An artist’s rendering of the heliosphere. Credit: Conceptual Image Lab at NASA’s Goddard Space Flight Center

A bubble around the system

The solar wind persistently transports charged particles in every direction. At a certain distance, they come into contact with the material of interstellar space. This process results in the formation of the heliosphere — a vast shell within which all the planets are situated.

Alongside the Sun, this bubble orbits the nucleus of the Milky Way galaxy. Over approximately four and a half billion years, it has traversed numerous regions within the galaxy. Currently, its boundary is situated at a distance approximately 120 times farther from our star than Earth’s orbit, and it was at this location that the Voyager 1 spacecraft crossed in 2012.

Three encounters with clouds

A team led by Merav Opper of Boston University reconstructed the heliosphere’s past trajectory using computer modeling. The results were published in the peer-reviewed journal Annual Review of Astronomy and Astrophysics.

Based on these calculations, over the course of several million years, the Solar System has traversed extremely cold clouds of gas and dust on at least three occasions. The pressure exerted by such clouds has compressed the heliosphere beyond Earth’s orbit, resulting in a temporary loss of planetary protection from the interstellar medium, as reported by NASA.

Traces in bottom sediments

The modeling indicates three periods: approximately two to three million years ago, six to seven million years ago, and thirteen to fourteen million years ago.

These dates align with geological data. Chemical elements characteristic of interstellar dust have been identified in deep-sea sediment cores, Antarctic snow, and lunar samples of precisely this age.

In the calculations, contact with a dense, cold cloud of hydrogen increased the concentration of water vapor in the atmosphere and altered processes in the upper layers of the atmosphere. The effects gradually reached the surface and, as a result, may have influenced past climate fluctuations, particularly ice ages. 

The paradox of the faint young Sun

Approximately three billion years prior, our Sun emitted merely 70 percent of its current luminosity. Under these illumination conditions, the Earth would have been entirely frozen, as reported by ScienceDaily.

A star, akin to the Sun, commences its lifecycle as a main-sequence star, subsequently progressing through the subgiant and giant phases. Following this, it expels its outer layers, resulting in the formation of a planetary nebula. Credit: ESO, S. Steinhöfel.

Geological evidence indicates otherwise. Stable liquid water existed on the planet well before that period. This discrepancy is recognized as the “weak young Sun” paradox. 

An experiment with protons

Astrophysicist Volodymyr Airapetian of NASA’s Goddard Space Flight Center has highlighted research concerning young stars analogous to the Sun. Data obtained from the Kepler space telescope indicate that these stars produce exceptionally intense flares daily and release streams of high-energy particles.

The team replicated the environment of ancient Earth within a sealed chamber by combining molecular nitrogen, ammonia, carbon dioxide, and carbon monoxide. The mixture was subjected to proton bombardment to emulate the effects of solar flares. This process yielded nitrous oxide, a greenhouse gas that is 300 times more potent than carbon dioxide. The research was documented in the peer-reviewed journal The Astrophysical Journal Letters.

Certain molecules were eradicated by intense ultraviolet radiation. Nevertheless, calculations indicated that even a tenth of these molecules would suffice to raise the temperature of the equatorial regions by five degrees Celsius above zero. Moderately elevated temperatures, marginally above the freezing point, were found to be even more favorable for the formation of lengthy chains of amino acids than higher temperatures.

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 Sun in October 2019 during a minimum in activity (left) and in April 2014 during a previous maximum (right). Images courtesy of the Solar Dynamics Observatory. Credit: NASA’s Solar Dynamics Observatory/Joy Ng

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. At the beginning of the cycle, solar activity gradually increases. More and more spots and active regions appear on the surface of the Sun, its magnetic field becomes more complex, the movement of hot plasma on the surface becomes more turbulent, and, as a result, flares and ejections of matter into space become more frequent. Particles and magnetic structures from some of these events reach the  Earth and interact with its magnetosphere. This can cause geomagnetic storms, create problems for satellites and radio communications, and the auroras become brighter and can be visible even at unusually low latitudes. After the maximum, activity gradually decreases. The number of sunspots, flares, and coronal ejections on average becomes smaller, and the Sun moves to solar minimum. Then the cycle begins again. And so on from cycle to cycle. 

