Sunspots, the Solar Cycle and the Solar Wind
Dark Patches on a Bright Surface
Sunspots are darker regions on the photosphere. They appear dark only by contrast — they are still extremely hot (around 3,500–4,000 K) but look dark against the ~5,800 K photosphere surrounding them. Sunspots form because intense, concentrated magnetic flux tubes punch through the photosphere and suppress convection. Without the steady upward flow of hot plasma, the sunspot cools relative to its surroundings.
Sunspots have a two-zone structure: the dark central umbra is where the magnetic field is strongest and convection is almost entirely stopped; surrounding it is the lighter, filamentary penumbra where the field is inclined and convection is only partially inhibited. The penumbra shows radial “fingers” of brighter plasma separated by darker filaments.
Sunspots have two zones: the dark central umbra (strongest magnetic field, convection halted) and the lighter outer penumbra (inclined field, convection partially inhibited). Sunspots appear dark because their temperature (~3,500–4,000 K) is lower than the surrounding photosphere (~5,800 K).
Measuring the Sun's Rotation Using Sunspots
Because sunspots are carried across the solar disc by the Sun's rotation, tracking the same sunspot group over successive days allows astronomers to calculate the Sun's rotation period. By measuring the angular displacement of a sunspot in degrees per day across the disc, and knowing the Sun completes 360° in one rotation, the period can be derived. The Sun rotates differentially — its equator rotates in approximately 25 days (sidereal), while higher latitudes rotate more slowly. As seen from Earth, taking into account Earth's own orbital motion, the average synodic rotation period is about 27 days.
In a practical sunspot-tracking exercise, students project the Sun's image onto paper (using the solar projection method) and mark sunspot positions on the same fixed template each day. The displacement of a spot between observations, divided by the number of days elapsed, gives the daily angular motion. Dividing 360° by this daily rate gives the rotation period. Because the Sun is a gas rather than a solid, this period varies with latitude — a behaviour unique to gaseous and plasma bodies called differential rotation.
It is easy to misread a sunspot data graph by measuring between the wrong peaks, or to confuse the sidereal and synodic periods. When a graph shows sunspot position (in degrees from disc centre or in angular position) against time, identify the same feature (e.g. a particular spot reaching disc centre) on successive transits across the Sun. The time between two consecutive central meridian crossings of the same feature gives the synodic rotation period. Do not use spots near the poles — their slower differential rotation would give an incorrect value for the equatorial period.
The 11-Year Solar Cycle
The number of sunspots visible on the Sun varies regularly in an 11-year solar cycle. During solar maximum, hundreds of sunspots may be visible at once and solar activity — flares, prominences and coronal mass ejections — is intense. During solar minimum, the disc may be virtually spot-free for weeks. The cycle is driven by the winding and tangling of the Sun's magnetic field as it undergoes differential rotation: over 11 years the field becomes increasingly tangled, then “resets” as opposite polarities reconnect. The complete magnetic cycle (returning to the original polarity) actually takes about 22 years (the Hale cycle), because the polarity of sunspot pairs flips from one 11-year cycle to the next.
Historical sunspot records reveal periods of anomalous low activity. The most famous is the Maunder Minimum (approximately 1645–1715), during which very few sunspots were recorded. This period coincided with unusually cold winters in Europe and North America — though the degree to which reduced solar output drove the climate cooling, versus volcanic eruptions and other factors, remains an active area of research.
The 11-year solar cycle moves from solar minimum (few sunspots, low activity) to solar maximum (many sunspots, high flare and CME frequency). The Maunder Minimum (1645–1715) was an extended period of near-zero sunspot activity.
The Solar Wind and Space Weather
The solar wind is a continuous outward flow of charged particles — mainly protons and electrons — streaming from the Sun's corona at 300–700 km/s. It flows throughout the heliosphere and reaches Earth roughly 2–4 days after leaving the Sun. Earth is protected by its magnetosphere — the region of space dominated by Earth's magnetic field — which deflects most of the solar wind around the planet. Trapped particles in the Van Allen Belts are two doughnut-shaped zones of high-energy particles held in place by the magnetosphere.
When solar wind particles reach Earth's polar regions, they spiral down magnetic field lines into the upper atmosphere. Here they collide with oxygen and nitrogen atoms, exciting the atoms to higher energy states. When the atoms return to their ground state they emit visible light, creating the aurora borealis (northern lights) and aurora australis (southern lights). During high solar activity, aurorae can be seen at much lower latitudes than usual.
During intense solar events (flares or coronal mass ejections), the surge of charged particles and electromagnetic radiation can cause serious disruption on Earth and in orbit. Geomagnetic storms induced by CMEs can disrupt power grids by inducing large currents in long transmission lines (the 1989 Quebec blackout affected six million people for nine hours). Satellite damage occurs from high-energy particles degrading solar panels, electronics and sensors. GPS and radio communications become unreliable because ionisation in the upper atmosphere disrupts radio wave propagation. Aircraft on polar routes may receive increased radiation doses from energetic particles; routes are sometimes diverted during major solar storms. Astronauts in the International Space Station are directed to sheltered modules during intense solar particle events.
The solar wind directly shapes cometary tails. Comets develop two tails as they approach the Sun: the dust tail, which curves gently along the comet's orbit under radiation pressure, and the ion (plasma) tail, which points directly away from the Sun because it is blown by the solar wind. Observing a comet's ion tail always pointing directly away from the Sun is direct evidence for the existence of the solar wind — a clue that led to Ludwig Biermann's solar wind hypothesis in 1951, before it was directly measured by spacecraft.