Solar-Terrestrial Effects
The Sun and Earth are connected by a continuous stream of plasma and magnetic fields. When the Sun erupts, a chain of physical processes links the solar surface to our magnetosphere, ionosphere, and ground-level infrastructure. Understanding this chain is essential for protecting the technologies modern society depends on.
The Sun-Earth Connection Chain
A typical sequence begins with a solar flare — an intense burst of electromagnetic radiation from an active region on the Sun. Flares are classified by their X-ray intensity as A, B, C, M, or X class, with X being the most powerful.
Many flares are accompanied by a coronal mass ejection (CME), a massive cloud of magnetised plasma hurled into interplanetary space. While the flare's radiation arrives at Earth in about 8 minutes (the speed of light), the CME travels much more slowly — typically 400–2,000 km/s — and takes one to three days to reach us.
When the CME's magnetic field has a strong southward component (negative Bz), it can couple efficiently with Earth's northward field through magnetic reconnection on the dayside magnetopause. This opens the magnetosphere, allowing solar wind energy and particles to flood in. The result is a compression of the magnetosphere, intensification of the ring current (a toroidal belt of ions circling Earth), and a geomagnetic storm as measured by indices such as Kp and Dst.
Effects on Technology
Satellites: Drag and Electronics
Geomagnetic storms heat the upper atmosphere, causing it to expand. This increases atmospheric drag on satellites in low Earth orbit, which can alter orbits and even cause premature re-entry. In February 2022, a moderate geomagnetic storm caused SpaceX to lose 38 of 49 newly launched Starlink satellites due to increased drag before they could raise their orbits.
Energetic particles also cause single-event upsets (SEUs) — bit-flips in satellite memory and logic circuits. These can corrupt data, trigger safe-mode resets, or in severe cases permanently damage components. Spacecraft charging from intense particle bombardment can produce electrical arcing that destroys electronics.
GPS and Navigation
GPS signals travel through the ionosphere, where storm-driven irregularities create rapid fluctuations in signal phase and amplitude (scintillation). This degrades positional accuracy from metres to tens of metres, affecting aviation approach procedures, precision agriculture, surveying, and autonomous vehicles.
Power Grids
Rapidly changing magnetic fields during a geomagnetic storm induce geomagnetically induced currents (GICs) in long conductors such as power lines and pipelines. GICs can saturate the magnetic cores of high-voltage transformers, leading to overheating, increased reactive power demand, and in extreme cases, permanent transformer damage.
The most dramatic modern example is the March 1989 Quebec blackout. A severe geomagnetic storm (Kp 9) caused GICs that tripped protective relays on Hydro-Québec's grid, plunging the entire province of Quebec into darkness for nine hours. The Carrington Event of 1859 — the most intense geomagnetic storm on record — induced currents strong enough to shock telegraph operators and set fire to telegraph paper. A Carrington-class event today could cause continent-wide transformer damage and extended blackouts, with estimated economic impacts in the trillions of dollars.
HF Radio Communications
High-frequency (HF) radio signals bounce off the ionosphere to achieve long-range communication. Solar flares produce intense X-ray and EUV radiation that dramatically increases ionisation in the D layer of the ionosphere on the dayside of Earth. This enhanced ionisation absorbs HF signals, causing partial or complete radio blackouts that can last from minutes to hours. These blackouts are classified on the NOAA R-scale from R1 (minor) to R5 (extreme). Trans-oceanic aviation, maritime, and emergency communications all depend on HF radio as a backup when satellite links are unavailable.
Aurora: Nature's Light Show
Auroras form when charged particles — primarily electrons — are accelerated along magnetic field lines into the upper atmosphere, where they collide with oxygen and nitrogen atoms. Excited oxygen atoms emit green light (557.7 nm, the most common aurora colour) and red light (630.0 nm, at higher altitudes where the atmosphere is thinner). Nitrogen produces blue and violet hues.
During quiet conditions, the auroral oval sits at roughly 65–70° geomagnetic latitude — visible from places like Tromsø (Norway), Fairbanks (Alaska), and Yellowknife (Canada). As geomagnetic activity increases, the oval expands equatorward. During moderate storms (Kp 5–6), aurora can be seen from the northern United States and central Europe. During extreme storms (Kp 8–9), aurora has been observed as far south as Mexico and the Caribbean.
The Kp index provides a rough guide to visibility latitude: Kp 5 pushes the aurora to about 50° geomagnetic latitude, Kp 7 to about 45°, and Kp 9 to about 40° or below. Local conditions (light pollution, cloud cover, and the time of night) matter as much as the Kp value for actual visibility.
Space Radiation Hazards
Solar energetic particle (SEP) events can produce dangerous radiation levels in space. Astronauts aboard the International Space Station shelter in more heavily shielded sections during major events. Future missions to the Moon and Mars, where there is no magnetospheric protection, will face even greater risks and will need dedicated storm shelters.
At aviation altitudes (9–12 km), passengers and crew are exposed to elevated cosmic radiation compared to ground level. During solar particle events, radiation dose rates at polar flight routes can increase by a factor of 10 or more. Airlines routinely monitor space weather and may re-route polar flights to lower latitudes during major events to reduce crew and passenger exposure, at the cost of additional fuel and flight time.
The NOAA S-scale (S1 through S5) classifies solar radiation storm intensity. At S1, there are minor impacts on HF radio at polar latitudes. At S5, astronauts face unavoidable high radiation doses, satellite electronics may be permanently damaged, and complete HF radio blackouts occur at polar latitudes.