A geomagnetic storm is a temporary but sometimes severe disturbance of Earth’s magnetic field, triggered when charged material from the Sun slams into the magnetosphere. The most common trigger is a coronal mass ejection, a massive cloud of magnetized plasma hurled outward from the Sun’s corona, which can cross the roughly 150 million kilometers to Earth in as little as a day or two. When the magnetic field embedded in that plasma is oriented opposite to Earth’s own field, the two connect and merge, funneling enormous amounts of energy into the near-Earth environment. The consequences ripple outward from there: surging electric currents in power grids, degraded GPS signals, expanded auroral displays, and satellite orbits that decay faster than expected.
What Sets One Off
The Sun is the instigator. During its roughly 11-year activity cycle, the frequency and violence of eruptions rise and fall. Coronal mass ejections are the heavy hitters. A study tracking major geomagnetic storms between 1996 and 2000 found that about 60% could be traced to a single identifiable CME on the Sun, while another 22% were associated with multiple CMEs that combined en route. The average transit time for a CME to reach near-Earth space was about 64 hours, and the peak of the resulting geomagnetic storm typically followed roughly 78 hours after the solar eruption. Faster CMEs arrive sooner; the relationship between speed and transit time is roughly linear, with the quickest events cutting that journey well below two days.1The American Astronomical Society / IOP Science. Identification of Solar Sources of Major Geomagnetic Storms between 1996 and 2000
Not every CME causes a geomagnetic storm. The plasma cloud has to be aimed roughly toward Earth, and its embedded magnetic field has to be oriented the right way. If the field points northward, matching Earth’s own field at the dayside boundary, it mostly slides past. If it points southward, reconnection happens: field lines from the solar wind link up with Earth’s field lines and peel open the magnetosphere, letting energy pour in. That southward-pointing field orientation is the single most important factor determining whether a given CME will cause a mild disturbance or a headline-making storm.
What Happens Inside the Magnetosphere
Once reconnection opens the door, charged particles from the solar wind stream along magnetic field lines into Earth’s inner magnetosphere. Electrons and ions spiral around Earth in opposite directions, creating a vast ring of electrical current circling the planet at altitudes of roughly 10,000 to 60,000 kilometers. This ring current is the main driver of the magnetic depression measured at the ground during a storm. Scientists quantify storm intensity using indices that track how much Earth’s horizontal magnetic field dips. A moderate storm might push the index to around negative 100 nanotesla; a severe event can go well beyond negative 200 nT.
Those indices, though, are imperfect stand-ins for what is actually happening in space. In-situ satellite measurements have shown that the standard ground-based index can overestimate the ring current’s energy content by up to about four times, because signals from other current systems (flowing along field lines and through the ionosphere) bleed into the ground-level measurement. There is also a timing mismatch: the index tends to peak several hours before the actual energy content of the ring current does, which matters when forecasters try to gauge how bad things are getting in real time.2Journal of Geophysical Research: Space Physics. Challenging the Use of Ring Current Indices During Geomagnetic Storms
Power Grids and the Invisible Current Underfoot
The most consequential ground-level effect of a geomagnetic storm is the electric field it induces in the Earth’s crust. Rapidly fluctuating magnetic fields overhead generate geoelectric fields at the surface, and those fields drive what are called geomagnetically induced currents, or GICs, through any long conductor that happens to be grounded at both ends. Power transmission lines are prime targets: quasi-direct current flows into the grid through transformer neutral connections, and even modest GIC levels can push transformer cores into saturation, causing them to draw excess reactive power, overheat, and potentially suffer permanent damage.3ScienceDirect. Classical and Recent Aspects of Power System Optimization
