Same wind. Different field.
Choose southward wind and let the response grow. Reverse the field. The pressure stays the same, but the response fades with a delay. What changed?
FIELD NOTE 13 / AURORA JOURNEY
From Sun to sky.
A stream of particles. A changing magnetic field. A sky full of light.
0.0 model min
Wind / energy transferMagnetic fieldAuroral electronsEmitted light
The Sun sends more than light.
The solar wind is a moving plasma, mostly protons and electrons, carrying a magnetic field. Most of that flow is diverted around Earth’s magnetic environment.
Distances, particle sizes and motion are schematic. The Sun–Earth gap is compressed.
- Sun → Earth at this speed
- — days
- Solar wind pressure
- — nPa
- Dayside boundary from Earth’s centre
- — Earth radii
Watch the response lag behind.
Change Bz while it runs. The atmosphere’s response has a memory.
Wind drivingMagnetosphereAuroral response
Illustrative response indices, from 0 to 1; these are not measured brightness, stored joules or a forecast. The model begins without a built-up response.
The journey is playing. Change any control as it runs.
Make a prediction. Change one thing. See what happens.
Model notes & references ↗ZOOM IN / ONE ELECTRON
Will it reach the air?
An electron circles a field line while moving along it. As the field gets stronger, it can turn back. Change the pitch angle to discover the dividing line.
—
—
One selected electron, independent of the full aurora above. The field line is straightened, the spiral enlarged, and the flight slowed enormously. A new identical electron begins each loop.
- Electron speed
- —
- Initial gyroradius
- —
- Time to atmosphere
- —
0.2 keV20 keV
This electron’s energy is independent of the bulk solar wind speed.Along the field · 0°More sideways · 45°
The angle between the electron’s velocity and the direction toward the atmosphere along the field.Energy changes the speed and spiral radius. In this collision-free magnetic model it does not change which pitch angles can reach the atmosphere.
Oxygen · green
557.7 nm 2.22 eVA keV electron can drive many excitations. A selected atomic transition still emits photons with its own characteristic energy.
Explore the aurora colour lab ↗FOLLOW THE SCIENCE
The journey is an
exchange of energy.
For the familiar nightside aurora, a useful trail to follow is the energy. The solar wind interacts with the magnetosphere; particles already within that environment are energized and some reach the upper atmosphere. This is not a single solar electron travelling directly from the Sun to a green photon. NOAA: the origin of auroral electrons ↗
The electrons transfer energy in collisions. Oxygen and nitrogen then emit light as excited states relax. The gas is doing the glowing. Most visible auroral structures occur roughly 80–500 km above the surface, far above ordinary clouds. NOAA: how auroras form ↗
THREE QUESTIONS TO TRY
Change a cause. Look for a consequence.
Push the boundary inward.
Keep speed and Bz fixed. Double the proton density. The pressure doubles, but the boundary moves inward by only about 11%. Why such a small movement?
Make an electron turn back.
Begin at 4°. Increase the pitch angle past about 8.1°. Now increase its energy. Does more energy alone let it through this magnetic mirror?
A magnetic environment shaped by a wind.
THE DAY SIDE
Compressed toward Earth.
Solar wind pressure pushes against the magnetic environment. Most solar wind flows around it, first crossing a bow shock. The magnetopause is the boundary of the magnetosphere, not a solid surface.
THE NIGHT SIDE
Stretched into a tail.
The field extends far downstream. The magnetotail can store energy and release it through reconnection and other dynamics. Our drawing compresses that enormous region to keep the connection visible.
On average, the dayside magnetopause is about ten Earth radii from Earth’s centre. Its actual position changes with the incoming conditions. NOAA: Earth’s magnetosphere ↗
Why does the magnetic direction matter?
Near the dayside boundary, a southward interplanetary magnetic field opposes Earth’s field. Magnetic reconnection changes field connectivity and allows efficient transfer of solar-wind energy into the magnetosphere. Reconnection can also release energy in the tail. Electric fields and plasma waves help accelerate the electrons.
A faster wind can matter, but speed alone cannot tell the whole story. Sustained southward field is especially effective. A northward field does not mean the real aurora must disappear: other coupling routes, past activity and other magnetic components still matter. NOAA: conditions that drive geomagnetic storms ↗
A magnetic force bends a path; it does not directly add kinetic energy.
The magnetic term is perpendicular to velocity. Electric fields do work on charged particles. A changing magnetic configuration is part of that electromagnetic energy exchange.
F⃗ = q(E⃗ + v⃗ × B⃗) · dK/dt = qE⃗ · v⃗
A spiral can become a magnetic mirror.
In a slowly varying field, an electron’s circular motion and motion along the field are connected. Entering a stronger field makes more of its momentum perpendicular to the field. Its forward motion can fall to zero and reverse, even though its total speed is unchanged.
The small range of starting directions that reaches the atmosphere is called the loss cone. Collisions and interactions with plasma waves can move particles into that range; electric fields can change it too. The electron experiment isolates the collision-free case without an accelerating electric field. NASA: gyration, bouncing and trapping ↗
sin² α / B = constant
sin² αloss = Bstart / Batmosphere = 1 / 50
αloss ≈ 8.13°
sin² αloss = Bstart / Batmosphere = 1 / 50
αloss ≈ 8.13°
The same field geometry guides precipitation into ovals around the magnetic poles. A side view shows only slices through those ovals, rather than two luminous dots at the exact poles.
