Once every eleven years, the Sun exhibits a marked increase in activity before reverting to a more quiescent state. We will elucidate the origins of this cyclical rhythm and the reasons why predicting the behavior of our star remains challenging.
The deceptive constancy of our star
Every morning, the Sun ascends beyond the horizon, mirroring its emergence from a millennium ago. It appears unchanging to us; however, in actuality, its activity exhibits cyclical variations approximately every eleven years. Initially, its activity increases over about four consecutive years. During this phase, the Sun’s magnetic field becomes increasingly twisted, the movement of the incandescent plasma on its surface intensifies, leading to more frequent solar flares and eruptions of matter into space. Some of these phenomena reach Earth, thereby affecting satellite operations, while the auroras become more luminous and can be observed at atypically low latitudes. Subsequently, over seven years, all activity gradually diminishes to a minimum, perpetuating the cycle anew.
This pattern was identified as early as the 19th century, although sunspots had been observed much earlier, with the earliest known depiction dating to 1607, by Johannes Kepler. Their quantity exhibited regular fluctuations; however, the underlying cause of this phenomenon remained unidentified for nearly a century. It is rooted in the Sun’s magnetic field, which astronomers of earlier periods were unable to detect without the advanced instruments available today.
Credit: Johannes Kepler
The accidental discovery of the solar cycle
In 1826, German amateur astronomer Samuel Heinrich Schwabe initiated a search for an elusive hypothetical planet thought to orbit closer to the Sun than Mercury. To prevent missing its transit across the Sun’s disk, he meticulously documented all dark spots observed through his telescope daily. Although he did not discover the planet, he amassed seventeen years of consistent observations, employing the same method throughout, and discerned a periodic pattern within the data. The number of sunspots fluctuated in a cycle of approximately ten years. Samuel Heinrich Schwabe subsequently published his findings in the early 1840s in the esteemed astronomical journal Astronomische Nachrichten.
The Swiss astronomer Rudolf Wolf undertook the subsequent step. He meticulously examined observatory archives and assembled fragmented observations dating from 1745 — systematic records before that period did not exist. Utilizing this extended series, the interval was revised to eleven years. Additionally, Wolf established the numbering system that remains in use today, commencing with the cycle initiated in 1755. The current cycle is designated as number 25.
The reasons why sunspots became the primary measure of solar activity are evident in the era’s practices. They are observable even with basic telescopes, whereas solar flares were first documented only in 1859, and material outbursts from space were not detected until the 1970s. Sunspots continue to serve as a practical indicator today, although solar radio emissions are now also employed alongside them.
The methodology scientists use to monitor the solar cycle. Source: NASA Goddard Space Flight Center
What is the origin of this rhythm?
Sunspots have been observed since the time of Samuel Heinrich Schwabe, but their true nature was not understood until 1908. At that time, American astronomer George Ellery Hale studied the light emanating from sunspots and discovered characteristic changes in it that occur only in a very strong magnetic field. Since then, it has become clear that sunspots are merely a visible sign, and the cause of the cycle must be sought in the behavior of the magnetic field inside the Sun.
This behavior is atypical. The Sun’s surface is designated as the photosphere — namely, the visible stratum from which light is emitted. It comprises plasma, a state of matter in which atoms have disintegrated into ions and free electrons. Plasma exhibits high electrical conductivity, and the magnetic field propagates in unison with it, as if embedded within. This interplay is reciprocal; matter flows can drag the magnetic field, while a sufficiently intense magnetic field can impede their motion.
The plasma exhibits variable rotational velocities across different latitudes. Specifically, the equatorial zone completes a full rotation in approximately 25 days, whereas the polar regions take about 36 days. Consequently, within the same temporal framework, the plasma at equatorial latitudes undergoes more rotations than that at polar latitudes. Initially, the Sun’s magnetic field extends approximately from pole to pole; however, due to the differential rotation speeds, it progressively bends along the equator and intensifies. This process is referred to as the omega effect.
The second component of the mechanism functions concurrently with the first. In the Sun’s outer layer, hot plasma ascends, cools, and descends, while rotational forces twist these flows similar to the manner in which cyclones are twisted in Earth’s atmosphere. The reversed magnetic field generated by this twisting gradually reverts to its original configuration, with the north magnetic pole occupying the position previously held by the south pole. This process occurs approximately at the peak of solar activity. This phenomenon is known as the alpha effect and remains a subject of considerable debate among researchers. Collectively, these two components constitute the solar dynamo.