The geology beneath the grid matters enormously. Earth’s subsurface conductivity structure determines how strong those geoelectric fields get and which direction they point. Recent modeling has shown that the polarization of the electric field during a storm depends heavily on the local conductivity, and in some geological regions the two horizontal components of the field become strongly correlated, creating a preferred directionality. That means a power grid’s vulnerability depends not just on storm intensity but on how the grid’s lines happen to be oriented relative to the local geology. A 2023 comparison near Edmonton, Alberta demonstrated strong correspondence between the modeled geoelectric field and the actual current measured flowing through a transformer’s ground connection.4Space Weather. The Influence of 3‐D Earth Conductivity, Geoelectric Field Polarization, and Power Grid Topology on GIC Risk
The canonical modern example is the March 1989 storm that blacked out Hydro-Québec’s grid, leaving millions of people without power. A reanalysis of the event found that the substorm responsible for the blackout was externally triggered by a sudden compression of the magnetosphere, which produced a faster and more intense onset than a typical internally driven substorm would have. The resulting GIC spike was enough to trip protective relays and cascade into a system-wide collapse within about 90 seconds.5Space Weather. A 21st Century View of the March 1989 Magnetic Storm
The May 2024 storm, the strongest in over two decades, brought the issue back into sharp focus. At least five interplanetary CMEs arrived in quick succession, and a transformer in southern Sweden was damaged, disrupting the power link between Sweden and Poland. Grid operators confirmed GICs as the cause. Interestingly, the disturbance did not occur at the time of the storm’s peak geoelectric field at that location, highlighting that GIC risk is not a simple function of storm intensity: local current-system geometry and solar-wind turbulence also play critical roles.6Space Weather. The Geomagnetic Storm on 10–12 May 2024 and Its Effect on the Swedish Power Grid
Satellites, Drag, and the Starlink Lesson
High above the grid, geomagnetic storms heat and expand the upper atmosphere. The thermosphere, which extends from about 80 to 600 kilometers altitude, absorbs energy deposited by storm-driven currents and particle precipitation. As it heats up, it puffs outward, raising the atmospheric density at altitudes where low-Earth-orbit satellites fly. More air means more drag, and more drag means orbits decay faster than expected.7Space Weather. Satellite Orbital Drag During Magnetic Storms
The most dramatic recent illustration came in February 2022, when SpaceX launched 49 Starlink satellites into a low initial orbit. Two consecutive geomagnetic storms in the days surrounding the launch raised thermospheric density at 210 kilometers altitude by at least 20 to 30 percent above what had been seen in the previous nine days. Thirty-eight of the 49 satellites could not overcome the extra drag and reentered the atmosphere, a loss worth tens of millions of dollars and a vivid reminder that even relatively mild storms can have outsized consequences when timing is bad.8Space Weather. The Thermosphere Is a Drag: The 2022 Starlink Incident and the Threat of Geomagnetic Storms to Low Earth Orbit Space Operations
GPS Errors and Communication Blackouts
Geomagnetic storms wreak havoc on the ionosphere, the electrically charged layer of the atmosphere that GPS signals must pass through. During a storm, the total electron content of the ionosphere can surge, and turbulent plasma structures create rapid signal fluctuations called scintillation. The practical result is degraded positioning accuracy. Analysis of a severe storm over China showed that when scintillation reached moderate levels, the average GPS positioning error jumped above 0.8 meters, a serious problem for precision applications like surveying and autonomous vehicles.9Radio Science. Assessing the Kinematic GPS Positioning Performance Under the Effect of Strong Ionospheric Disturbance Over China and Adjacent Areas During the Magnetic Storm At lower latitudes, storms can cause outright loss of satellite signals as the disturbed ionosphere scatters them beyond recovery.10Journal of Global Positioning Systems. Analyzing the Influence of Global Ionospheric Scintillation on GPS PPP during the Geomagnetic Storm of 26-28 February 2023
High-frequency radio communication, which depends on ionospheric reflection, is also vulnerable. The May 2024 storm caused an HF radio blackout across the 2-to-12 MHz frequency band, driven by intense ionization in the lower ionosphere from an associated solar flare. Dayside total electron content more than doubled during the event.11PubMed Central. Super-Intense Geomagnetic Storm on 10-11 May 2024: Possible Mechanisms and Impacts Even at high latitudes, where GPS scintillation is often driven by smaller-scale plasma structures near the auroral zone, recent observations have linked enhanced phase scintillation to features called space hurricanes, vortex-like patterns of plasma flow in the polar ionosphere.12Space Weather. Ionospheric Scintillation and Geomagnetic Disturbance Caused by Space Hurricanes
