Solar cycle
-
What is the solar cycle?
The solar cycle is a roughly 11-year period characterized by fluctuations in the number and size of sunspots and solar prominences. During each cycle, the magnetic polarities of sunspots in the Sun's hemispheres reverse, leading to a complete magnetic cycle of 22 years. While sunspots were observed as early as 1600, the 11-year cycle was not identified until 1843 by Samuel Heinrich Schwabe. Periods of significantly reduced sunspot activity, such as the Maunder minimum, have been linked to cooler periods on Earth, though the exact mechanism is still unknown. Solar cycle 25 began in 2019 and is predicted to reach its maximum in 2025.
4 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How long does a typical solar cycle last?
A typical solar cycle, characterized by fluctuations in sunspots and solar prominences, lasts approximately 11 years. While the magnetic polarities reverse every 11 years, the complete magnetic cycle, where polarities return to their original state, takes 22 years. The current solar cycle, cycle 25, began in 2019 and is predicted to reach its maximum in 2025. Historically, periods of reduced sunspot activity, such as the Maunder Minimum (mid-17th to early 18th century) and the Dalton Minimum (late 18th to early 19th century), have coincided with cooler periods on Earth, though the direct causal link remains unclear.
4 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
What are sunspots, and how do they relate to the solar cycle?
Sunspots are regions on the Sun's surface characterized by intense magnetic fields, appearing darker due to their cooler temperatures compared to the surrounding photosphere. These magnetic disturbances are central to the solar cycle, an approximately 11-year period of fluctuations in solar activity.
The solar cycle involves a rise and fall in the number and size of sunspots. This cycle begins with spots emerging at higher latitudes and gradually drifting towards the equator as activity peaks. The magnetic polarity of these sunspots reverses with each new cycle, leading to a full magnetic cycle of 22 years. Solar flares and coronal mass ejections, powerful releases of energy and particles, are also more frequent during periods of high sunspot activity.
5 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How do scientists measure or observe changes during the solar cycle?
Scientists observe changes during the solar cycle primarily by monitoring sunspots. The number and distribution of sunspots, which are regions of strong magnetic fields on the Sun's surface, follow a roughly 11-year cycle. This cycle is also reflected in variations in solar magnetic activity, including solar flares and coronal mass ejections.
Additionally, the science of helioseismology uses the study of wave frequencies on the Sun's surface to map its internal structure. These frequencies can vary slightly with the sunspot cycle, and by analyzing them, scientists can also map the Sun's internal rotation at different depths. Satellites provide direct measurements of solar irradiance, showing small variations, while historical sunspot data offers indirect measures of solar activity over longer periods.
4 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How does the solar cycle affect Earth’s technology and climate?
The solar cycle, an approximately 11-year period of fluctuations in solar activity like sunspots, significantly impacts Earth's technology and climate. Increased solar activity, including solar flares and coronal mass ejections, can disrupt radio communications and create voltage surges in power lines. These phenomena, collectively known as space weather, can affect satellites, leading to radiation exposure and increased atmospheric drag, impacting their orbital lifetimes. Geomagnetic storms, triggered by solar wind disturbances, can cause widespread power outages by damaging electrical transformers.
While the Sun is the primary driver of Earth's climate, the exact mechanisms by which solar variability influences climate are not fully understood. However, historical periods of reduced solar activity, such as the Maunder Minimum, have been linked to cooler temperatures, like those experienced during the Little Ice Age.
9 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
What are some ways the solar cycle can impact astronauts and space missions?
The solar cycle can significantly impact astronauts and space missions by increasing radiation exposure. During periods of high solar activity, such as solar flares and coronal mass ejections, intense bursts of energetic particles are released from the Sun. These particles can pose a lethal radiation hazard to astronauts outside Earth's protective magnetic field, potentially causing radiation sickness, DNA damage, and increasing cancer risk.
Furthermore, these solar events can disrupt radio communications, affect satellite operations, and even cause voltage surges in long-distance conductors. Space missions, especially those venturing beyond Earth's orbit, require robust shielding and accurate predictions of solar activity to ensure astronaut safety and mission success.
6 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
Solar cycle, period of about 11 years in which fluctuations in the number and size of sunspots and solar prominences are repeated. Sunspot groups have a magnetic field with a north and a south pole, and, in each 11-year rise and fall, the same polarity leads in a given hemisphere while the opposite polarity leads in the other. In each rise and fall, the latitude of sunspot eruption starts around 30° and drifts to the equator, but the magnetic fields of the follower spots (sunspots usually come in pairs, called leader and follower) drift poleward and reverse the polar field. In the next 11-year period, the magnetic polarities are reversed but follow the same pattern. Therefore, the magnetic period is 22 years.
Although sunspots were known as early as 1600, no one noticed that their number changed with time until the German amateur astronomer Samuel Heinrich Schwabe announced the 11-year cycle in 1843. Swiss astronomer Rudolf Wolf studied historical sunspot records and proposed the scheme still used for numbering solar cycles, with solar cycle 1 beginning in 1755, the earliest year for which he found reliable sunspot numbers. The 22-year magnetic cycle was discovered in 1925 by the American astronomer George Ellery Hale.
In 1894 the English astronomer E. Walter Maunder pointed out that very few sunspots were observed between 1645 and 1715, a period now known as the Maunder minimum. This period coincided with the coldest part of the Little Ice Age (c. 1300–1850) in the Northern Hemisphere, when the River Thames in England froze over during winter, Viking settlers abandoned Greenland, and Norwegian farmers demanded that the Danish king recompense them for lands occupied by advancing glaciers. The event was confirmed by the American astronomer J.A. Eddy, using carbon isotope ratios in tree rings. During this time the 11-year cycle continued but with a much-reduced amplitude. The data suggest that other such events occurred even earlier in the previous millennium. The late 18th and early 19th centuries also had a brief period of decreased sunspot activity, the Dalton minimum, that also coincided with a period that was slightly cooler than normal. The physical mechanism that explains how changes in solar activity affect Earth’s climate is unknown, and these episodes, however suggestive, do not prove that lower sunspot numbers produce cooling.
" height="20" alt="HEW Logo">
Solar cycle 25 began in 2019 and will reach maximum in 2025, but that maximum is predicted to be weak, like that of solar cycle 24, which had only half the number of sunspots seen in solar cycle 23. This decrease in the number of sunspots has led some solar physicists to conclude that the Sun may be in a period of inactivity like the Dalton minimum.