1 Overview of solar activity
Solar activity is the collective term for the changing phenomena observed on the Sun, especially in its visible surface, atmosphere, and magnetic environment. It includes dark sunspots, bright flares, expulsions of plasma known as coronal mass ejections, prominences, variations in the solar wind, and the repeating solar cycle. These features do not occur randomly; they are linked by the Sun’s magnetic field and by the movement of hot ionized gas within the solar interior and atmosphere.
The study of solar activity is a major part of solar physics because the Sun is the dominant source of light, heat, and space weather in the Solar System. Changes in solar activity can influence near-Earth space, affecting communication systems, satellite operations, and the appearance of auroras. At the same time, solar activity offers a visible laboratory for understanding magnetic fields and plasma behavior under extreme conditions.
1.1 Definition and scope
In scientific usage, solar activity refers to observable changes that arise from magnetic and plasma processes on or above the Sun’s surface. The term is broad enough to include both small-scale events, such as localized brightenings, and large-scale phenomena, such as global cycle variations. It is often used to describe the Sun’s short-term eruptive behavior as well as longer periodic changes.
The scope of solar activity extends beyond the solar disk. Material and energy released by the Sun travel outward through the heliosphere, where they interact with planets, moons, and spacecraft. For this reason, solar activity is studied not only as a solar phenomenon but also as a driver of conditions throughout the Solar System.
1.2 Physical basis
Solar activity is rooted in the Sun’s internal dynamics and in the behavior of electrically conducting plasma. The Sun rotates differentially, meaning that its equator and higher latitudes rotate at different rates. Combined with convection, this motion stretches, twists, and amplifies magnetic field lines. When magnetic energy becomes concentrated, it can be released suddenly in flares or ejections.
The visible signs of solar activity are therefore outward expressions of deeper physical processes. The most active regions are usually associated with intense magnetic fields, complex plasma flows, and strong temperature contrasts between neighboring areas.
1.2.1 Solar magnetic field
The solar magnetic field is the principal organizing force behind activity. It is generated by a dynamo process in the solar interior, where moving plasma and rotation create and sustain magnetic structures. These fields emerge through the photosphere and shape the appearance of sunspots, loops, prominences, and coronal structures.
Because magnetic fields store energy and influence the motion of charged particles, they are central to eruptive events. When field lines reconnect or rapidly reorganize, they can release energy as radiation, heating, and high-speed plasma flow. This magnetic restructuring underlies many of the Sun’s most energetic displays.
1.2.2 Convection and plasma dynamics
Convection in the Sun’s outer interior transports heat upward through the motion of hot plasma. This churning motion, together with rotation, helps drive the solar dynamo. It also disturbs magnetic fields, pushing them into complex shapes and sometimes concentrating them at the surface.
Plasma dynamics in the atmosphere are equally important. Gas there is fully or partly ionized, so it responds strongly to electromagnetic forces. As a result, loops, jets, and eruptive structures often follow magnetic geometry rather than simple fluid motion.
1.3 Relationship to the solar atmosphere
Solar activity is most visible in the atmosphere rather than in the Sun’s deeper layers. The photosphere provides the familiar disk, while the chromosphere and corona reveal higher-temperature, magnetically shaped structures. Many active phenomena originate in the lower atmosphere and extend outward into the corona.
The atmosphere is layered but tightly connected. Disturbances in one region can trigger changes in another, allowing events such as flares or coronal mass ejections to develop across multiple altitudes. This connected structure makes the solar atmosphere a dynamic system rather than a set of isolated layers.
2 Solar cycle
Solar activity varies in a roughly cyclic pattern, with the best-known rhythm lasting about 11 years. During this cycle, the number of sunspots, flares, and eruptive events typically rises and falls. The cycle is tied to the evolution of the Sun’s global magnetic field and the redistribution of magnetic flux over time.
The solar cycle provides a framework for understanding long-term changes in space weather. It is not perfectly regular, however, and its timing and amplitude vary from one cycle to another. These irregularities remain an important subject of study.
2.1 11-year sunspot cycle
The 11-year cycle is commonly described by the rise and decline in sunspot counts. At the beginning of a cycle, few spots are visible. As the cycle progresses, spots become more numerous, reaching a peak known as solar maximum. Afterward, the number declines toward solar minimum.
Although the phrase “11-year cycle” is standard, the actual duration can vary. Some cycles are shorter or longer, and the interval between successive maxima or minima is not perfectly fixed. The pattern is therefore approximate rather than exact.
