Solar Flares, CMEs and Space Weather: When the Sun Threatens Our Infrastructure

On 1 September 1859, British astronomer Richard Carrington was sketching a particularly large group of sunspots through his telescope when he observed two intensely bright patches of white light appear near the sunspot group, intensify, and fade over the course of approximately five minutes. He was the only professional astronomer to witness the event in real time. Seventeen hours later, telegraph networks across Europe and North America failed simultaneously. Auroras were visible as far south as Cuba and Hawaii. Some telegraph operators received electric shocks from their equipment. In several cases, operators disconnected their batteries and continued sending messages using only the geomagnetically induced current flowing through their lines.

The Carrington Event remains the most powerful geomagnetic storm in recorded history. Its peak Dst index — a measure of ring current intensity and geomagnetic disturbance — is estimated at approximately -850 nT, compared to a major storm threshold of -100 nT. If an equivalent event occurred today, the consequences for a civilisation dependent on satellite navigation, high-voltage power transmission, and satellite communications would be qualitatively different from the telegraph disruptions of 1859 — and quantitatively catastrophic.

Key parameters

Parameter	Value
Solar flare X-ray transit to Earth	8 min 20 s
CME transit time to Earth	18–72 hours
Carrington Event (1859) estimated Dst	< −1,600 nT
Quebec blackout (1989) Dst	−589 nT
NOAA SWPC warning lead time	15–60 min (X-ray), 1–3 days (CME)
Class X flare frequency (solar max)	~10/year

The Solar Dynamo: Why the Sun is Electromagnetically Violent

The Sun is a magnetised plasma, and like all magnetised plasmas it is electromagnetically unstable. The solar dynamo — driven by differential rotation (the solar equator rotates every 25 days; the poles every 35 days) and convective motion in the outer third of the solar interior — continuously generates, amplifies, distorts, and destroys magnetic field structures throughout the corona.

Solar activity follows an approximately 11-year cycle, the Schwabe cycle, during which the number and intensity of sunspots, flares, and coronal mass ejections varies from solar minimum (quiet Sun, few spots) to solar maximum (active Sun, frequent eruptions). The 11-year cycle is itself part of a 22-year Hale cycle, in which the Sun’s global magnetic polarity reverses and then reverses again. Cycle 25, which began in December 2019, reached its solar maximum in late 2024–2025 — one of the most active in the past two decades.

Solar Flares: Magnetic Energy Release at X-ray Wavelengths

A solar flare is a sudden, intense brightening of the solar atmosphere caused by the rapid release of magnetic energy stored in the corona. When antiparallel magnetic field lines are driven together by convective motions in the photosphere, they can undergo magnetic reconnection — a topological change in which field line connectivity rearranges, releasing stored magnetic energy as kinetic energy of plasma, heat, and radiation.

The energy released in a major flare can reach 10²⁵ joules — equivalent to roughly 10 billion hydrogen bombs. This energy is emitted primarily in X-rays and extreme ultraviolet (EUV) radiation, with smaller contributions in gamma rays, radio waves, and accelerated particle beams.

Flares are classified by their peak X-ray flux at 1–8 Å wavelength as measured by GOES satellites:

Class	Peak flux (W/m²)	Approximate frequency
A	< 10⁻⁷	Multiple daily, solar minimum
B	10⁻⁷ – 10⁻⁶	Daily, moderate activity
C	10⁻⁶ – 10⁻⁵	Several per day, active Sun
M	10⁻⁵ – 10⁻⁴	Weekly at solar maximum
X	> 10⁻⁴	Several per year at solar max

The 1859 Carrington flare was an extreme X-class event. The most powerful flare in the satellite era was the X28+ event of 4 November 2003 — it saturated GOES detectors, preventing accurate measurement of its peak.

The immediate effect of a solar flare on Earth is ionospheric disruption. The X-ray and EUV pulse, arriving at the speed of light (~8 minutes from the Sun), ionises the sunlit ionosphere, increasing the electron density in the D region (60–90 km altitude) and causing shortwave radio blackouts on the sunlit hemisphere. GPS signals, which pass through the ionosphere and are sensitive to its refractive index, experience positioning errors. This ionospheric response is immediate and lasts minutes to hours.

Coronal Mass Ejections: The Slower, More Dangerous Threat

A coronal mass ejection (CME) is a magnetised plasma cloud erupted from the solar corona into interplanetary space. CMEs are often but not always associated with flares. The largest events eject up to 10¹³ kg of plasma — roughly the mass of 10,000 Empire State Buildings — at speeds of 250 to over 3,000 km/s.