A graph illustrating sunspot numbers, accompanied by a forecast for Cycle 24, produced in 2011. Author: David Hathaway. Source: NASA

This rhythm was noticed as early as the 19th century, although astronomers had observed sunspots themselves much earlier. One of the earliest dated records belongs to Johannes Kepler: in 1607 he observed the Sun using a pinhole camera. However, Kepler mistakenly thought that the dark spot on the Sun’s disk was Mercury passing in front of the Sun. It was not until 1843 that the German astronomer Heinrich Schwabe drew attention to the approximately 10-year periodicity of changes in the number of sunspots. Later, the period was refined to approximately 11 years. The explanation for this cycle is related to the Sun’s magnetic field, which changes along with the motion and differential rotation of the plasma inside the star. In the early 20th century, astronomers finally had the opportunity to directly study the magnetic fields of sunspots – something that astronomers of the past could not investigate without the instruments available to us today.

Sketches of sunspots published by Johannes Kepler in his book “Phaenomenon Singulare Seu Mercurius In Sole” in 1609.
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 esteeme  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. The Sun, of course, does not have a regular solid surface: the photosphere consists of very hot plasma. This is a state of matter in which the gas is largely ionized: along with neutral particles, it contains charged ions and free electrons. It is this mobile electrically conductive substance that forms the complex magnetic field of the Sun. Plasma conducts electric current well, and the magnetic field moves with it, as if frozen inside. This connection is two-way: the flows of matter seem to drag the magnetic field along with them, and a sufficiently strong field, on the contrary, slows down their movement.

The plasma exhibits variable rotational velocities across different latitudes. The equatorial zone completes a complete rotation in about 25 days, while at high, near-polar latitudes the rotation takes about 35 days. This difference in speed gradually deforms the Sun’s poloidal magnetic field: its field lines stretch in the direction of rotation and increasingly wind around the Sun, forming a toroidal component of the magnetic field. This part of the solar dynamo mechanism is called the omega effect.

A schematic representation delineating the three phases of the Sun’s magnetic field evolution throughout a cycle. On the left, the magnetic field extends longitudinally from pole to pole. In the central depiction, the field exhibits twisting along the parallels due to differential rotation speeds. On the right, the magnetic field appears entangled, with loops protruding outward and dark spots forming at their bases.

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 areas where the magnetic field is particularly strong, it affects the movement of the plasma and can make a magnetically saturated region buoyant. It rises through the Sun’s mass, carrying with it the magnetic flux. Near the surface, the magnetic field can emerge in arcs and loops, while their ends remain submerged in the plasma. A strong magnetic field suppresses the convective transfer of energy from the depths, so where such a loop emerges, the surface becomes colder than the surrounding area. This is how a sunspot appears. The more complex and tangled the field becomes, the more often loops of opposite direction come together and reconnect. During such magnetic reconnection, some of the stored magnetic energy is quickly released. This can cause powerful solar flares, and under certain conditions, coronal mass ejections, when huge amounts of plasma are ejected into interplanetary space.

A composite image of an active region on the Sun, captured by the Solar Dynamics Observatory. Credit: NASA/SDO

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. 

A collection of images of the Sun captured during the initial year of the Daniel K. Inouye Solar Telescope’s operation, released on May 19, 2023. Credit: NSF/AURA/NSO

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.

An eruptive prominence observed on August 31, 2012, that initiated a coronal mass ejection. A composite image captured in two ultraviolet bands. Credit: NASA’s Goddard Space Flight Center/SDO

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.

The apex of Solar Cycle 25 commenced in 2024, during which time the most intense solar flares occurred with extraordinary regularity. Credit: NASA/SDO/AIA/LMSAL

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.