2.2 Magnetic polarity cycle
Beyond the sunspot count, the Sun’s magnetic polarity changes in a regular sequence. Magnetic structures in one cycle tend to reverse their polarity in the next, so a full return to the original magnetic configuration takes about twice as long as the sunspot cycle. This deeper rhythm links visible activity to the global magnetic field.
The polarity cycle is important because it reveals that the solar cycle is not just a count of spots but a magnetic process. What appears as one cycle of activity is part of a larger magnetic oscillation.
2.2.1 22-year Hale cycle
The 22-year Hale cycle is the full magnetic cycle of the Sun, named for George Ellery Hale. During this period, the magnetic orientation of sunspot pairs and large-scale solar fields completes one full reversal and return. The 11-year sunspot cycle represents half of this longer pattern.
The Hale cycle helps explain recurring changes in solar magnetism, including the systematic reversal of polar fields. It also provides context for how solar activity organizes itself over decades.
2.3 Solar minima and maxima
Solar minimum is the phase when sunspots and eruptive activity are relatively scarce. The Sun still remains active, but large active regions are less common and the corona often appears simpler. Solar maximum is the period of greatest activity, with more sunspots, more flares, and a more complex magnetic environment.
These phases are useful for forecasting space weather patterns. During maximum, the likelihood of strong disturbances tends to increase, although individual events can still occur at any time in the cycle.
2.4 Cycle prediction and variability
Predicting the solar cycle remains difficult because the underlying dynamo is complex and only partly understood. Scientists use statistical methods, precursor indicators, and physical models to estimate future activity. Forecasts usually focus on cycle strength, timing, and the likelihood of extreme events.
Variability between cycles is one of the enduring features of solar activity. Some cycles are relatively weak and smooth, while others are more intense or irregular. This diversity complicates prediction and continues to motivate research into the Sun’s magnetic behavior.
3 Sunspots
Sunspots are dark-looking regions on the solar surface caused by strong magnetic fields that suppress convective heat transport. They are cooler than surrounding areas, which makes them appear darker against the bright photosphere. Sunspots are among the oldest and most carefully studied indicators of solar activity.
Their number, size, and arrangement vary with the solar cycle. Because they are easy to observe, sunspots have long served as a primary measure of solar activity and a key tool for historical reconstruction.
3.1 Formation and structure
Sunspots form when concentrated magnetic flux rises through the solar surface. The magnetic field inhibits the upward flow of hot plasma, reducing the local temperature. As a result, the affected area emits less light and appears dark.
A typical sunspot has a structured internal pattern and may persist for days or weeks. Larger spots or groups often reflect more complex magnetic organization, and these are more likely to be associated with flares and eruptions.
3.2 Umbra and penumbra
The dark central region of a sunspot is the umbra, where the magnetic field is strongest and the temperature is lowest. Surrounding it is the penumbra, a somewhat brighter zone marked by radial or filament-like texture. The contrast between these parts gives sunspots their familiar layered appearance.
The umbra and penumbra are not merely visual features; they reflect differences in magnetic geometry and energy transport. Their structure helps researchers infer the properties of the magnetic field within the spot.
3.3 Sunspot groups
Sunspots often appear in groups rather than as isolated features. These groups may include spots of opposite magnetic polarity and a variety of surrounding magnetic structures. Complex groups are especially significant because they are frequently sites of enhanced eruptive activity.
The arrangement of a sunspot group can indicate the level of magnetic stress in the region. More intricate patterns generally correspond to a greater likelihood of flares or other disturbances.
3.4 Sunspot counts and indices
Sunspot counts are among the oldest quantitative measures of solar activity. Modern indices combine direct counts with weighting schemes that account for group complexity and spot size. These records help track cycle progression and compare activity across long time spans.
Such indices are valuable because they provide continuity over centuries. They also allow scientists to connect historical observations with modern space-weather monitoring.
4 Solar flares
Solar flares are sudden bursts of electromagnetic radiation released from active regions on the Sun. They can brighten across many wavelengths, from radio waves to X-rays, and may occur alongside other eruptive events. Flares are among the most energetic forms of solar activity observable from Earth.
Their occurrence is closely linked to magnetic fields in complex sunspot regions. Although short-lived, flares can produce immediate effects in the near-Earth environment by altering the ionosphere and disrupting radio propagation.
4.1 Mechanism of flare production
Flares are thought to arise when stressed magnetic fields undergo rapid reconfiguration, often through magnetic reconnection. In this process, field lines break and reconnect in new arrangements, releasing stored magnetic energy. The released energy heats plasma, accelerates particles, and emits intense radiation.