The light belongs to the air.
| Emitter | Wavelength | Photon energy | What it tells you |
|---|---|---|---|
| Atomic oxygen · green | 557.7 nm | 2.22 eV | A particular oxygen transition |
| Atomic oxygen · red | 630.0 nm | 1.97 eV | A different, longer-lived oxygen state |
| Molecular nitrogen ion · violet | 427.8 nm | 2.90 eV | One band among many nitrogen emissions |
Red oxygen emission is favoured higher up, where fewer collisions interrupt its long-lived excited state. Green oxygen emission survives at lower altitudes too. The real atmosphere has overlapping gases and emissions, rather than sharply separated colour layers. A brighter display can contain more photons without changing the energy of a particular line. NASA: auroral colours and altitude ↗
The mathematics, assumptions & references
Calculated wind relationships
Pdyn = mpnv²
Rmp = 10 RE (Pdyn / 2 nPa)−1/6
ttravel = 1 AU / v
Esouth = v max(0, −Bz)
Rmp = 10 RE (Pdyn / 2 nPa)−1/6
ttravel = 1 AU / v
Esouth = v max(0, −Bz)
Density is proton number density; alpha particles, thermal pressure and magnetic pressure in the wind are omitted. There is no ½ in this momentum-flux definition. A dipole field falls as r⁻³, so its magnetic pressure falls as r⁻⁶. Balancing that with the wind pressure gives the sixth-root scaling, normalized here to 10 Earth radii at 2 nPa. This is a teaching estimate, not a magnetopause prediction. Reconnection erosion and dipole tilt are omitted.
Travel time uses 149,597,870.7 km and constant speed; it ignores solar-wind acceleration, stream interactions and Earth’s orbital motion. Bz represents only the north–south component in a simplified Sun–Earth frame. Esouth is a convection-electric-field proxy for southward driving, not the field accelerating an individual auroral electron. With v in km/s and Bz in nT, multiply their product by 0.001 for mV/m.
An illustrative delayed response
D = Esouth / (Esouth + 3 mV/m)
dM/dt = (D − M) / 12 min
dA/dt = (M − A) / 3 min
dM/dt = (D − M) / 12 min
dA/dt = (M − A) / 3 min
D is wind driving, M is a magnetospheric response index, and A is an auroral response index. All begin at zero except the imposed driving. Two exact first-order responses demonstrate buildup and delayed fading. The time constants and normalization are chosen for teaching; they are not fitted to observations. These variables are not joules, particle fluxes, probabilities, Kp or calibrated brightness. Density influences the pressure estimate but does not enter this deliberately limited driving proxy. Northward-only driving goes to zero in this model; the real magnetosphere has additional coupling and background activity.
The clock runs at four model minutes per real second at 1×. The large-scale paths show connectivity and energy transfer, not integrated particle trajectories or an MHD solution. Magnetic topology, bow-shock position, auroral oval size and light intensity are schematic. Earth is enlarged in the magnetic-field view. Tail and electron markers show different parts of the energy pathway, not one tracked particle from the Sun. The illustrative glow follows A; changing the one-electron experiment does not switch off the whole aurora.
One electron in a converging flux tube
The tube has path length L = 4,000 km and field B(s) = B₀(1 + 49s), where s runs from 0 to 1, B₀ = 1 μT and the absorbing atmospheric endpoint is at 50 μT. Its plotted width varies as B⁻½, conserving magnetic flux. This straightened, prescribed tube is not an altitude profile or a solution for Earth’s actual field. No field-aligned electric field, scattering, gravity or radiation loss acts before the endpoint.
γ = 1 + K / (mec²) · v = c√(1 − γ⁻²)
p = γmev · rL = p sin α / (|e|B)
s(u) = cos α₀ u − (49 sin² α₀)u²/4 · u = vt/L
p = γmev · rL = p sin α / (|e|B)
s(u) = cos α₀ u − (49 sin² α₀)u²/4 · u = vt/L
Conservation of total energy and the adiabatic invariant p⊥²/B gives the parallel motion used here. A mirror occurs at B/B₀ = 1/sin² α₀, if that point lies before the atmosphere. The calculated flight time, speed and initial gyroradius use SI constants and relativistic momentum. For a mirrored electron, the time readout is one-way to its turning point. The drawing stretches each complete flight or out-and-back pass over six real seconds at 1×; drawn turns and gyroradii are enlarged and are not a resolved gyro-orbit. Energy or pitch edits evaluate a new electron at the same animation phase, with no pause or time reset.
At the atmospheric endpoint, the collision and photon sequence is illustrative. The selected emission shows hc/λ, not a full excitation cascade. Ionization, excitation thresholds, secondary electrons, atmospheric density/composition profiles, collisional quenching rates and energy-dependent stopping depths are not simulated. Selecting an emission neither changes the electron’s kinetic energy nor claims that every collision emits that line.
All views pause together and resume in place. Hidden pages freeze the clock; reduced-motion preferences start paused. The history retains the latest 120 model minutes. No live solar-wind data or location forecast is connected. Earth’s texture is NASA/GSFC Blue Marble imagery with illustrative geographic orientation.