Submarine Cables and Buried Pipelines
The same geoelectric fields that threaten power grids also affect any other long grounded conductor. Modern submarine fiber-optic cables, despite carrying data as light, still contain a copper conductor that feeds electrical power to the repeaters spaced along the cable’s length. During a storm, geomagnetically induced voltages add to or subtract from that power feed, potentially pushing repeaters outside their operating range.13Space Weather. An Examination of Geomagnetic Induction in Submarine Cables The risk is real enough that researchers have begun developing dedicated simulation tools to predict induced voltages along specific cable routes during severe storms.14Space Weather. Validating SCUBAS Predictions of Geomagnetically Induced Voltage in Submarine Cables Using Legacy Superstorm Observations
Buried oil and gas pipelines face a different but related problem. GICs flowing through the pipe can override the cathodic protection systems that prevent corrosion. During days of high geomagnetic activity, the induced currents can be large enough to accelerate corrosion and potentially shorten the pipeline’s useful life, depending on the electrical conductivity of the surrounding soil.15Journal of Applied Geophysics. Currents induced by geomagnetic storms on buried pipelines as a cause of corrosion
The Carrington Event and the Question of Scale
The largest geomagnetic storm in recorded history struck in September 1859. A CME reached Earth extraordinarily quickly after a white-light solar flare observed by Richard Carrington and Richard Hodgson. The resulting magnetic disturbance was so intense that a negative horizontal-field swing of roughly 3,000 nT was estimated at Rome, and the auroral oval expanded to mid-latitudes. Telegraph systems across roughly 200,000 kilometers of wire were disrupted, many for eight hours or more.16PubMed Central. Duration and extent of the great auroral storm of 1859 In Boston, telegraph operators disconnected their batteries and found they could send messages using only the current induced by the storm itself.17Advances in Space Research. The super storms of August/September 1859 and their effects on the telegraph system
What would a Carrington-scale event do to today’s infrastructure? One recent modeling study estimated that a 250-year return-period storm would cause daily economic losses from transformer thermal heating alone of about $1.8 billion in the United States, disrupting power for roughly 5.1 million people and 135,000 businesses. The authors emphasized that these figures are conservative lower bounds, excluding voltage collapse, cascading failures, and the long restoration times that damaged high-voltage transformers would require.18AGU Advances. Major space weather risks identified via coupled physics–engineering–economic modeling
Auroras, Unusual Colors, and STEVE
The most visually spectacular effect of a geomagnetic storm is the expansion of auroral displays to latitudes where they are rarely seen. During the May 2024 storm, people across much of the continental United States and southern Europe photographed vivid curtains of light. The familiar green comes from oxygen atoms emitting at 557.7 nanometers, while reds appear at higher altitudes from a different oxygen transition. During intense storms, unusual colors can emerge. Extended magenta-colored aurora observed during a recent event was traced to emissions from ionized nitrogen molecules, driven partly by heavy particle precipitation and resonant scattering of upwelling ions.19Scientific Reports. Extended magenta aurora as revealed by citizen science
Geomagnetic storms have also drawn attention to STEVE (Strong Thermal Emission Velocity Enhancement), a narrow luminous arc that appears at subauroral latitudes. STEVE’s continuum emission is produced by fast ion flows and is distinct from both aurora and airglow.20Geophysical Research Letters. A Mechanism for the STEVE Continuum Emission The green “picket fence” structures sometimes seen alongside STEVE are caused by oxygen emission at relatively low excitation energies: the spectral signature shows strong oxygen and nitrogen emissions but lacks the higher-energy ionized-nitrogen lines typical of auroral particle precipitation, suggesting the exciting particles have energies below about 19 electron volts.21Geophysical Research Letters. Subauroral Green STEVE Arcs: Evidence for Low‐Energy Excitation STEVE remains an active research area partly because citizen science photographers first brought it to scientists’ attention, a reminder that geomagnetic storms keep revealing phenomena we had not previously catalogued.