The precise details of flare onset and energy release remain active areas of research. Nonetheless, the magnetic explanation is central to current understanding.
4.2 Flare classification
Flares are classified by the intensity and wavelength of their emission. The classification system helps compare events and estimate their likely impacts. Stronger flares tend to be associated with more pronounced effects on the near-Earth environment.
4.2.1 X-ray classes
X-ray classification is based on the peak soft X-ray flux measured near Earth. The main classes are A, B, C, M, and X, with each step representing a tenfold increase in intensity. X-class flares are the strongest in the standard scale.
This system is widely used in solar monitoring because X-ray emission responds quickly to flare activity. It provides a practical measure for operational space-weather forecasting.
4.2.2 Optical and radio signatures
Flare activity can also be identified in visible light and radio emission. Optical signatures may include brightening in the chromosphere or white-light flares in extreme cases. Radio signatures can show bursts, continuum changes, or frequency-dependent disturbances.
These multiwavelength observations are important because they reveal different physical aspects of the same event. Together they offer a more complete picture of flare evolution.
4.3 Flare effects
Flares can increase ultraviolet and X-ray radiation reaching Earth, which alters the ionosphere and can disrupt high-frequency radio communication. They may also contribute to particle radiation hazards in near-Earth space. In combination with coronal mass ejections, flares can form part of major space-weather events.
The immediate visible effect of a flare on Earth is usually indirect, but its consequences can be significant for technology. Satellites, navigation systems, and radio links may all be affected.
4.4 Flare forecasting
Forecasting flares involves monitoring active regions for magnetic complexity, rapid change, and other warning signs. Scientists use magnetograms, ultraviolet imagery, and statistical models to estimate flare probability. Prediction is challenging because the timing of magnetic release is difficult to determine precisely.
Although exact forecasting remains limited, operational alerts can identify regions with elevated risk. This supports satellite operators, communication planners, and space-weather services.
5 Coronal mass ejections
Coronal mass ejections, or CMEs, are large expulsions of plasma and magnetic field from the solar corona. They can carry enormous amounts of mass outward into interplanetary space. When directed toward Earth, they may disturb the magnetosphere and trigger geomagnetic storms.
CMEs are closely watched because of their potential to produce widespread space-weather effects. Their speed, direction, and magnetic orientation all influence their impact.
5.1 CME formation
CMEs are believed to form when coronal magnetic structures become unstable and erupt. This may happen after magnetic twisting, shearing, or reconnection in active regions or filament channels. The eruption lifts coronal material outward, often accompanied by flare activity.
The exact trigger can vary from event to event. Some CMEs are associated with clear surface activity, while others develop more gradually from coronal instability.
5.2 CME structure and speed
A CME typically contains a dense leading edge, a cavity, and a central core or filament material. Its speed can range from relatively slow to extremely fast, with faster events generally causing stronger disturbances if they interact with Earth. The internal magnetic field of the CME is especially important for geoeffectiveness.
The structure of a CME may also evolve as it travels. Expansion, deformation, and interaction with the solar wind can all modify its appearance and effect.
5.3 CME and geomagnetic storms
When a CME reaches Earth, it can compress the magnetosphere and transfer energy into near-Earth space. If the CME’s magnetic field couples efficiently with Earth’s field, a geomagnetic storm may occur. Such storms can intensify auroras and disturb technological systems.
Not every CME causes a major storm. Direction, speed, and magnetic orientation determine whether the encounter becomes mild or severe.
5.4 Observation and tracking
CMEs are observed using coronagraphs and space-based imagers that block the bright solar disk to reveal the faint corona. Tracking methods estimate the CME’s trajectory, speed, and arrival time at Earth or elsewhere in the heliosphere. These measurements are essential for forecasting.
Multi-instrument observations improve reliability. By combining images, radio data, and in situ measurements, researchers can better model CME propagation.
6 Solar prominences and filaments
Prominences are large, luminous structures of cooler plasma suspended in the hot corona by magnetic fields. When seen against the bright solar limb, they appear as glowing arches or loops. The same material, viewed against the solar disk, is called a filament.
Prominences and filaments are closely related to active regions and magnetic field configurations. They can remain stable for long periods or erupt suddenly into space.
6.1 Prominence morphology
Prominences come in a variety of shapes, including arches, sheets, and thread-like structures. They are composed of relatively cool, dense plasma held aloft by magnetic support. Their fine structure reflects the complexity of the magnetic field.
