In brief
A solar storm is a major disturbance of the space environment triggered by a flare or a coronal mass ejection. It takes 15 minutes to 3 days to reach Earth depending on its nature, and can produce auroras, radio blackouts, GPS degradation and satellite damage.
Three events, three effects
The term 'solar storm' actually covers three distinct phenomena, which can occur alone or in cascade.
- Solar flare: sudden release of magnetic energy in the solar corona. Emits intense X-ray and ultraviolet radiation that reaches Earth in 8 minutes. Scale: A, B, C, M, X (logarithmic, each letter × 10).
- Coronal mass ejection (CME): bubble of magnetised plasma launched into interplanetary space at 500-3000 km/s. Takes 15 hours to 3 days to reach Earth.
- Geomagnetic storm: consequence of an effective CME hitting the magnetosphere. Lasts hours to days. This is the 'storm' in the usual sense; it is what produces auroras.
The trigger
Everything starts in the solar corona, where magnetic field lines from sunspots twist and tangle. When tension exceeds a critical limit, the lines reconnect abruptly: stored magnetic energy is released within minutes as radiation (flare) and/or expelled plasma (CME). A single major reconnection can release the equivalent of billions of atomic bombs.
The probability of a geo-effective event depends on the active region's position on the solar disc. A limb CME is tangential: little chance of hitting us. A centre-disc CME (a 'geoeffective' region) is almost certain to hit Earth three days later.
Propagation
Between the Sun and Earth, the CME rides the solar wind. Two satellites in L1 orbit (ACE and DSCOVR, 1.5 million km from Earth towards the Sun) detect its arrival about 30 to 60 minutes before it reaches the magnetosphere. These 30-60 minutes are the forecasters' warning window.
Key parameter on these satellites: Bz, the north-south component of the interplanetary magnetic field. Positive Bz (northward) lets the magnetosphere close up without reconnection; strongly negative Bz (southward) enables massive coupling. The more negative Bz and the higher solar-wind speed, the more intense the storm.
Effects on Earth
A major geomagnetic storm touches far more than the night sky.
- HF telecommunications: radio signal absorption in the polar region. Transpolar aviation routes are sometimes rerouted.
- GPS: accuracy degradation from a few metres to tens of metres for hours, especially at high latitudes.
- Power grids: induced currents in high-voltage lines, capable of tripping transformers. The Quebec blackout of March 1989 (9-hour outage for 6 million households) is the historical example.
- Satellites: pointing anomalies, faster orbital decay (thermosphere expanded by heating), long-term solar-panel degradation.
- Auroras: the most visible and harmless effect for the general public.
Historical storms
A few events are landmark.
- Carrington event, 1-2 September 1859: the largest known storm, sustained Kp 9+ over several days. Telegraphs caught fire, auroras were seen as far south as Cuba. A modern Carrington would cause trillions of euros of damage (Lloyd's, NASA studies).
- March 1989: Kp 9, Quebec blackout, Concorde rerouted.
- Halloween 2003: series of CMEs, auroras visible from Texas.
- August 1972: fastest CME observed (2850 km/s), caused spontaneous detonation of magnetic mines in Vietnam.
- 10-11 May 2024: the strongest storm since 2003, Kp 9 for several hours, auroras visible across the entire Northern Hemisphere down to Mexico.
Can you prepare?
For the general public, 'preparing' mostly means getting a timely alert to go out and observe. Pulsar notifications trigger the moment conditions cross your city's visibility threshold, with typical 30-minute to 3-hour lead time based on L1 detection.
For critical industries (telecom, satellite operators, power grids), preparation involves safe-mode procedures: folding steerable antennas, dropping transmitted power, safing satellites, isolating the most exposed transformers. This B2B segment is what Pulsar Space Intelligence serves.