Birds That Lose Their Way
Geomagnetic storms do not just affect infrastructure. Many migratory birds appear to navigate partly using Earth’s magnetic field, and storm-driven distortions of that field seem to confuse them. A large radar-based study of nocturnal bird migration across the United States found a 9 to 17 percent decrease in migration intensity during severe space weather events, in both spring and fall, after controlling for ordinary atmospheric weather. During fall migration, birds showed reduced effort flying against the wind, suggesting that active navigation was depressed and birds were drifting more passively with the airflow. The effect was strongest under overcast skies, when celestial cues like stars were also obscured, pointing to a double disruption of the two main orientation systems birds rely on at night.22PubMed Central. Space weather disrupts nocturnal bird migration
Controlled laboratory experiments have added texture to that finding. When European robins were exposed to simulated magnetic storm conditions, they reduced their nighttime migratory restlessness but increased activity in the early morning, as if shifting their migration to daytime hours when visual landmarks and the Sun’s position could compensate for a degraded magnetic sense.23PubMed Central. Magnetic storms disrupt nocturnal migratory activity in songbirds The ecological consequences of widespread storm-induced disorientation remain poorly understood, but given that billions of birds migrate across North America alone each season, even a modest disruption in navigation could have ripple effects on arrival timing, energy budgets, and breeding success.
How Much Warning Do We Get
Forecasting geomagnetic storms is a two-stage problem. The first stage relies on remote-sensing observations of the Sun: if a large CME is detected erupting from the solar disk and appears aimed Earthward, forecasters can issue a watch, typically one to three days before arrival. But this initial warning is coarse. The crucial detail, whether the CME’s magnetic field will be oriented southward when it arrives, is extremely difficult to determine from solar observations alone.24Space Weather. Monitoring the Solar Wind Before It Reaches L1
The second stage kicks in when the CME reaches the L1 Lagrange point, about 1.5 million kilometers sunward of Earth, where spacecraft like NOAA’s DSCOVR and NASA’s ACE measure the solar wind in real time. At typical solar wind speeds, this buys roughly 30 to 60 minutes of lead time. Algorithms that ingest L1 magnetic field data can then calculate the probability that a geomagnetic storm will follow and estimate its intensity.25Space Weather. Bayesian prediction of geomagnetic storms: Wind data, 1996–2010 More recently, deep-learning models trained on historical L1 data and storm-index records have aimed to push prediction horizons out to multiple hours, though uncertainty grows substantially with lead time.26Space Weather. Forecasting Geomagnetic Storm Disturbances and Their Uncertainties Using Deep Learning
For power grid operators, that narrow window matters. Mitigation strategies include reducing power flows on vulnerable long-distance transmission lines, opening transformer neutral connections to block GIC, and keeping reactive-power reserves high. The challenge is that these actions carry costs and require confident forecasts to justify. Strengthening collaboration between researchers, grid operators, and policymakers remains one of the most-cited recommendations for reducing economic risk from future storms.27PLoS One. Geomagnetic disturbances and grid vulnerability: Correlating storm intensity with power system failures
Miyake Events and the Deep Past
Tree rings offer an unexpected window into extreme solar activity far beyond the era of telescopes and magnetometers. Annually resolved measurements of radiocarbon in tree rings have revealed rare, sharp spikes in carbon-14 production, now known as Miyake events, that likely reflect bursts of cosmic radiation from the Sun far more intense than anything observed in the modern instrumental record.28Proceedings of the Royal Society A. Modelling cosmic radiation events in the tree-ring radiocarbon record The best-studied event, around 774 CE, deposited several times more carbon-14 than a typical solar cycle’s worth. If a Miyake-class event were accompanied by a proportionally large CME directed at Earth, the resulting geomagnetic storm would dwarf Carrington. Whether that scales linearly is debated, but the existence of these events makes clear that the historical record of storms we have witnessed firsthand covers only a thin slice of what the Sun is capable of.
What Storms Look Like at Mars
Earth’s strong magnetic field is both shield and antenna: it protects the surface from direct particle bombardment but channels energy into concentrated current systems that cause infrastructure trouble. Mars, which lost its global magnetic field billions of years ago, faces a different reality. The MAVEN spacecraft has observed CME impacts and solar energetic particle events at Mars, including a series of moderate events during solar cycle 24. Without a magnetosphere to deflect incoming plasma, the solar wind interacts directly with the Martian atmosphere, stripping away ions over time. The storms observed were moderate by historical standards, consistent with the generally weaker conditions of solar cycle 24 compared to some of its predecessors.29Journal of Geophysical Research: Space Physics. MAVEN observations of the solar cycle 24 space weather conditions at Mars For future human missions to Mars, the absence of a protective magnetic field means that storm-driven radiation becomes a direct crew safety concern rather than primarily an infrastructure one.