Morphology often changes over time. Some prominences gradually evolve, while others show rapid motion before eruption.
6.2 Filaments on the solar disk
A filament is the disk-view counterpart of a prominence. Because it is cooler than the surrounding photosphere, it appears as a dark, elongated feature crossing the solar disk. Filaments often trace magnetic polarity boundaries.
Their visibility on the disk makes them useful indicators of coronal structure. A filament that disappears from the disk may later be seen as a prominence at the limb.
6.3 Eruptive prominences
Some prominences become unstable and erupt outward, sometimes contributing to a CME. The eruption can involve a rapid lift-off of magnetic loops and entrained plasma. Such events are important because they connect slow coronal structuring with fast solar transients.
Eruptive prominences are often monitored closely in space-weather forecasting. Their instability can signal a broader magnetic reorganization.
6.4 Relation to other solar events
Prominences, filaments, flares, and CMEs often occur in related magnetic environments. An eruption may involve more than one phenomenon, though not every prominence eruption produces a strong flare or CME. The relationships are therefore statistical rather than absolute.
These features together illustrate how the corona stores and releases magnetic energy. They are among the clearest examples of solar magnetism in action.
7 Solar wind
The solar wind is a continuous flow of charged particles streaming outward from the corona into interplanetary space. It fills the heliosphere and shapes the environment through which planets move. Although usually invisible, it is a fundamental component of solar activity.
Its properties vary with solar conditions. Changes in speed, density, and magnetic structure can influence space weather and planetary magnetospheres.
7.1 Composition and origin
The solar wind is composed mainly of electrons, protons, and alpha particles, along with embedded magnetic fields. It originates in the outer solar atmosphere, where the high temperature allows particles to escape the Sun’s gravity. The exact details of its acceleration remain a major topic in solar physics.
The solar wind carries the Sun’s magnetic influence far from the solar surface. In this way, the corona extends its reach across the Solar System.
7.2 Fast and slow solar wind
Two broad categories are commonly distinguished: fast and slow solar wind. Fast wind is generally associated with coronal holes and can reach higher speeds. Slow wind tends to be denser and more variable, often emerging from regions near the solar equator or complex magnetic boundaries.
These categories are not rigid, but they are useful for describing the wind’s behavior. Their differences help explain the changing character of space weather.
7.3 Interaction with planetary magnetospheres
When the solar wind encounters a planet with a magnetic field, it compresses and shapes the magnetosphere. This interaction can drive currents, energize particles, and produce auroras. The strength of the effect depends on solar-wind speed, density, and magnetic orientation.
The solar wind also affects planets without strong global magnetic fields by interacting directly with their upper atmospheres. Thus, it is a universal driver of planetary space environments.
7.4 Solar wind variability
Solar wind conditions fluctuate on many timescales, from minutes to years. Transient structures such as shocks, streams, and CME-driven disturbances create sudden changes, while the solar cycle influences long-term trends. These variations are crucial for predicting space-weather conditions.
Monitoring solar-wind variability has become a routine part of space-weather analysis. Continuous measurements provide early warning of approaching disturbances.
8 Solar corona and chromosphere
The corona and chromosphere are outer layers of the solar atmosphere where much of solar activity becomes visible. The corona is extremely hot and tenuous, while the chromosphere is denser and cooler. Both are strongly shaped by magnetic fields.
These layers are essential to understanding how the Sun releases energy. Many dynamic events begin in the chromosphere and extend into the corona, where they can expand into the heliosphere.
8.1 Coronal heating
One of the central problems in solar physics is why the corona is so much hotter than the surface beneath it. Several mechanisms have been proposed, including wave heating and magnetic reconnection. The solution likely involves more than one process working together.
Coronal heating is directly relevant to activity because it supplies the energetic environment in which flares, loops, and wind acceleration occur. It remains a major area of theoretical and observational study.
8.2 Active regions
Active regions are localized areas of strong magnetic field, often associated with sunspots and bright coronal loops. They are hotspots of solar activity and common sites of flares and CMEs. Their appearance changes as magnetic structures evolve over days or weeks.
Because active regions are so dynamic, they serve as important laboratories for studying magnetic energy storage and release. They are among the most closely watched features on the solar disk.
8.3 Coronal holes
Coronal holes are regions where the corona appears darker in certain wavelengths because the magnetic field is more open and the plasma density is lower. These areas are major sources of fast solar wind. They can persist for long periods and recur with solar rotation.
Although visually subtle, coronal holes have significant heliospheric influence. Their association with fast wind makes them relevant to the recurrence of space-weather disturbances.
