Solar Activity: Flares, Sunspots, and Space Weather
Linas JuozėnasShare
Knowledge Ark · Universe · Chapter 08 / Article 02
When the Sun stirs our skies.
A curtain of aurora reveals a connection that reaches across space. Follow the Sun’s changing magnetism, distinguish its eruptions, and discover how solar activity touches life in a technological world.
The space around Earth is an active environment
The Sun can look serene while magnetic energy builds in its atmosphere. When that energy is released, the consequences depend on what leaves the Sun—and how it reaches us.
Space weather describes changing conditions in the space environment and their effects on planets, spacecraft, and technology. Solar activity is a major driver.[1]
The previous article followed the energy that keeps the Sun shining. Here we follow its magnetic activity outward, from dark sunspots to glowing skies and the systems that help us anticipate a storm.
Why does the Sun become restless?
The Sun is made largely of plasma, matter containing charged particles. Its rotation and convective motions move this electrically conducting material and help regenerate magnetic fields. This process is the solar dynamo. Differential rotation—the equator turning faster than the poles—helps stretch the field, while rotating convection also shapes it. The complete workings of the dynamo remain an active research problem.[2]
Sunspots make magnetism visible
Strong magnetic fields can suppress the usual convective supply of heat near the surface, creating relatively cool sunspots. They look dark against the brighter photosphere but still emit substantial light. A developed spot has a darker central umbra and a surrounding, filamentary penumbra. Penumbral fields are generally weaker and more inclined toward the horizontal than umbral fields.[3]
An uneven rhythm of about eleven years
Sunspot numbers rise and fall over an approximately eleven-year activity cycle. Cycles differ in length and strength. The polarity pattern reverses between successive cycles, giving a magnetic cycle of about 22 years. Solar maximum generally brings more active regions and eruptions; minimum means reduced activity, not an inactive star.[4], [2]
What actually leaves the Sun?
Magnetic fields in an active region can become stressed and release energy through magnetic reconnection, a rearrangement of the field. A single eruptive event may produce several signals. Flares and coronal mass ejections often occur together, but either can occur without the other.[6]
Flare radiation: an abrupt increase in light
A solar flare is a burst of electromagnetic radiation across many wavelengths. Its light reaches Earth about eight minutes after emission. A flare can last minutes to hours; its duration is different from its light-travel time. Once a near-Earth instrument detects the flare, those photons have already arrived here.[7]
Energetic particles: a much faster population
Solar energetic particles are accelerated far beyond the energies of ordinary solar-wind particles. Reconnection can accelerate them, and shocks driven by fast CMEs are especially important in large, prolonged particle events. They are not simply the front edge of the bulk CME material.[8]
The first very fast particles can reach Earth’s vicinity within tens of minutes or less, while the arrival of other particles extends over longer periods. A radiation storm can persist for hours or days.[9]
Coronal mass ejections: matter carrying magnetic fields
A coronal mass ejection, or CME, is an expanding cloud of plasma and magnetic field. Exceptionally fast Earth-directed CMEs can arrive in roughly 15–18 hours; slower ones may take several days. A CME must intersect Earth’s environment to affect it directly, and its magnetic properties help determine the outcome.[10]
How strong is a solar event?
A flare class measures one part of the event
Flares are classified by their peak X-ray flux measured near Earth in the 0.1–0.8 nanometre band. The classes run A, B, C, M, X. At the same numerical level, each successive letter represents ten times the flux: M1 is ten times C1. Within a class, X2 has twice the peak flux of X1. The X class has no upper ceiling.
This measurement describes an X-ray peak. It is not the total energy of an eruption, a measurement of CME speed, or a forecast of the eventual geomagnetic disturbance.[7]
| Scale | What it describes | Rating basis |
|---|---|---|
| R · Radio blackouts | Flare-related disruption of high-frequency radio on Earth’s sunlit side. | Peak solar X-ray flux. |
| S · Solar radiation storms | Elevated energetic-proton conditions. | Proton flux above a specified energy threshold. |
| G · Geomagnetic storms | Disturbances in Earth’s magnetic environment. | The planetary Kp index of magnetic activity. |
NOAA’s scales each run from 1, minor, to 5, extreme. The ratings describe different hazards; they are not successive stages of one storm.[11]
Why does Earth sometimes respond strongly?
Earth’s magnetic field shapes a surrounding region called the magnetosphere. The solar wind compresses its sunward side and stretches a long tail away from the Sun. A geomagnetic storm occurs when unusually strong energy transfer from the solar wind disturbs this system.
