What Is The 11 Year Solar Cycle?
The solar cycle is the periodic, approximately 11-year variation in activity on the Sun’s surface. The solar cycle is caused by the Sun’s magnetic field flipping polarity, with the north pole becoming the south pole and vice versa every 11 years (https://www.space.com/solar-cycle-frequency-prediction-facts). This cycle of magnetic field reversal results in an ebb and flow of sunspot activity, solar flares, and ejections of solar material.
Understanding the solar cycle is important because it impacts space weather and can affect satellite operations, communications systems, power grids and other infrastructure on Earth. Periods of maximum solar activity, known as solar maxima, can increase radiation exposure for astronauts and spacecraft. The cycles also influence the amount of ultraviolet radiation reaching Earth. Scientists study the solar cycle to help predict space weather events and their potential impacts.
History of Discovery
The Sun’s approximately 11-year cycle of activity was first discovered in 1843 by Heinrich Schwabe, an amateur astronomer in Dessau, Germany. Schwabe had been observing sunspots on the solar surface from 1826 to 1843 and noticed a periodic variation in the number of sunspots over time [1].
This cycle became known as the Schwabe cycle, and it was not until 1851 that astronomers Rudolf Wolf and Alfred Sabine recognized that the cycle was approximately 11 years long. Wolf and Sabine developed a formula to quantify the number of sunspots and established the Wolf number, which continues to be used today as an index of solar activity [2].
Later astronomers studied past records of sunspots and determined that the approximately 11-year solar cycle has been taking place for centuries with varying degrees of activity. The periodic nature of the solar cycle pointed to an underlying mechanism inside the Sun driving the cycles.
Cause of the Solar Cycle
The 11-year solar cycle is caused by changes in the Sun’s magnetic field. The Sun is made up of hot electrically charged gas, known as plasma. This plasma flows and churns inside the Sun, generating the Sun’s powerful magnetic field through a process called the solar dynamo. The solar dynamo arises from two main processes inside the Sun: convection and rotation.
Convection occurs as hot plasma rises from the solar interior to the surface. As it rises and cools, it sinks back down again, creating convection currents. This churning, bubbling plasma helps generate and strengthen the magnetic field. Meanwhile, the Sun’s rotation twists and coils the magnetic field lines. This twisting action builds up the magnetic field and stores up magnetic energy over time.
Every 11 years or so, this stored magnetic energy reaches a peak point and the magnetic field lines get tangled and burst apart, flipping the poles and marking the solar maximum. The solar magnetic field then gradually builds up again until it flips back once more after the next 11 years, continuing this solar cycle (https://spaceplace.nasa.gov/solar-cycles/en/). This solar dynamo of convection, rotation, and flipping magnetic field lines powers the solar cycle and associated sunspot activity.
Tracking the Solar Cycle
The solar cycle is tracked by monitoring the number of sunspots visible on the Sun. Sunspots are areas of intense magnetic activity that appear darker than the surrounding surface. The number of sunspots visible on the Sun rises and falls in an approximately 11-year cycle. During solar maximum, there are high numbers of sunspots, and during solar minimum there are low numbers of spots.
Tracking is conducted by a network of ground-based solar observatories and space-based telescopes. Some of the major observatories involved in monitoring solar activity include the Solar Optical Observing Network and the Global Oscillation Network Group. These facilities conduct daily observations of the Sun and record properties like sunspot numbers, solar flares, and coronal mass ejections. Long-term records of solar data allow scientists to study solar cycles going back hundreds of years.
Sunspot counts provide a quantitative way to identify when the Sun transitions between solar maximum and minimum. According to NASA, sunspot counts over 100 indicate solar maximum conditions, while counts below 10 indicate minimums (NASA). By keeping detailed records of sunspot numbers over time, the solar cycle can be predicted and tracked.
Impacts on Space Weather
The changes in the Sun’s activity during the solar cycle strongly influence space weather. During solar maximum, when sunspots and solar flares are more common, the Sun emits more ultraviolet radiation and can produce more frequent Coronal Mass Ejections (CMEs) – large expulsions of plasma and magnetic field from the Sun’s corona.
These CMEs and increased radiation lead to more disturbances in the near-Earth environment or space weather. One major effect is an increase in geomagnetic storms. According to NOAA, geomagnetic storms can cause “disruption of radio communications, GPS errors, power outages, and aurora, at high latitudes” [1].
Solar activity and space weather also pose hazards for satellites, astronauts and technologies in space. Increased radiation from solar flares can damage satellite electronics and solar panels. CMEs and energetic particles can degrade spacecraft materials and endanger astronauts by penetrating protective shielding.