8.4 Chromospheric activity indicators
The chromosphere shows several indicators of activity, including emission lines, plages, and network brightening. These features help astronomers track magnetic concentration and heating. Chromospheric observations often complement photospheric and coronal data.
Because the chromosphere bridges the lower atmosphere and the corona, it is especially valuable for studying how surface magnetic fields develop into larger-scale activity.
9 Observations and measurement
Solar activity has been observed for centuries, first by eye and later with increasingly sophisticated instruments. Modern measurements combine ground-based telescopes, space observatories, and numerical data products. Together they allow continuous monitoring of the Sun’s changing state.
Observational techniques differ by wavelength and purpose. Some instruments record visible light, while others track ultraviolet, X-ray, or radio emission. Each reveals a different layer or process.
9.1 Historical observations
Early observations of sunspots provided the first systematic evidence that the Sun changes over time. Long-term records helped reveal cyclic behavior and established the importance of solar monitoring. Historical accounts of auroras and unusual sky events also contributed to the study of solar influence.
These records are valuable because they extend knowledge beyond the era of modern instrumentation. They allow comparisons across centuries and help place current activity in a longer context.
9.2 Ground-based solar telescopes
Ground-based observatories use filters, spectrographs, and imaging systems to study the solar surface and atmosphere. They can monitor sunspots, prominences, flares, and magnetic fields with high spatial detail. Atmospheric conditions on Earth limit some observations, but ground instruments remain essential.
Many solar telescopes are specialized to reduce the impact of scattered light and to isolate particular wavelengths. They often work in coordination with spacecraft observations.
9.3 Space-based observatories
Space observatories avoid atmospheric distortion and can observe ultraviolet and X-ray wavelengths that are blocked from the ground. They are especially useful for tracking flares, coronal structures, and solar wind sources. Continuous operation in space has transformed solar physics.
Space missions provide data that are central to space-weather prediction. They also allow multiwavelength studies of activity from the photosphere to the heliosphere.
9.4 Common data products and indices
To compare activity across time and between instruments, scientists use standardized indices and data products. These measures condense complex observations into usable numbers. They are essential for research, forecasting, and historical analysis.
9.4.1 Sunspot number
The sunspot number is a long-standing index based on the quantity and grouping of visible sunspots. It is one of the most widely used measures of solar cycle phase. Because of its long record, it is especially important for studying historical trends.
The index is not a direct measure of energy output, but it serves as a convenient proxy for overall activity. Its continuity makes it a cornerstone of solar monitoring.
9.4.2 F10.7 radio flux
F10.7 radio flux measures solar radio emission at a wavelength of 10.7 centimeters. It correlates well with general solar activity and is widely used in atmospheric and space-weather modeling. Unlike sunspot counts, it captures aspects of the Sun that are not visible in the same way.
This index is valued because it is stable, routinely measured, and broadly informative. It often serves as an input for operational forecasting.
9.4.3 Kp and geomagnetic indices
Kp and related geomagnetic indices summarize disturbances in Earth’s magnetic environment. They are not direct measures of solar output, but they reflect the effects of solar activity on the magnetosphere. Higher values generally indicate stronger geomagnetic disturbance.
These indices help connect solar observations with terrestrial consequences. They are important for assessing the severity of space-weather events.
10 Space weather impacts
Space weather refers to the changing conditions in space caused largely by solar activity. It can affect Earth’s upper atmosphere, satellites, communication systems, navigation, and power infrastructure. The most noticeable natural result is the aurora, but many effects are technological rather than visual.
The severity of space-weather impacts depends on the type of solar event and the vulnerability of the affected system. Some disturbances are minor, while others are operationally significant.
10.1 Auroras
Auroras occur when charged particles guided by Earth’s magnetic field collide with atoms and molecules in the upper atmosphere. They produce glowing curtains and arcs of light, often near polar regions. Strong solar disturbances can expand auroral visibility to lower latitudes.
Auroras are among the most recognizable signs of solar activity’s influence on Earth. They provide a visible link between solar eruptions and atmospheric response.
10.2 Radio blackouts
Solar flares can cause sudden ionospheric disturbances that absorb or scatter radio signals, especially at high frequencies. These radio blackouts may last from minutes to hours, depending on flare strength and geometry. They can interfere with aviation, maritime, and emergency communications.
Because the ionosphere responds rapidly to flare radiation, radio conditions can change almost immediately after an event. This makes flare monitoring operationally important.
