In drawings and paintings the sun often appears as a perfect circle of light, projecting its energy equally in all directions. However, if we look closer we observe occasional eruptions in specific directions. Solar eruptions occur when the energy stored in the sun’s magnetic field discharges suddenly, rapidly heating matter in its vicinity to a temperature of millions of degrees. This results in a release of a vast amount of energy across the entire electromagnetic spectrum: from gamma rays to x-rays and to radio waves. While our atmosphere shields us from harmful radiation, when these emissions of radiation reach Earth, they can disrupt media broadcasts. But this brings us to an intriguing question - why does the sun have a magnetic field?
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Scalding Magnets
When gas reaches extremely high temperatures, it transforms into plasma - an ionized gas state. During the ionization process electrons are released from the gas atoms, leading to the formation of free particles with an electric charge: negative electrons and positive atom nuclei. The laws of electrodynamics teach us that when a charged particle is in motion it generates a magnetic field around it and conversely, a charged particle within a magnetic field will be propelled by it and will create an electric current. In fact, this principle forms the basis of the dynamo generator.
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A simulation of the Parker spacecraft navigating close to the Sun
(Illustration: NASA)
The sun is a giant plasma sphere that rotates on its axis. The surface temperature on the sun is lower than the temperature within it. The temperature differences cause hot matter from deeper layers to rise while colder matter from the outer layers descends - a process known as convection, which is also responsible for the swirling motion of boiling water in a kettle. The complex motion of the charged plasma particles, driven by convection and the sun’s axial rotation generates magnetic fields, which in turn have an effect on the motion of the charged particles. The complex motion that is caused by the combination of convection, the sun’s spin - which is different at varying speeds across latitude lines on its surface - and the local magnetic fields contribute to the intricate dynamics of the sun's magnetic field. In fact, the depth at which the sun's magnetic field originates remains a subject of ongoing research.
A Spotted Sunray
Solar eruptions typically occur in “active areas”, where the sun’s magnetic field is especially strong and its shape is warped. In these areas “sunspots” often develop, which are akin to dark spots that appear on the sun’s surface for a brief time and then vanish. In the spot areas, the distortion in the magnetic field halts the convection from the deeper layers outwards, leading to temperature decreases and the appearance of darker hues.
A video by NASA detailing the changes in the sun’s magnetic field:
Three Millenia Of Sun Monitoring
The first documented observation of sunspots dates back to the Chinese “Book of Changes” ( I Ching), written over 2,800 years ago. Starting from the 28th century B.C. Chinese astronomers methodically documented sunspots.
The first recorded report of sunspots in the Western world was by the Greek philosopher Theophrastus, around 300 B.C. The first to observe sunspots through a telescope - or atleast, the first to document it, was English astronomer Tomas Harriot in December 1610. In 1775, Danish astronomer Christian Horrebow noticed that sunspot numbers and sizes change in a cycle of a few years. In an article published in 1801, British astronomer William Herschel claimed to have found a link between the number and shape of sunspots and the temperature measured on the surface of the Earth, and even between the number of sunspots and wheat prices in Britain. However, these claims were later disproven. In 1843 German astronomer Heinrich Schwabe presented a clear cycle in the average number of sunspots, while in 1852 Swiss astronomer Rudolf Wolf established a numbering system for solar cycles, dating back to mid-18th century, with the cycle that began in 1755 considered as the first cycle in the count. The average duration of a solar cycle is approximately eleven years and it is defined as the period between two periods of low solar activity.
By analyzing the frequency of the carbon 14 isotope in tree rings, in 2004 scientists successfully reconstructed solar activity levels for the past 11,400 years, since the last ice age. The researchers found that during ninety percent of this prolonged period, the level of solar activity was lower than that observed between the 1940’s and the early 2000’s, and nearly all previous periods of enhanced activity were shorter than this period. In the last solar cycle, cycle number twenty four, that lasted from December 2008 to December 2019, solar activity was significantly lower than what was considered usual in recent decades, similar to the levels documented between the late 19th century and the early 20th century.
Is there a link between solar activity levels and global warming on Earth? Most researchers believe that the sun’s contribution to the observed warming trend in recent decades is insignificant. For instance, despite Earth experiencing a gradual warming since the mid-20th century, the intensity of solar radiation reaching us has remained relatively stable since at least the mid-19th century. Moreover, if solar activity were the main driver of global warming, we would expect warming across all layers of the atmosphere, whereas the upper atmosphere is actually exhibiting a cooling trend.
When Solar Wind Reaches The Earth
Aside from eruptions and sunspots, the activity of the sun's magnetic field can also lead to coronal mass ejections. The corona is a type of very hot aura surrounding the sun, which is mainly composed of ionized hydrogen. Due to the high temperature of the corona, it continuously releases charged particles into space, creating a phenomenon known as “solar wind”. These charged particles are attracted to Earth's magnetic poles and can become trapped in Earth's magnetic field near the poles. When these charged particles collide with molecules in the upper layers of the atmosphere, a typical light is emitted, creating the phenomenon we know as the northern lights (Aurora Borealis)
When the sun’s magnetic activity intensifies solar wind, it may lead to a geomagnetic storm - a temporary disturbance in the Earth’s magnetic field. In extreme cases such storms can disrupt satellite operations, high-voltage power lines, media broadcasts, radar and navigation systems, and more.
Thanks to Earth’s atmosphere and magnetosphere, life on Earth is relatively shielded from the effects of geomagnetic storms. However, astronauts or individuals flying at high altitudes, outside this protective envelope or in its outskirts, could experience severe medical implications, such as formation of mutations leading to cancer, following exposure to geomagnetic storms. Some argue that such storms also affect animals relying on Earth’s magnetic field for navigation and migration, though research in this area is still in its infancy.
It’s Always The Sun
The current solar cycle, number twenty-five, commenced in December 2019 and is projected to last until 2030. While most projections anticipate it to be a relatively weak cycle, similar to the previous one, observations from its first three years suggest higher-than-expected activity levels, albeit still lower than those of cycles that occurred in the 20th century. Thus, for example, two notably strong eruptions occurred during recent August. Nevertheless, the peak of solar activity is still ahead of us, and is anticipated to occur toward the end of 2024 or during 2025.
While researchers in the field generally agree that solar activity levels have little effect on global warming, solar cycles can still impact our daily lives through disruptions to communication and navigation systems and damage to high-voltage power lines. Beyond its practical significance, the study of solar activity holds great theoretical importance. Continued observation and monitoring of the sun and its magnetic activity will deepen our understanding of the mechanisms underlying the formation and development of the sun's magnetic field—and that of other stars in the universe.