A fast CME (>1,500 km/s) takes 18–24 hours to travel from the Sun to Earth. A typical moderate CME takes 2–4 days. This transit time is the most operationally valuable parameter in space weather forecasting — it defines the warning time available before the magnetic structure of the CME arrives at Earth’s magnetosphere.

The geomagnetic storm is produced not by the CME’s kinetic energy directly but by the interaction of the CME’s magnetic field with Earth’s magnetospheric field. If the CME carries a southward-directed magnetic field component (Bz < 0), it reconnects with Earth’s northward magnetospheric field on the dayside, driving energy into the magnetosphere. The ring current — a toroidal current of energetic ions at approximately 2–7 Earth radii — intensifies, producing the negative Dst perturbation that characterises a geomagnetic storm.

The critical variable — Bz orientation — cannot be measured until the CME reaches the L1 Lagrange point, approximately 1.5 million km upstream of Earth, roughly 15–60 minutes before magnetospheric impact. All current operational forecasting of geomagnetic storm intensity therefore has a maximum reliable lead time of 15–60 minutes for the most important parameter.

Infrastructure Consequences

A repeat of the Carrington Event would interact with modern infrastructure in ways that did not exist in 1859.

Power grids: Geomagnetically induced currents (GICs) are quasi-DC currents driven into power transmission lines and pipelines by the rapidly changing magnetic field during a geomagnetic storm. In long transmission lines, GICs saturate transformer cores, causing reactive power demand spikes, harmonic distortion, and — in extreme events — thermal damage to transformer windings. High-voltage grid transformers are not stockpiled items; they are custom-built, weigh hundreds of tonnes, and have lead times of 12–18 months.

The March 1989 geomagnetic storm caused the collapse of the Hydro-Québec power grid within 92 seconds, leaving 6 million people without power for up to nine hours. Sustained GICs are estimated to have caused permanent damage to 21 transformers in the continental US. A Carrington-scale event could potentially take down significant portions of power grids across North America and northern Europe, with recovery times measured in months rather than hours.

Satellites: The intensified Van Allen belts following a major CME irradiate satellite electronics and solar panels. The October/November 2003 “Halloween storms” caused anomalies on 47 of 59 monitored spacecraft, with two satellites — ADEOS-2 and Mars Odyssey (instrument damage) — sustaining permanent damage. The outer Van Allen belt can intensify by a factor of 1,000 within hours, far exceeding the design tolerance of many satellites.

GPS: The ionospheric disturbance during a major storm causes GPS position errors of tens of metres for single-frequency receivers and can make accurate positioning impossible for hours. Aviation navigation, precision agriculture, financial transaction timestamps, and power grid synchronisation all depend on GPS.

Radio communications: High-frequency (HF, 3–30 MHz) communications used by aviation, maritime operations, and emergency services are disrupted by ionospheric absorption during flares and phase distortion during storms.

Space Weather Forecasting

The primary operational space weather forecasting agencies are NOAA’s Space Weather Prediction Center (SWPC), ESA’s Space Weather Service Network, and the UK Met Office Space Weather Operations Centre.

NOAA operates the Advanced Composition Explorer (ACE) and DSCOVR satellites at the L1 Lagrange point as solar wind monitors — providing the 15–60 minute warning window before CME arrival. The Solar Dynamics Observatory (SDO), launched in 2010, provides near-real-time imagery of the solar corona at multiple EUV wavelengths, enabling rapid flare detection and post-eruption CME trajectory estimation.

ESA’s SOHO (Solar and Heliospheric Observatory), operational since 1996, provides coronagraph imagery that tracks CME launch and initial propagation. The STEREO-A spacecraft, in a heliocentric orbit near Earth’s orbit, provides a different viewpoint on solar eruptions.

Predicting CME arrival time at Earth-accuracy to ±6–12 hours is achievable with current physics-based propagation models (ENLIL, developed by Dusan Odstrcil at NOAA/CU Boulder). Predicting Bz orientation — the critical geomagnetic storm parameter — ahead of CME arrival at L1 remains an unsolved problem in solar physics.

The two most practically important advances needed in space weather forecasting: reliable 24–48 hour Bz prediction, and an upstream solar wind monitor positioned further from Earth than L1 (perhaps at L5, where ESA’s Vigil mission, targeted for the late 2020s, would provide flank solar wind observations). These advances would transform the warning time available for grid operators, satellite controllers, and aviation managers from minutes to hours.

The Carrington Event was a data point, not a historical anomaly. The statistical recurrence interval for Carrington-scale events is estimated at 150 years by some analyses, 100 years by others. The last major event was in 1989. The question of when the next one occurs is a matter of when, not if.

For the radiation environment these events create for astronauts and spacecraft electronics, see radiation in space: Van Allen belts, cosmic rays, and the human exposure problem.