Sunshine Cycles: How the Sun’s Activity Affects Earth

The sun is not a static, unchanging star, but a dynamic body with natural variations in its activity and energy output. These fluctuations follow predictable patterns, known as solar cycles. These cycles have implications that extend far beyond the sun itself, influencing the space environment and directly affecting Earth.

The Primary Solar Cycle

The most prominent pattern is the Schwabe Cycle, lasting approximately 11 years. This cycle is tracked by counting sunspots—cooler, magnetically complex regions that appear as dark blemishes on the sun’s surface. The number of sunspots systematically increases and decreases, defining the two main phases: solar maximum and solar minimum. Scientists have numbered these cycles since 1755, and the world is currently in Solar Cycle 25.

A solar maximum is the peak of activity, characterized by a high number of sunspots and a significant increase in solar flares and coronal mass ejections (CMEs). Following the maximum, activity wanes toward a solar minimum, a quiet phase with few sunspots where flares and CMEs are infrequent. The solar minimum marks the end of one cycle and the beginning of the next.

Beyond the 11-Year Cycle

While the 11-year cycle is the most famous, it is part of a larger system of solar variability. The Hale Cycle spans approximately 22 years, or two Schwabe cycles. This cycle is defined by the reversal of the sun’s main magnetic field, where the north and south magnetic poles flip near the peak of each 11-year solar maximum.

On longer timescales, the Gleissberg cycle describes a variation in the strength of the 11-year cycles and has a period of roughly 80 to 90 years. Another long-term variation is the Suess-de Vries cycle, which has a period of about 200 years. This cycle is prominent in records of cosmogenic isotopes, like carbon-14 in tree rings, which are influenced by the sun’s magnetic field over long periods.

What Drives These Cycles

The engine behind these solar cycles is believed to be the solar dynamo, a process within the sun’s interior. This mechanism converts kinetic energy from the movement of hot, electrically charged gas, known as plasma, into magnetic energy. Two processes work together: differential rotation and convection, which twist and amplify the sun’s magnetic field.

The sun exhibits differential rotation, meaning its equatorial regions spin faster than its polar regions. This uneven rotation takes the sun’s north-south magnetic field lines and stretches them around the sun, creating powerful, east-west oriented magnetic fields. These magnetic bands are stored in a region deep inside the sun called the tachocline.

This process is combined with convection, the churning motion of hot plasma rising and cool plasma sinking. As these convective cells rise, they twist the magnetic field lines into tight tubes. When these tubes become buoyant, they rise to the surface, breaking through as sunspot pairs and driving the events of a solar maximum.

Earthly Impacts of Solar Activity Cycles

The fluctuations of the solar cycle drive space weather, which has tangible effects on Earth and our technology. During solar maximum, the increased frequency of solar flares and CMEs intensifies the aurora borealis and australis. These beautiful light displays are caused by energetic particles from the sun colliding with gases in Earth’s upper atmosphere.

Increased solar activity also poses risks to technological systems:

• Geomagnetic storms can induce electrical currents in power grids, potentially damaging transformers and causing blackouts.
• The upper atmosphere expands, increasing drag on satellites and causing them to lose altitude, while high-energy particles can damage electronics and pose a radiation risk to astronauts.
• Solar flares can alter the ionosphere, disrupting high-frequency (HF) radio communications used for long-distance travel.
• GPS and other navigation signals can be disrupted by atmospheric disturbances during strong solar storms.

There is also a connection between solar cycles and Earth’s climate, though it is complex. The total energy output from the sun varies only slightly over an 11-year cycle. However, historical periods of prolonged solar inactivity have coincided with cooler climate periods in some regions. The Maunder Minimum (1645-1715), a time with virtually no sunspots, occurred during the coldest part of the “Little Ice Age” in Europe.