10.3 Satellite anomalies
Satellites may experience anomalies due to increased radiation, surface charging, or changes in atmospheric drag caused by solar activity. High-energy particles can damage electronics or disrupt onboard systems. Atmospheric expansion during periods of increased solar heating can also alter low-Earth orbit conditions.
Operators monitor solar conditions to reduce risk. Protective measures may include safe modes, operational delays, or orbit adjustments.
10.4 Power grid disturbances
Large geomagnetic storms can induce currents in long conductors such as power lines and pipelines. In power grids, these currents may stress transformers and other equipment. Although serious events are uncommon, they are among the most economically important effects of solar activity.
Grid operators use space-weather alerts to anticipate disturbance periods. Preparedness helps reduce the chance of equipment damage or service interruptions.
10.5 Radiation hazards for astronauts and aircraft
Energetic particles associated with solar events can create radiation risks for astronauts, especially outside Earth’s protective atmosphere and magnetic field. Aircraft flying over polar routes may also encounter elevated radiation during strong events. These hazards are mainly a concern during intense solar particle events.
Monitoring and forecasting support mission planning and flight operations. Shielding and route adjustments can reduce exposure when conditions warrant.
11 Historical and scientific study
The study of solar activity has developed over centuries, moving from simple visual observation to advanced space-based research. Each stage has added new detail about the Sun’s magnetic behavior and its effects on the Solar System. The field now combines astronomy, plasma physics, and space science.
Historical records remain valuable because they show how the Sun behaves over long intervals. Modern models, meanwhile, seek to explain the underlying mechanisms and improve prediction.
11.1 Early discoveries
Sunspots were observed in antiquity, but systematic study expanded after the invention of the telescope. Early astronomers recognized that the spots moved across the solar disk and changed from day to day. Later, observers discovered the periodicity of sunspot activity and its broader significance.
These discoveries established that the Sun is not a static body. They also laid the foundation for the idea that solar changes could affect Earth.
11.2 Development of solar physics
Solar physics developed as spectroscopy, photography, and magnetism became central scientific tools. Researchers learned to identify distinct atmospheric layers, measure magnetic fields, and connect surface features with energetic events. The emergence of radio and space astronomy opened further windows on the Sun.
As the field matured, solar activity became understood as a magnetically driven process rather than a collection of unrelated phenomena. This shift unified many observations under one physical framework.
11.3 Modern research methods
Modern research uses multiwavelength imaging, helioseismology, magnetography, numerical simulation, and spacecraft measurements. These methods make it possible to study the solar interior, surface, atmosphere, and heliosphere as an interconnected system. Computer models help test ideas about the solar dynamo, flare initiation, and CME propagation.
The combination of observation and simulation is especially important because many relevant processes cannot be reproduced directly in the laboratory. Solar activity therefore remains a field where theory and measurement must work closely together.
11.4 Open questions and current models
Despite major progress, several questions remain unresolved. Scientists continue to investigate the details of coronal heating, flare triggering, CME formation, and the origin of cycle variability. Predicting the strength and timing of individual events also remains difficult.
Current models seek to connect magnetic field evolution with observable activity across many scales. The challenge is to describe a complex, turbulent, and changing star using limited measurements from a distance.
</INTERNAL_LINK_CANDIDATES> Sunspot (dark magnetic region on the Sun’s surface) Solar flare (sudden release of electromagnetic energy from an active region) Coronal mass ejection (large expulsion of plasma and magnetic field from the corona) Solar wind (continuous stream of charged particles flowing outward from the Sun) Solar cycle (repeating long-term variation in solar activity) Solar magnetic field (the magnetic field that drives solar activity) Magnetic reconnection (restructuring of magnetic field lines that releases energy) Photosphere (the Sun’s visible surface) Chromosphere (the solar atmospheric layer above the photosphere) Corona (the Sun’s hot outer atmosphere) Prominence (a suspended cloud of cool plasma in the corona) Filament (a prominence seen against the solar disk as a dark feature) Geomagnetic storm (disturbance of Earth’s magnetic field caused by solar activity) Aurora (light display produced by charged particles in a planet’s upper atmosphere) Space weather (conditions in space influenced by the Sun) Heliosphere (the region of space dominated by the solar wind) Solar dynamo (the process generating the Sun’s magnetic field) Coronagraph (instrument used to observe the corona by blocking the bright solar disk) Sunspot number (standard index counting visible sunspots) F10.7 radio flux (measure of solar radio emission used as an activity index) </INTERNAL_LINK_CANDIDATES>