Speed and magnetic-field strength matter, but so does orientation. A sustained southward component of the incoming field can enable efficient reconnection with Earth’s dayside field. That connection transfers energy into the magnetosphere and strengthens currents. This is why an impressive eruption image alone cannot predict the strength of a storm at Earth.[12]
Auroras reveal energy reaching the atmosphere
Auroras form when energetic particles, often electrons, follow magnetic field lines into the upper atmosphere. Their collisions excite oxygen and nitrogen, which release energy as light. During strong activity, the auroral region can extend much farther from the poles.[13]
Coronal holes provide another route to disturbance
Coronal holes are regions with open magnetic fields that release fast solar-wind streams. When a fast stream catches slower wind ahead, the interaction compresses material and magnetic fields and can drive geomagnetic activity. Long-lived coronal holes can recur as the Sun rotates; they are common around solar minimum, so quieter sunspot conditions do not eliminate space weather.[14]
How does space weather affect technology?
Radio communication
Flare X-rays increase ionization in the lower ionosphere on Earth’s sunlit side. This can absorb high-frequency radio signals that would otherwise support long-distance communication. Energetic solar protons cause a different absorption problem in polar regions, potentially affecting routes that rely on high-latitude radio links.[15]
Satellite navigation
Signals from navigation satellites cross the ionosphere before reaching a receiver. Small-scale irregularities in electron density can make their strength and phase fluctuate, a process called scintillation. Severe fluctuations can prevent receivers from maintaining a reliable lock, degrading positioning or making it unavailable.[16]
Satellites in low Earth orbit
Storm-related heating can expand the upper atmosphere, increasing gas density at satellite altitudes. The resulting drag changes orbital motion and makes accurate predictions more difficult. This is an atmospheric response: a satellite can experience increased drag without being struck by a solid object.[17]
Power networks on the ground
Rapid changes in the magnetic field induce electric fields in the ground. These can drive currents through long conducting networks, including power lines, stressing transformers and complicating grid operation. The effects depend on local magnetic changes, ground conductivity, and network design—not only on a storm’s global rating.[18]
Spacecraft electronics and people above the atmosphere
Charged particles can also produce surface or internal charging in spacecraft, cause individual electronic errors, and contribute to cumulative radiation damage. These mechanisms are distinct from atmospheric drag, and depend on particle energies, orbital environment, shielding, and design.[19]
Earth’s atmosphere provides strong protection for people on the ground. Exposure is a different concern for astronauts and, during strong particle events, for people at high aviation altitudes, especially near the poles. Radiation monitoring and mission planning therefore address a different hazard from a ground-level power-grid disturbance.[9]
How do forecasters know what is coming?
Forecasting begins with observations of the Sun. Images reveal active regions and eruptions; magnetic maps show the field at the visible surface. Coronagraphs block the bright solar disk so that expanding material can be followed outward. These observations help estimate a CME’s direction, width, and speed.[10]
Models estimate the journey
Propagation models such as WSA–Enlil combine the background solar wind with an estimated CME launch. They calculate how the disturbance travels toward Earth and other locations. Uncertain starting measurements and simplified eruption geometry produce uncertain arrival times, so a forecast should be read as an evolving estimate.[20]
Upstream instruments measure what is approaching
Solar-wind monitors upstream of Earth measure speed, density, and magnetic-field direction. They reveal whether the incoming field favors a strong geomagnetic response. The lead time is usually minutes to tens of minutes, depending on solar-wind speed.[13]
There is no universal warning time for “a solar storm.” Light, energetic particles, and bulk plasma arrive differently, and each requires its own observations. A useful forecast states both what is expected and how certain that expectation is.
What have major storms taught us?
| Event | What was recorded | What it helps us understand |
|---|---|---|
| 1859 · Carrington-era storms | Widespread auroras, magnetic disturbances, and disruption of telegraph systems. | Solar activity could affect long conducting networks even before modern electronics.[21] |
| March 1989 · Québec | A geomagnetic storm caused the Hydro-Québec power-system collapse. | Rapid local magnetic changes and network response matter alongside global storm indices.[22] |
| 2003 · Halloween storms | A sequence of eruptions affected spacecraft, radio links, navigation, and operations on Earth. | Multiple eruptions can create a prolonged period of different, overlapping hazards.[23] |
| May 2024 · Intense activity | Unusually widespread auroras accompanied atmospheric expansion and challenges for satellite operators. | A storm can be visually spectacular while also changing the environment of orbiting spacecraft.[24] |
Calling an event “Carrington-like” can hide uncertainty. Historical measurements are incomplete, and estimates of the 1859 storm’s intensity differ. Those uncertainties carry into estimates of how often comparable extremes occur. The record supports taking strong events seriously; it does not supply a reliable date for the next one.[25]
These cases make the science practical. Measurements, engineering, and operational decisions connect: the useful question is how a particular system responds to a particular disturbance.
What does space weather look like beyond Earth?
The same active Sun influences other planets, but each responds according to its atmosphere and magnetic environment. Mars has no Earth-like global magnetic field. Solar particles can therefore produce much more widespread atmospheric effects.