The magnetic field beneath the Sun’s surface exhibits an uneven distribution. In regions where the magnetic field is most intense, the surrounding material exhibits reduced density compared with other areas, resulting in buoyant rises that accompany the field. These loops extend outward, with both termini remaining situated deep within the solar interior. Within such loops, the magnetic field is sufficiently strong to inhibit hot plasma from below, leading to a noticeable cooling effect on the surface in those regions. The increased complexity of tangled magnetic fields leads to more frequent interactions between loops traveling in opposite directions, which reconnect and release the accumulated excess energy as solar flares and coronal mass ejections.
The concept of such a mechanism was first proposed by the Irish physicist Joseph Larmor in 1919. However, the theory still does not elucidate why one of its cycles precisely endures eleven years. Individual cycles vary from nine to fourteen years, with an average of eleven. Currently, it remains infeasible to accurately predict either the duration or the intensity of the subsequent cycle.
Why the area is not actually dark
On the Sun itself, sunspots remain the most conspicuous manifestation of this phenomenon, and their designation leads to a common misconception. The term “spot” implies a defect or a void, although in reality it refers to the same incandescent material, but cooler than the adjacent regions. The photosphere is heated to approximately 5,500 degrees Celsius, whereas a sunspot reaches about 3,500 degrees in temperature. Consequently, the material within a sunspot emits roughly one-fifth of the light emitted by the surrounding surface, and this contrast is what renders the sunspot dark.
The smallest sunspots measure approximately 1,600 kilometers in diameter, whereas the largest can extend up to 160,000 kilometers — approximately twelve times the diameter of Earth. The most intense flares and eruptions originate above these substantial sunspot clusters. These phenomena reach us through multiple pathways.
The radiation emitted by the flare arrives at Earth in approximately eight minutes. It remains imperceptible to the human eye due to its predominant emission within the X-ray and ultraviolet spectra. Nonetheless, it ionizes the upper atmospheric layers, resulting in disruptions to shortwave radio communications on the sunlit side of the Earth and impairing the accuracy of satellite navigation as signals traverse the modified ionosphere.
Emissions of this substance propagate at a considerably slower rate, taking several days to reach Earth’s vicinity. They are responsible for magnetic storms and the resulting auroras, which become visible at much lower latitudes than usual under such conditions. Due to abrupt variations in the magnetic field, supplementary currents are induced in extensive power lines, leading to transformer overloads. The upper atmosphere heats and expands as a consequence of the storm, causing satellites in low Earth orbit to encounter a denser environment and consequently lose altitude at an accelerated rate. Additionally, high-energy charged particles can impair satellite electronics, causing issues ranging from isolated memory errors to the gradual deterioration of solar panels. Such events are markedly more frequent during solar maxima than during solar minima.
This remains, however, merely a preliminary guideline. Predicting a specific flare is unattainable, and severe storms have been documented on numerous occasions even during the declining phase, when overall activity was diminishing.
What is currently occurring with the Sun?
The present cycle has demonstrated the considerable challenges associated with making precise predictions. In 2019, a panel of experts from the United States National Oceanic and Atmospheric Administration (NOAA), in collaboration with NASA, forecasted that the peak would occur in July 2025; however, it actually materialized earlier, in the autumn of 2024. Moreover, the predicted intensity was also inaccurate. The forecasted activity index was 115, whereas the actual index surpassed 160. These figures do not directly correspond to the count of sunspots on the solar disk, as the index also considers their groupings. The peak itself is identified by the average value over the year, making its exact date indeterminate until after it has occurred.
Approximately concurrently, the polarity of the Sun’s poles underwent a reversal. Based on measurements from the Japanese Hinode spacecraft, this event likely occurred in October 2024 in the southern polar region, with the northern polar region experiencing the reversal approximately one month later.
The activity is presently declining and is projected to continue decreasing until reaching a minimum around the year 2030. Subsequently, Cycle 26 will commence, and at its peak — approximately in the mid-2030s — the polarity will reverse once more, reverting the magnetic field to its pre-2024 state. It has been observed that the interval between two activity minima is approximately eleven years, as evidenced by the frequency of sunspots and solar flares. During this period, the solar poles undergo a single polarity reversal. To complete another such reversal and return to the initial configuration, an additional cycle is necessary; thus, a complete magnetic cycle is estimated to span approximately twenty-two years.
