🌌 Aurora Borealis
Watch electrons from the solar wind spiral along Earth's magnetic field lines, crash into oxygen and nitrogen atoms at 80–300 km altitude, and excite them into their characteristic aurora colours. Adjust solar wind intensity, particle energy, and geomagnetic latitude.
The Physics of Auroras
Solar-wind electrons (and some protons) travel along open field lines into the polar cusps. They spiral around field lines due to the Lorentz force F = qv × B (gyro-radius r = mv⊥/qB). Colliding with O and N₂ in the thermosphere, they excite electrons to higher orbitals. De-excitation emits photons: green (O at 557.7 nm, 100 km), red (O at 630 nm, >200 km), and blue/purple (N₂, <100 km). High Kp index = stronger solar wind = aurora visible at lower latitudes.
Aurora Colours Explained
Green (557.7 nm) — Most
common. Oxygen atom excited at 100–150 km altitude by electron
impact. The ¹S → ¹D transition emits green light with a lifetime of
~0.7 s.
Red (630 nm) — High-altitude
aurora (>200 km). Oxygen ¹D → ³P transition, but very slow (110 s
lifetime) — only occurs where the atmosphere is sparse enough to
avoid collisional quenching.
Purple/Blue — Low-altitude
(<100 km). Molecular nitrogen N₂ excited electronic states. Very
energetic particles are needed to reach this depth.
Kp index — Planetary
geomagnetic disturbance index (0–9). Kp ≥ 5 is a geomagnetic storm.
During extreme events (Kp 9, like the 2024 May storm) aurora is
visible down to 40° latitude.
About Aurora Borealis
The Aurora Borealis simulation models how charged particles from the solar wind travel along Earth's magnetic field lines, spiral down into the polar upper atmosphere, and collide with oxygen and nitrogen atoms at altitudes of 80–300 km. The Lorentz force (F = qv x B) causes electrons to gyrate around field lines, and upon colliding with atmospheric atoms they excite electrons to higher energy states; when those atoms relax they emit photons at characteristic wavelengths — green at 557.7 nm from oxygen at 100–150 km, red at 630 nm from oxygen above 200 km, and blue-purple from molecular nitrogen below 100 km. Users can adjust the Kp geomagnetic disturbance index, particle energy in keV, and geomagnetic latitude to observe how these factors control aurora intensity, colour, and visibility.
Auroras are a direct visible indicator of space weather: the same geomagnetic storms that paint polar skies can disrupt satellite communications, GPS accuracy, and high-latitude power grids. The 2024 May geomagnetic storm (Kp 9) pushed the auroral oval so far equatorward that the aurora was visible across much of continental Europe and the United States.
Frequently Asked Questions
What causes the aurora borealis?
Auroras are produced when charged particles — mainly electrons — from the solar wind are funnelled by Earth's magnetic field into the polar regions and collide with oxygen and nitrogen atoms in the upper atmosphere at altitudes of 80–300 km. These collisions excite the atoms' electrons to higher energy states; as the electrons return to their ground state they release the excess energy as visible light. The specific colour depends on which atom is excited and at what altitude the collision occurs.
How do the simulation controls work?
The Kp index slider (1–9) controls solar wind intensity and particle flux — higher Kp produces brighter aurora and expands the auroral oval to lower latitudes. The particle energy slider (1–50 keV) sets how deeply particles penetrate the atmosphere, shifting the dominant colour from red (low energy, high altitude) through green to blue-purple (high energy, low altitude). The geomagnetic latitude slider moves the auroral oval, and the speed control lets you slow or accelerate the animation. The preset buttons load physically realistic parameter sets from Quiet Sun to X-Class Flare.
Why are there different aurora colours at different altitudes?
Each colour is a fingerprint of a specific atomic transition at a specific altitude. Green light (557.7 nm) comes from the O(1S) to O(1D) transition in oxygen atoms at 100–150 km; it is the most common aurora colour. Red light (630 nm) comes from the O(1D) to O(3P) transition in oxygen above 200 km, but only appears where the atmosphere is sparse enough that atoms are not collisionally de-excited before they can radiate — the lifetime of this transition is about 110 seconds. Blue and purple hues below 100 km come from electronically excited states of molecular nitrogen N2, which require more energetic particles to reach that depth.