During strong solar activity in May 2024, NASA’s MAVEN spacecraft observed ultraviolet auroras across Mars, while the Curiosity rover recorded increased radiation at the surface. These observations connected an eruption at the Sun with measurements at another world.[26]
For exploration beyond Earth, understanding the Sun–planet connection helps interpret scientific measurements and plan spacecraft and crew operations. NASA’s space-weather research includes these wider Solar System applications.[1]
The story returns to a familiar principle: distance alone does not determine an outcome. Magnetic fields, atmospheres, the path of the disturbance, and the system encountering it all matter.
Sources and further reading
Primary research and institutional explainers supporting the physical processes, forecasting methods, and historical examples in this article.
- NASA — Space WeatherThe Sun’s influence on the space environment, technology, and exploration.
- NASA Marshall — The Solar DynamoHow flows of conducting plasma, differential rotation, and convection regenerate the Sun’s magnetic field; the dynamo remains an active research problem.
- NASA Marshall — Photospheric FeaturesSunspot umbrae and penumbrae, their temperatures and magnetic fields, and the granulation visible in the photosphere.
- NASA Marshall — The Sunspot CycleThe roughly eleven-year sunspot cycle and the latitude bands that shift toward the equator in the butterfly diagram.
- NASA — Is the Sun Causing Global Warming?Distinguishes solar variability from the causes of recent global warming.
- NASA — Solar Storms and FlaresAn illustrated introduction distinguishing flare light, energetic particles, and coronal mass ejections, including their different travel times.
- NOAA Space Weather Prediction Center — Solar Flares (Radio Blackouts)Flare durations and classification by peak GOES X-ray flux in the 0.1–0.8 nanometre band, from A through X.
- David V. Reames — Solar Energetic Particles (2017)A specialist account of particle acceleration by magnetic reconnection and CME-driven shocks, including evidence for the origins of large particle events.
- NOAA Space Weather Prediction Center — Solar Radiation StormEnergetic solar protons can arrive within tens of minutes or less, while particle storms can persist for hours to days.
- NOAA Space Weather Prediction Center — Coronal Mass EjectionsExpanding clouds of plasma and magnetic field, their travel times to Earth, and shocks that can accelerate charged particles.
- NOAA — Space Weather ScalesSeparate R, S, and G ratings describe radio blackouts, solar radiation storms, and geomagnetic storms.
- NOAA — Geomagnetic StormsSolar-wind energy transfer, southward magnetic fields, high-speed streams, and changes in Earth’s magnetic environment.
- NOAA — Aurora TutorialAuroral light production, recurrent solar-wind disturbances, and the different limits of short- and longer-range forecasts.
- NOAA Space Weather Prediction Center — Coronal HolesOpen magnetic regions supply fast solar-wind streams; persistent coronal holes are common around solar minimum.
- NOAA — HF Radio CommunicationsDayside radio absorption from flare X-rays and high-latitude absorption from energetic solar protons.
- NOAA — Ionospheric ScintillationSmall-scale electron-density structure alters radio signals and can disrupt satellite navigation.
- NOAA — Satellite DragUpper-atmosphere heating, increased density at satellite altitudes, and changes to low Earth orbits.
- NOAA — Electric Power TransmissionHow changing magnetic fields induce currents in conducting networks, with effects shaped by the ground and grid.
- NOAA — Spacecraft Environmental Anomalies Expert System–Real TimeDistinguishes surface charging, internal charging, single-event upsets, and accumulated radiation effects.
- NASA CCMC — WSA–Enlil at SWPCMagnetic maps and coronagraph-derived CME parameters drive a solar-wind propagation model, whose forecasts retain input and geometry uncertainties.
- Green and Boardsen (2006) — Duration and Extent of the Great Auroral Storm of 1859Historical auroral, magnetic, and telegraph observations of the 1859 disturbances.
- Boteler (2019) — A 21st Century View of the March 1989 Magnetic StormReconstructs the storm and its effects on the Hydro-Québec system and other power networks.
- NOAA — Remembering the Great Halloween Solar StormsReviews the 2003 eruptions and their effects on spacecraft, communications, and infrastructure.
- ESA — The May 2024 Solar Storm: Your Questions AnsweredExplains the auroras, atmospheric expansion, and operational challenges during the storm.
- Love et al. (2024) — On the Uncertain Intensity Estimate of the 1859 Carrington StormShows how uncertain historical storm intensity affects estimates of extreme-event frequency.
- NASA Watches Mars Light Up During Epic Solar StormMeasurements of radiation and widespread Martian auroras during solar activity in May 2024.
The Solar System’s Dynamics and Future
- The Sun’s Structure and Life Cycle
- Solar Activity: Flares, Sunspots, and Space Weather · You are here
- Planetary Orbits and Resonances
- Asteroid and Comet Impacts
- Planetary Climate Cycles
- The Red Giant Phase: Fate of the Inner Planets
- Kuiper Belt and Oort Cloud
- Potential Habitable Zones Beyond Earth
- Human Exploration: Past, Present, and Future
- Long-Term Solar System Evolution