Aurora

Formation: how auroras are made

The Sun continuously emits the solar wind — a stream of charged particles and plasma flowing outward through the Solar System. During periods of heightened solar activity, particularly around solar flares and coronal mass ejections (CMEs), the solar wind intensifies and can disturb Earth's magnetosphere. Under normal conditions the magnetosphere deflects most of this particle flow around the planet, but at the polar regions, where magnetic field lines converge and dip toward the surface, some charged particles — mainly electrons and protons — are funnelled inward along those lines and descend into the upper atmosphere.

As these energetic particles collide with atoms and molecules of oxygen and nitrogen, they transfer energy to the gas, exciting the electrons of those atoms to higher energy states. When the electrons return to their ground states they release that energy as photons of light — producing the glow of the aurora. The process is analogous to what happens inside a fluorescent tube or neon lamp, with the atmosphere itself acting as the luminous medium.

Because the magnetic field acts as a guide rather than a uniform funnel, the aurora forms not at a single point but along an auroral oval — a roughly circular band around each magnetic pole. When geomagnetic activity increases, this oval expands toward lower latitudes, bringing auroras into view for observers far from the polar regions. Auroral activity is strongly tied to the Sun's approximately 11-year cycle, peaking near solar maximum when solar flares and CMEs are most frequent and powerful.

Colors, altitudes, and the physics of emission

The color of an aurora is not arbitrary: it is determined by which atmospheric gas is being excited, at what altitude, and by the quantum properties of the specific electronic transition involved. The result is a layered structure in which different colors dominate at different heights.

Green is by far the most common auroral color, appearing in roughly 90 percent of visible displays. It is produced by atomic oxygen emitting at a wavelength of 557.7 nm — the OI green line — at altitudes of approximately 100 to 200 km, with the greatest intensity around 110 to 150 km. The excited state responsible for this emission has a lifetime of roughly 0.7 seconds; below about 95 km the atmosphere is dense enough that collisions de-excite the oxygen atom before it can radiate, quenching the green line. Combined with the human eye's peak sensitivity near 555 nm, this makes green both the physically dominant and perceptually brightest color in most auroral scenes.

Red auroras arise from the same species — atomic oxygen — but via a different metastable transition at 630.0 nm. This excited state has a lifetime of approximately 100 seconds, meaning it requires very low collision frequencies to survive long enough to emit. That condition is only met at altitudes of roughly 200 to 300 km and above. Entirely red displays are rare, particularly at lower latitudes, but red tops or caps above green curtains are a more common sight during active storms.

Blue, violet, and purple hues are produced by molecular and ionized nitrogen (N2 and N2+). The first negative system of N2+ emits prominently near 391 nm and 428 nm in the blue-violet, while N2's second positive system contributes bands spanning red to blue that together often appear visually purple. These emissions occur predominantly below about 100 km, where nitrogen is abundant and fast-radiating nitrogen band systems can still emit before collisional quenching. Pure blue aurora is extremely rare; violet or purple fringes appear more often along the lower edges of bright curtains during intense geomagnetic storms.

Pink and magenta bands, frequently seen as a narrow fringe beneath green curtains during strong storms, result from the combined emission of blue-violet nitrogen and green or red oxygen light. They are most prominent at the lowermost border of bright auroras, typically below 100 km. The overall auroral spectrum is therefore a superposition of discrete atomic oxygen lines at 557.7 and 630.0 nm and structured molecular nitrogen band systems in the blue-to-red range — a line-and-band spectrum that serves as a precise diagnostic of upper-atmospheric composition and the altitude of particle precipitation.

Space weather forecasting and the Kp index

Because auroras are driven by solar activity, forecasting when and where they will appear is fundamentally a space-weather problem. The primary tool used by operational centers worldwide is the planetary Kp index — a dimensionless 3-hourly measure from 0 to 9 of disturbances in Earth's magnetic field, derived from a network of mid-latitude magnetometers. Values of 0–1 indicate very quiet conditions; 2–3 unsettled; 4 active; and Kp 5 or above constitutes a geomagnetic storm. NOAA maps Kp onto a G-scale for public communication: Kp 5 = G1 (minor), Kp 6 = G2 (moderate), Kp 7 = G3 (strong), Kp 8–9– = G4 (severe), and Kp 9o = G5 (extreme).

Higher Kp values correspond to a larger, more equatorward auroral oval, making Kp a practical visibility proxy. As a rough European guide, a Kp of 5 brings aurora to northern Scotland and southern Scandinavia; Kp 7 reaches northern England and northern Germany; Kp 9 can make the lights visible from southern England and central France. In the southern hemisphere, a Kp of 3 may suffice for southern Tasmania, while Kp 5 can bring aurora to the Victorian coastline and Kp 9 pushes it into the tropics.

Operational aurora forecasting works in three layers. The most immediate is nowcasting — using real-time solar-wind and interplanetary magnetic field (IMF) data from spacecraft at the Sun–Earth L1 point, combined with auroral imagery, to provide short-term probability estimates and oval locations. NOAA's 30-minute aurora forecast is the principal operational product at this timescale. Short-term forecasts of 0–3 days are built from CME tracking, coronal-hole high-speed stream modeling, and propagation models; NOAA SWPC's 3-day Kp and G-scale forecast is the standard reference. Longer outlooks to about 27 days exploit the Sun's rotation period and the recurrence of coronal holes, but are treated as trend indicators rather than precise predictions.