What physics equations govern particle motion in the aurora?
The fundamental equation is the Lorentz force F = q(v x B), which causes a charged particle moving across a magnetic field to curve into a circular path. The gyro-radius (also called the Larmor radius) is r = mv_perp / (qB), where m is the particle mass, v_perp is the velocity component perpendicular to the field B, and q is the charge. Particles also experience a mirror force along converging field lines that can reflect them back before they reach the atmosphere; only those with a pitch angle inside the loss cone actually precipitate. Birkeland currents (field-aligned currents) carry the energy from the magnetosphere into the ionosphere along these field lines.
What is the Kp index and what does it mean for aurora visibility?
The Kp index is a global measure of geomagnetic disturbance, running from 0 (very quiet) to 9 (extreme storm), averaged from ground-based magnetometer stations every three hours. A Kp of 0–2 means aurora is confined to latitudes above 67 degrees geomagnetic; at Kp 5 (storm threshold) it reaches roughly 60 degrees; at Kp 7 it can reach 50 degrees; and at Kp 9 during extreme events like the Carrington-class storms, aurora has been seen at 40 degrees latitude or lower. During the May 2024 geomagnetic storm — one of the strongest in two decades — the aurora was visible across Spain, Italy, and the southern United States.
Is a common misconception that the aurora is caused by the sun hitting the atmosphere directly?
Yes — the sun does not shine directly into the polar atmosphere to create auroras. The particles responsible travel along Earth's magnetic field lines after being guided by the magnetosphere, not in straight lines from the sun. Most of the solar wind is deflected around Earth by the magnetopause; only particles that enter through the polar cusps or are energised in the magnetotail reconnection region make it to the atmosphere. The process is electromagnetic, not optical — visible sunlight plays no role in creating the aurora.
Who first explained the scientific origin of auroras?
Norwegian physicist Kristian Birkeland conducted pioneering laboratory experiments in the early 1900s using a magnetised sphere (his "terrella") and electron beams to reproduce aurora-like glows, proposing that electric currents flowing along magnetic field lines caused the phenomenon. The field-aligned currents he predicted — now called Birkeland currents — were confirmed by satellite measurements in 1967 by Olof Ivar Sandholt and definitively by the TRIAD satellite in 1973. Earlier, in 1896, Norwegian mathematician Carl Stormer began computing electron trajectories in a dipole field, laying the theoretical groundwork for understanding particle precipitation.
What other phenomena and simulations are connected to aurora physics?
Aurora physics is closely related to the broader solar wind-magnetosphere interaction: the Solar Wind and Magnetosphere simulation on this site shows the bow shock, magnetopause, and magnetotail that channel particles toward the poles. Van Allen radiation belts store and accelerate the same particles before they precipitate. Geomagnetic storms also connect to space weather engineering topics such as satellite drag, induced currents in power lines, and HF radio blackouts. Plasma physics in tokamak fusion reactors involves many of the same magnetic confinement and particle gyration principles as the magnetosphere.
How is aurora science used in engineering and technology today?
Understanding aurora and geomagnetic activity is essential for space weather forecasting, which protects satellites, astronauts, and ground infrastructure. NOAA's Space Weather Prediction Center issues Kp-based alerts so power grid operators can take protective action before geomagnetically induced currents (GICs) damage transformers. Satellite operators adjust orbital drag models during storms. GPS receivers correct for ionospheric delays — which are strongest during geomagnetic activity — using dual-frequency signals. High-frequency radio communications, including aviation routes over the poles, can be disrupted by aurora-related ionospheric absorption and are monitored in real time using aurora forecasts.
What are current research frontiers in aurora science?
Researchers are actively investigating "pulsating aurora" — rapidly flickering patches caused by whistler-mode chorus waves scattering electrons in the radiation belts — and "STEVE" (Strong Thermal Emission Velocity Enhancement), a narrow mauve arc distinct from traditional aurora that was identified with the help of citizen scientists around 2016 and whose full physical origin is still debated. Multi-satellite missions such as ESA's Swarm and NASA's MMS (Magnetospheric Multiscale) are measuring the fine-scale electric and magnetic structures inside Birkeland currents at resolutions previously impossible. Machine learning is also being applied to predict geomagnetic storm intensity from upstream solar wind measurements with greater lead time than traditional empirical models.