the Planets found, Within his reach, are turn’d in Ether round.
— Richard Blackmore, In Creation: A Philosophical Poem in Seven Books
“Magnetic fields are a little like rubber bands. They consist of continuous loops of lines of force that have both tension and pressure. Like rubber bands, magnetic fields can be strengthened by stretching them, twisting them, and folding them back on themselves. This stretching, twisting, and folding is done by the fluid flows within the Sun,” The Marshall Space Flight Center explains.
Stefani had his doubts whether or not tidal forces from the planets could alter an event as powerful as the solar dynamo. However, once he realized the Tayler instability could provide the trigger for the process, Stefani and his team began developing a computer simulation to model the process.
“I asked myself: What would happen if the plasma was impacted on by a small, tidal-like perturbation? The result was phenomenal. The oscillation was really excited and became synchronized with the timing of the external perturbation,” Stefani explains.
Sun, Spot, Sun!
The motion of the sun is complex, with multiple effects contributing to its intricate dance. As the sun rotates, the equator moves faster than the material near the poles. In a process known as the omega effect, lines of the sun’s magnetic field are pulled and stretched near the equator, creating a bend in the direction of the solar equator.
A little-understood alpha effect then affects the magnetic lines, pushing them toward their original alignment, resulting in a twisting of the lines of force.
These actions create the cool, dark areas we know as sunspots. While most of the surface of the Sun glows around 5,500 degrees Celsius (9,900 Fahrenheit), sunspots remain at a relatively cool 3,200 Celsius (5,800 Fahrenheit). Sunspots are still fairly bright, only appearing dark against the torrid backdrop of the solar surface.
This new model, folding tidal forces into the complex processes of the solar dynamo, could explain several questions astronomers and physicists have about the solar dynamo, and how it affects our parent star.
The Parker Solar Probe is currently in orbit around the Sun, in a mission to study our stellar companion up close. This program could answer a multitude of mysteries concerning Sun over the next few years.