Besides Kp, skilled observers and automated tools also monitor solar-wind speed and density and especially the north–south component of the IMF, called Bz. A sustained southward Bz strongly favors geomagnetic activity and aurora because it allows efficient energy transfer from the solar wind into the magnetosphere. M- and X-class solar flares are tracked for their associated CMEs, with estimated arrival times at Earth providing a warning window of roughly one to three days.

Notable recent events: 2024–2025 solar maximum

Solar cycle 25 reached its peak around 2024–2025, producing a run of significant geomagnetic storms that brought auroras to latitudes not commonly reached in recent decades. The most remarkable was a series of fast CMEs from an unusually active sunspot region in early May 2024 that drove conditions to Kp 9 (G5 — extreme), the highest category on NOAA's scale. Operational Kp logs recorded multiple consecutive 3-hour intervals at Kp 8–9 during this period, and auroras were photographed across the continental United States, large parts of Europe, and comparable mid-latitudes in Asia. The event was widely described by operational space-weather centers as the strongest geomagnetic storm since at least the Halloween storms of 2003.

Across the remainder of 2024, several additional CME-driven storms reached Kp 7–8 (G3–G4), producing reliable mid-latitude auroras across North America and Europe. These events typically followed M- or X-class flares recorded in GOES X-ray flux data, with associated Earth-directed CMEs arriving after propagation times of one to three days. Throughout 2025, with solar activity remaining elevated, recurrent coronal holes and further CMEs generated multiple moderate-to-strong storms (Kp 6–7, G2–G3), and persistent coronal-hole streams contributed extended periods with Kp 4–5 or above.

Auroras beyond Earth

Every planet in the outer Solar System with a strong magnetic field supports auroral activity, though the physics differs substantially from planet to planet. Auroras have also been detected on bodies without global dipole fields — Mars and Venus — driven by crustal or induced magnetic fields and solar-wind interactions. Even a nearby brown dwarf approximately 18 light-years away shows a bright red aurora, demonstrating that auroral processes extend to substellar objects.

Jupiter hosts the most powerful auroras in the Solar System — up to roughly 1,000 times brighter than Earth's. They are dominated by far-ultraviolet and infrared emissions largely invisible to the human eye, and also produce strong radio signals. Crucially, Jupiter's auroras do not depend solely on the solar wind: the planet's rapid rotation and enormous magnetosphere generate intense auroral activity even during periods of weak solar-wind input. A major additional energy source is the plasma torus created by volcanic material from the moon Io, which is ionized, trapped in Jupiter's magnetosphere, and drives vivid auroral footprints where the field lines connecting to Io, Europa, and Ganymede intersect the Jovian atmosphere. Hubble Space Telescope UV imaging and in situ measurements by the Voyager and Juno missions have provided detailed maps of Jupiter's persistent main oval and moon-associated auroral spots.

Saturn also produces impressive auroras, brighter than Earth's though far less powerful than Jupiter's, and strongest in UV and infrared. Occasional visible-light observations have recorded pink glows with purple streaks. Saturn's auroras are primarily solar-wind-driven but receive additional contributions from plasma sourced by its moons and rings — notably from the geysers of Enceladus — and wrap around the poles as glowing rings sometimes punctuated by spots linked to moon-magnetosphere interactions. Cassini's multi-spectral observations provided the most detailed characterisation of their morphology and variability.

Uranus and Neptune both have strongly tilted and offset magnetic fields — their dipole axes are significantly misaligned from their rotation axes and displaced from the planetary centres — which means their auroras can appear at unusual latitudes rather than forming tidy polar ovals. At the great distances of these ice giants from the Sun, the solar wind is weaker, and the resulting auroras are faint and detected primarily in ultraviolet. Voyager 2 provided the first evidence of auroral emissions at both worlds during its flybys; subsequent Hubble UV imaging has added limited further detections. Both remain poorly understood compared with the gas giants.

Beyond the giants, auroral-like glows have been identified on Mercury and on comet 67P/Churyumov-Gerasimenko, further expanding the known range of auroral environments in the Solar System.

Auroral substorms and the oval

Within the overall auroral cycle driven by the solar wind, shorter episodes called auroral substorms punctuate the display. A substorm has three phases — expansion, breakup, and recovery — during which the aurora suddenly brightens, spreads outward, and then gradually fades. Substorms are caused by the explosive release of energy stored in Earth's magnetic tail, and they can transform a quiet auroral arc into a rapidly moving, pulsating curtain within minutes.

The auroral oval itself — the ring-shaped zone where aurora is most reliably seen — is not fixed in size or shape. Under quiet conditions it hugs the magnetic poles over high-latitude regions such as Canada, Alaska, Iceland, northern Norway, Sweden, Finland, Russia, and southern Greenland in the north, and the Antarctic continent and surrounding ocean in the south. During strong geomagnetic activity the oval expands significantly. A notable 1958 storm produced a display that stretched approximately 1,250 miles from Oregon to New Hampshire. The May 2024 G5 event similarly pushed the visible oval deep into mid-latitudes on multiple continents.