Impact on Earth’s Climate
The solar cycle has a small but observable impact on Earth’s climate through its effects on cosmic rays, cloud formation, UV radiation, and total solar irradiance. During periods of increased solar activity like solar maxima, the increased strength of the solar wind blocks more galactic cosmic rays from entering Earth’s atmosphere. Cosmic rays interact with atmospheric molecules to promote cloud formation, so reduced cosmic rays can lead to less cloud cover and warmer temperatures on Earth (NASA). Higher solar activity also increases UV radiation reaching Earth’s surface, which can cause ozone depletion and interact with the climate.
Total solar irradiance, the amount of solar energy reaching Earth, varies by about 0.1% over the course of the 11 year solar cycle. This fluctuation in incoming radiation is too small to be the main driver of climate change, but likely contributes a slight warming during solar maxima (NSF). While not the primary factor influencing Earth’s climate, the solar cycle does have measurable short-term effects through changes to cloud cover, UV exposure, and total sunlight.
Predicting Upcoming Solar Cycles
Current predictions for Solar Cycle 25, which started in December 2019, indicate that it will peak between January and October 2024 with a maximum sunspot number between 137 and 187 (the average is 179) according to NOAA. This means Solar Cycle 25 is expected to peak faster and stronger than the previous cycle.
Looking beyond Cycle 25, predictions become more uncertain but scientists expect the cycles to continue. NASA models predict Cycle 26 will peak around July 2031 and Cycle 27 around October 2038.
However, accurately predicting cycles more than a few years in advance is difficult. Factors like changes in the Sun’s polar magnetic fields can alter cycle strength and timing. Forecast models continue to improve but still have large uncertainties. Surprises like the historic Maunder Minimum where cycles essentially stopped are always possible.
The Maunder Minimum
The Maunder Minimum refers to a prolonged period of very low solar activity between the years 1645 and 1715. This period coincided with severely cold winters in the Northern Hemisphere, which became known as the “Little Ice Age”. The Maunder Minimum is named after English astronomer Edward W. Maunder, who discovered the dearth of sunspots during this time.
During the Maunder Minimum, very few sunspots were observed. According to one reconstruction of solar activity, an average of only about 50 sunspots appeared per year, instead of the typical 40,000-50,000 sunspots per year. This low sunspot activity correlated with a decrease in solar irradiance and solar wind strength.
The cause of the Maunder Minimum is not fully understood, but may have been related to changes in the Sun’s large-scale magnetic field. Some scientists have proposed that reduced convection inside the Sun led to a cessation of the solar dynamo that generates the magnetic field and sunspots.[1]
On Earth, the Maunder Minimum coincided with an abnormally cold climatic period. River Thames in London froze over repeatedly, glaciers advanced in the Alps, sea ice expanded, and agricultural growing seasons were impacted across Europe and North America. The correlation between low solar activity and cold winters provided early evidence of the Sun’s influence on climate.[2]
While the impacts were regional, not global, the Maunder Minimum provides an example of how changes in solar output can influence Earth’s climate on decadal to centennial timescales.
[1] https://www.aanda.org/articles/aa/full_html/2015/09/aa26652-15/aa26652-15.html
[2] https://en.wikipedia.org/wiki/Maunder_Minimum
Other Stars with Cycles
Research shows that stellar magnetic activity cycles are common in Sun-like stars, though the periods of these cycles vary. A study by Oláh et al. analyzed 1095 solar-type stars observed with the Kepler spacecraft and found that 60% of them showed activity cycles like the Sun’s 11-year cycle (Oláh et al., 2016). The lengths of these cycles ranged from 2.5 to 25 years.
Evidence of stellar cycles has also been seen in other types of stars as well. Older, evolved stars tend to have longer cycles, such as the 4.5-year cycle observed in the evolved subgiant Epsilon Eridani (Oláh et al., 2016). Shorter cycles around 1-2 years occur more frequently in younger, faster-rotating stars.
While the Sun’s 11-year cycle is fairly well understood, there are still many open questions about what factors determine the lengths of stellar cycles and why they vary between stars. Further long-term observations and modeling will help reveal more insight into the magnetic evolution of stars across cosmic time.
Future Research
While scientists have made significant progress in understanding solar cycles, many open questions remain. Some key areas for future research include:
Improving models and predictions – Current models still have uncertainty in predicting the strength and timing of future solar cycles. Work is ongoing to incorporate more influencing factors like the Sun’s magnetic field to make predictions more accurate (source).
Understanding cycle-to-cycle variability – While the average solar cycle length is 11 years, some cycles last 9 years while others go on for over 12 years. Research aims to uncover why this variability occurs.
Studying the relationship between solar cycles and climate – There are indications that solar cycles impact Earth’s climate and weather patterns, but the exact mechanisms are still being explored.
Developing new technologies – Next-generation solar telescopes like the Daniel K. Inouye Solar Telescope and space observatories will provide unprecedented detail on solar dynamics and activity. This can shed light on the underpinnings of the solar cycle.
By advancing these research fronts, scientists hope to gain a comprehensive picture of the solar cycle, allowing better prediction and understanding of the Sun’s impacts on Earth and space weather.
