Space weather refers to the conditions in space driven by the Sun and their effects on everything in the solar system, including Earth. Just as atmospheric weather involves wind, rain, and temperature changes, space weather involves streams of charged particles, bursts of radiation, and massive magnetic eruptions from the Sun. These events can disrupt power grids, degrade GPS signals, expose airline passengers to elevated radiation, and produce the aurora borealis. Though the Sun sits nearly 100 million miles away, its activity shapes daily life in ways most people never consider.
The Sun Drives Everything
The Sun constantly releases a stream of charged particles called the solar wind. Under normal conditions, this slow wind travels at roughly 400 kilometers per second through the plane where Earth orbits. Regions on the Sun’s surface called coronal holes can produce faster streams, pushing speeds to 500 to 800 kilometers per second. When these fast and slow streams collide, they create interaction regions with extremely high particle densities and strong magnetic fields, which can rattle Earth’s magnetic environment when they arrive.
Solar activity follows an approximately 11-year cycle of rising and falling intensity, measured by the number of sunspots visible on the Sun’s surface. We are currently in Solar Cycle 25, with peak activity expected around July 2025 and a predicted peak of 115 sunspots. During solar maximum, the Sun produces far more flares and eruptions, making disruptive space weather events more frequent.
Solar Flares vs. Coronal Mass Ejections
Two types of solar eruptions matter most for space weather, and they work differently. A solar flare is an intense burst of light and radiation that travels at the speed of light, reaching Earth in about eight minutes. The energy from a strong flare can immediately cause radio blackouts on the sunlit side of Earth, interfering with shortwave radio and radio navigation systems.
A coronal mass ejection, or CME, is a physically different event. Magnetic field lines in the Sun’s outer atmosphere twist and kink, generating superheated plasma. When those magnetic connections snap, billions of tons of solar material launch into space, sometimes traveling faster than 600 miles per second. A CME typically takes one to three days to reach Earth. One well-documented CME in late 2008 impacted Earth roughly three days after leaving the Sun. A single solar event can produce both a flare and a CME, but the two are not the same thing: the flare is a flash of radiation, while the CME is a physical cloud of magnetized particles barreling through space.
What Happens When a CME Hits Earth
Earth’s magnetic field acts as a shield, deflecting most incoming solar material. During a mild CME impact, the magnetosphere absorbs the blow and life continues uninterrupted. During a powerful CME, the magnetic field gets compressed and distorted, setting off a chain of effects that ripple down to the ground.
The collision energizes currents high in the atmosphere called the auroral electrojet, which can reach millions of amperes. These currents disturb Earth’s magnetic field, which in turn induces electric fields along the planet’s surface. During a moderate geomagnetic storm, those electric fields measure 1 to 5 volts per kilometer. The largest ever recorded reached 20 volts per kilometer. These ground-level electric fields push quasi-direct currents into the power grid through the grounded connections of high-voltage transformers. Even small amounts of this current can saturate a transformer’s iron core, meaning the core can no longer contain its magnetic energy. The result: overheating of internal components, insulation damage, harmonic distortion in the electrical supply, and voltage instability. In a severe enough storm, this cascade can threaten grid collapse across wide areas.
GPS, Satellites, and Communication
Space weather also distorts the layer of charged particles in Earth’s upper atmosphere called the ionosphere. GPS signals pass through this layer on their way from satellites to your phone or car. When solar activity stirs up the ionosphere, it creates rapid fluctuations in signal strength known as scintillation. These fluctuations degrade positioning accuracy, cause GPS receivers to lose track of satellites, and in severe cases lead to complete signal loss. Equatorial and low-latitude regions are especially vulnerable because scintillation events there tend to produce deeper signal fades.
Strong solar flares produce immediate radio blackouts by flooding the ionosphere with X-ray energy. At the most extreme level on NOAA’s scale, high-frequency radio communication goes completely dark across the entire sunlit hemisphere for hours. For aviation, maritime operations, and emergency services that rely on HF radio, these blackouts are more than an inconvenience.
Radiation Risks at High Altitude
Earth’s atmosphere and magnetic field shield people on the ground from nearly all solar radiation. At cruising altitude, that protection thins considerably. During solar particle events, when the Sun hurls high-energy protons toward Earth, radiation exposure on commercial flights increases. A CDC study of flight attendant radiation doses found that maximum exposure during a single flight segment reached as high as 1.2 millisieverts during one solar particle event. For context, a standard chest X-ray delivers about 0.1 millisieverts. Multiple flight segments during an active event compound the dose.
For astronauts outside the protection of a spacecraft, the risk is far greater. NOAA’s most extreme solar radiation storm rating describes an “unavoidable high radiation hazard” for anyone conducting a spacewalk.
The Aurora: Space Weather You Can See
The most visible sign of space weather is the aurora. When charged particles from the solar wind funnel along Earth’s magnetic field lines and collide with gases in the upper atmosphere, those gases glow. The specific color depends on which gas is struck and at what altitude.
- Green: Oxygen molecules excited between 60 and 120 miles altitude
- Red: Oxygen above 120 miles
- Blue: Nitrogen between 60 and 120 miles
- Pink: Nitrogen below 60 miles, creating the reddish-purple glow along the aurora’s lower edge
During powerful geomagnetic storms, the aurora expands far beyond its usual polar regions and becomes visible at much lower latitudes. The stronger the storm, the farther south (or north, in the Southern Hemisphere) the lights reach.
How Space Weather Is Measured
NOAA’s Space Weather Prediction Center uses three scales to communicate space weather severity, similar to how hurricane categories work for tropical storms.
- G-scale (Geomagnetic Storms): Ranges from G1 (minor) to G5 (extreme). A G5 storm can cause widespread voltage control problems and potential power grid collapse.
- S-scale (Solar Radiation Storms): Ranges from S1 to S5. An S5 event poses extreme radiation hazards to astronauts and elevated risk to high-altitude aircraft passengers and crew.
- R-scale (Radio Blackouts): Ranges from R1 to R5. An R5 event means complete high-frequency radio blackout across the entire sunlit side of Earth, lasting hours.
The Worst Case: The Carrington Event
The most powerful space weather event in recorded history struck on September 1 to 2, 1859. A massive solar flare observed by astronomer Richard Carrington was followed by a CME that reached Earth in under 18 hours, an extraordinarily fast transit. The resulting geomagnetic storm drove Earth’s magnetic field to a disturbance of roughly negative 1,760 nanoteslas, dwarfing anything measured since. For comparison, a severe modern geomagnetic storm might produce disturbances of negative 300 to 500 nanoteslas.
Telegraph systems across the United States and Europe failed. Currents induced in telegraph wires were strong enough to arc and start fires at telegraph stations. Auroras were visible as far south as the Caribbean. If a storm of that magnitude hit today, with our vastly greater dependence on electrical grids, satellites, and GPS, the consequences would be far more disruptive. Estimates from various analyses suggest potential economic damages in the trillions of dollars, largely from prolonged power grid failures and the months-long lead time required to manufacture and replace damaged high-voltage transformers.